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  97.     <ul>

  98. <li><a class="reference internal" href="#">6. Expressions</a><ul>

  99. <li><a class="reference internal" href="#arithmetic-conversions">6.1. Arithmetic conversions</a></li>

  100. <li><a class="reference internal" href="#atoms">6.2. Atoms</a><ul>

  101. <li><a class="reference internal" href="#atom-identifiers">6.2.1. Identifiers (Names)</a></li>

  102. <li><a class="reference internal" href="#literals">6.2.2. Literals</a></li>

  103. <li><a class="reference internal" href="#parenthesized-forms">6.2.3. Parenthesized forms</a></li>

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  106. <li><a class="reference internal" href="#set-displays">6.2.6. Set displays</a></li>

  107. <li><a class="reference internal" href="#dictionary-displays">6.2.7. Dictionary displays</a></li>

  108. <li><a class="reference internal" href="#generator-expressions">6.2.8. Generator expressions</a></li>

  109. <li><a class="reference internal" href="#yield-expressions">6.2.9. Yield expressions</a><ul>

  110. <li><a class="reference internal" href="#generator-iterator-methods">6.2.9.1. Generator-iterator methods</a></li>

  111. <li><a class="reference internal" href="#examples">6.2.9.2. Examples</a></li>

  112. <li><a class="reference internal" href="#asynchronous-generator-functions">6.2.9.3. Asynchronous generator functions</a></li>

  113. <li><a class="reference internal" href="#asynchronous-generator-iterator-methods">6.2.9.4. Asynchronous generator-iterator methods</a></li>

  114. </ul>

  115. </li>

  116. </ul>

  117. </li>

  118. <li><a class="reference internal" href="#primaries">6.3. Primaries</a><ul>

  119. <li><a class="reference internal" href="#attribute-references">6.3.1. Attribute references</a></li>

  120. <li><a class="reference internal" href="#subscriptions">6.3.2. Subscriptions</a></li>

  121. <li><a class="reference internal" href="#slicings">6.3.3. Slicings</a></li>

  122. <li><a class="reference internal" href="#calls">6.3.4. Calls</a></li>

  123. </ul>

  124. </li>

  125. <li><a class="reference internal" href="#await-expression">6.4. Await expression</a></li>

  126. <li><a class="reference internal" href="#the-power-operator">6.5. The power operator</a></li>

  127. <li><a class="reference internal" href="#unary-arithmetic-and-bitwise-operations">6.6. Unary arithmetic and bitwise operations</a></li>

  128. <li><a class="reference internal" href="#binary-arithmetic-operations">6.7. Binary arithmetic operations</a></li>

  129. <li><a class="reference internal" href="#shifting-operations">6.8. Shifting operations</a></li>

  130. <li><a class="reference internal" href="#binary-bitwise-operations">6.9. Binary bitwise operations</a></li>

  131. <li><a class="reference internal" href="#comparisons">6.10. Comparisons</a><ul>

  132. <li><a class="reference internal" href="#value-comparisons">6.10.1. Value comparisons</a></li>

  133. <li><a class="reference internal" href="#membership-test-operations">6.10.2. Membership test operations</a></li>

  134. <li><a class="reference internal" href="#is-not">6.10.3. Identity comparisons</a></li>

  135. </ul>

  136. </li>

  137. <li><a class="reference internal" href="#boolean-operations">6.11. Boolean operations</a></li>

  138. <li><a class="reference internal" href="#assignment-expressions">6.12. Assignment expressions</a></li>

  139. <li><a class="reference internal" href="#conditional-expressions">6.13. Conditional expressions</a></li>

  140. <li><a class="reference internal" href="#lambda">6.14. Lambdas</a></li>

  141. <li><a class="reference internal" href="#expression-lists">6.15. Expression lists</a></li>

  142. <li><a class="reference internal" href="#evaluation-order">6.16. Evaluation order</a></li>

  143. <li><a class="reference internal" href="#operator-precedence">6.17. Operator precedence</a></li>

  144. </ul>

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  235.   <section id="expressions">

  236. <span id="id1"></span><h1><span class="section-number">6. </span>Expressions<a class="headerlink" href="#expressions" title="Link to this heading">¶</a></h1>

  237. <p id="index-0">This chapter explains the meaning of the elements of expressions in Python.</p>

  238. <p><strong>Syntax Notes:</strong> In this and the following chapters, extended BNF notation will

  239. be used to describe syntax, not lexical analysis.  When (one alternative of) a

  240. syntax rule has the form</p>

  241. <pre>

  242. <strong id="grammar-token-python-grammar-name">name</strong> ::=  othername

  243. </pre>

  244. <p>and no semantics are given, the semantics of this form of <code class="docutils literal notranslate"><span class="pre">name</span></code> are the same

  245. as for <code class="docutils literal notranslate"><span class="pre">othername</span></code>.</p>

  246. <section id="arithmetic-conversions">

  247. <span id="conversions"></span><h2><span class="section-number">6.1. </span>Arithmetic conversions<a class="headerlink" href="#arithmetic-conversions" title="Link to this heading">¶</a></h2>

  248. <p id="index-1">When a description of an arithmetic operator below uses the phrase “the numeric

  249. arguments are converted to a common type”, this means that the operator

  250. implementation for built-in types works as follows:</p>

  251. <ul class="simple">

  252. <li><p>If either argument is a complex number, the other is converted to complex;</p></li>

  253. <li><p>otherwise, if either argument is a floating point number, the other is

  254. converted to floating point;</p></li>

  255. <li><p>otherwise, both must be integers and no conversion is necessary.</p></li>

  256. </ul>

  257. <p>Some additional rules apply for certain operators (e.g., a string as a left

  258. argument to the ‘%’ operator).  Extensions must define their own conversion

  259. behavior.</p>

  260. </section>

  261. <section id="atoms">

  262. <span id="id2"></span><h2><span class="section-number">6.2. </span>Atoms<a class="headerlink" href="#atoms" title="Link to this heading">¶</a></h2>

  263. <p id="index-2">Atoms are the most basic elements of expressions.  The simplest atoms are

  264. identifiers or literals.  Forms enclosed in parentheses, brackets or braces are

  265. also categorized syntactically as atoms.  The syntax for atoms is:</p>

  266. <pre>

  267. <strong id="grammar-token-python-grammar-atom">atom     </strong> ::=  <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-literal"><code class="xref docutils literal notranslate"><span class="pre">literal</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-enclosure"><code class="xref docutils literal notranslate"><span class="pre">enclosure</span></code></a>

  268. <strong id="grammar-token-python-grammar-enclosure">enclosure</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-parenth_form"><code class="xref docutils literal notranslate"><span class="pre">parenth_form</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-list_display"><code class="xref docutils literal notranslate"><span class="pre">list_display</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-dict_display"><code class="xref docutils literal notranslate"><span class="pre">dict_display</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-set_display"><code class="xref docutils literal notranslate"><span class="pre">set_display</span></code></a>

  269.                | <a class="reference internal" href="#grammar-token-python-grammar-generator_expression"><code class="xref docutils literal notranslate"><span class="pre">generator_expression</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-yield_atom"><code class="xref docutils literal notranslate"><span class="pre">yield_atom</span></code></a>

  270. </pre>

  271. <section id="atom-identifiers">

  272. <span id="identifiers-names"></span><h3><span class="section-number">6.2.1. </span>Identifiers (Names)<a class="headerlink" href="#atom-identifiers" title="Link to this heading">¶</a></h3>

  273. <p id="index-3">An identifier occurring as an atom is a name.  See section <a class="reference internal" href="lexical_analysis.html#identifiers"><span class="std std-ref">Identifiers and keywords</span></a>

  274. for lexical definition and section <a class="reference internal" href="executionmodel.html#naming"><span class="std std-ref">Naming and binding</span></a> for documentation of naming and

  275. binding.</p>

  276. <p id="index-4">When the name is bound to an object, evaluation of the atom yields that object.

  277. When a name is not bound, an attempt to evaluate it raises a <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a>

  278. exception.</p>

  279. <p id="index-5"><span id="private-name-mangling"></span><strong>Private name mangling:</strong> When an identifier that textually occurs in a class

  280. definition begins with two or more underscore characters and does not end in two

  281. or more underscores, it is considered a <em class="dfn">private name</em> of that class.

  282. Private names are transformed to a longer form before code is generated for

  283. them.  The transformation inserts the class name, with leading underscores

  284. removed and a single underscore inserted, in front of the name.  For example,

  285. the identifier <code class="docutils literal notranslate"><span class="pre">__spam</span></code> occurring in a class named <code class="docutils literal notranslate"><span class="pre">Ham</span></code> will be transformed

  286. to <code class="docutils literal notranslate"><span class="pre">_Ham__spam</span></code>.  This transformation is independent of the syntactical

  287. context in which the identifier is used.  If the transformed name is extremely

  288. long (longer than 255 characters), implementation defined truncation may happen.

  289. If the class name consists only of underscores, no transformation is done.</p>

  290. </section>

  291. <section id="literals">

  292. <span id="atom-literals"></span><h3><span class="section-number">6.2.2. </span>Literals<a class="headerlink" href="#literals" title="Link to this heading">¶</a></h3>

  293. <p id="index-6">Python supports string and bytes literals and various numeric literals:</p>

  294. <pre>

  295. <strong id="grammar-token-python-grammar-literal">literal</strong> ::=  <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-stringliteral"><code class="xref docutils literal notranslate"><span class="pre">stringliteral</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-bytesliteral"><code class="xref docutils literal notranslate"><span class="pre">bytesliteral</span></code></a>

  296.              | <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-integer"><code class="xref docutils literal notranslate"><span class="pre">integer</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-floatnumber"><code class="xref docutils literal notranslate"><span class="pre">floatnumber</span></code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-imagnumber"><code class="xref docutils literal notranslate"><span class="pre">imagnumber</span></code></a>

  297. </pre>

  298. <p>Evaluation of a literal yields an object of the given type (string, bytes,

  299. integer, floating point number, complex number) with the given value.  The value

  300. may be approximated in the case of floating point and imaginary (complex)

  301. literals.  See section <a class="reference internal" href="lexical_analysis.html#literals"><span class="std std-ref">Literals</span></a> for details.</p>

  302. <p id="index-7">All literals correspond to immutable data types, and hence the object’s identity

  303. is less important than its value.  Multiple evaluations of literals with the

  304. same value (either the same occurrence in the program text or a different

  305. occurrence) may obtain the same object or a different object with the same

  306. value.</p>

  307. </section>

  308. <section id="parenthesized-forms">

  309. <span id="parenthesized"></span><h3><span class="section-number">6.2.3. </span>Parenthesized forms<a class="headerlink" href="#parenthesized-forms" title="Link to this heading">¶</a></h3>

  310. <p id="index-8">A parenthesized form is an optional expression list enclosed in parentheses:</p>

  311. <pre>

  312. <strong id="grammar-token-python-grammar-parenth_form">parenth_form</strong> ::=  &quot;(&quot; [<a class="reference internal" href="#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate"><span class="pre">starred_expression</span></code></a>] &quot;)&quot;

  313. </pre>

  314. <p>A parenthesized expression list yields whatever that expression list yields: if

  315. the list contains at least one comma, it yields a tuple; otherwise, it yields

  316. the single expression that makes up the expression list.</p>

  317. <p id="index-9">An empty pair of parentheses yields an empty tuple object.  Since tuples are

  318. immutable, the same rules as for literals apply (i.e., two occurrences of the empty

  319. tuple may or may not yield the same object).</p>

  320. <p id="index-10">Note that tuples are not formed by the parentheses, but rather by use of the

  321. comma.  The exception is the empty tuple, for which parentheses <em>are</em>

  322. required — allowing unparenthesized “nothing” in expressions would cause

  323. ambiguities and allow common typos to pass uncaught.</p>

  324. </section>

  325. <section id="displays-for-lists-sets-and-dictionaries">

  326. <span id="comprehensions"></span><h3><span class="section-number">6.2.4. </span>Displays for lists, sets and dictionaries<a class="headerlink" href="#displays-for-lists-sets-and-dictionaries" title="Link to this heading">¶</a></h3>

  327. <p id="index-11">For constructing a list, a set or a dictionary Python provides special syntax

  328. called “displays”, each of them in two flavors:</p>

  329. <ul class="simple">

  330. <li><p>either the container contents are listed explicitly, or</p></li>

  331. <li><p>they are computed via a set of looping and filtering instructions, called a

  332. <em class="dfn">comprehension</em>.</p></li>

  333. </ul>

  334. <p id="index-12">Common syntax elements for comprehensions are:</p>

  335. <pre>

  336. <strong id="grammar-token-python-grammar-comprehension">comprehension</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate"><span class="pre">assignment_expression</span></code></a> <a class="reference internal" href="#grammar-token-python-grammar-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a>

  337. <strong id="grammar-token-python-grammar-comp_for">comp_for     </strong> ::=  [&quot;async&quot;] &quot;for&quot; <a class="reference internal" href="simple_stmts.html#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> &quot;in&quot; <a class="reference internal" href="#grammar-token-python-grammar-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> [<a class="reference internal" href="#grammar-token-python-grammar-comp_iter"><code class="xref docutils literal notranslate"><span class="pre">comp_iter</span></code></a>]

  338. <strong id="grammar-token-python-grammar-comp_iter">comp_iter    </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-comp_if"><code class="xref docutils literal notranslate"><span class="pre">comp_if</span></code></a>

  339. <strong id="grammar-token-python-grammar-comp_if">comp_if      </strong> ::=  &quot;if&quot; <a class="reference internal" href="#grammar-token-python-grammar-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> [<a class="reference internal" href="#grammar-token-python-grammar-comp_iter"><code class="xref docutils literal notranslate"><span class="pre">comp_iter</span></code></a>]

  340. </pre>

  341. <p>The comprehension consists of a single expression followed by at least one

  342. <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause and zero or more <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code> clauses.

  343. In this case, the elements of the new container are those that would be produced

  344. by considering each of the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code> clauses a block,

  345. nesting from left to right, and evaluating the expression to produce an element

  346. each time the innermost block is reached.</p>

  347. <p>However, aside from the iterable expression in the leftmost <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause,

  348. the comprehension is executed in a separate implicitly nested scope. This ensures

  349. that names assigned to in the target list don’t “leak” into the enclosing scope.</p>

  350. <p>The iterable expression in the leftmost <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause is evaluated

  351. directly in the enclosing scope and then passed as an argument to the implicitly

  352. nested scope. Subsequent <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clauses and any filter condition in the

  353. leftmost <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause cannot be evaluated in the enclosing scope as

  354. they may depend on the values obtained from the leftmost iterable. For example:

  355. <code class="docutils literal notranslate"><span class="pre">[x*y</span> <span class="pre">for</span> <span class="pre">x</span> <span class="pre">in</span> <span class="pre">range(10)</span> <span class="pre">for</span> <span class="pre">y</span> <span class="pre">in</span> <span class="pre">range(x,</span> <span class="pre">x+10)]</span></code>.</p>

  356. <p>To ensure the comprehension always results in a container of the appropriate

  357. type, <code class="docutils literal notranslate"><span class="pre">yield</span></code> and <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> expressions are prohibited in the implicitly

  358. nested scope.</p>

  359. <p id="index-13">Since Python 3.6, in an <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> function, an <code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code>

  360. clause may be used to iterate over a <a class="reference internal" href="../glossary.html#term-asynchronous-iterator"><span class="xref std std-term">asynchronous iterator</span></a>.

  361. A comprehension in an <code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code> function may consist of either a

  362. <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> clause following the leading

  363. expression, may contain additional <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code>

  364. clauses, and may also use <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions.

  365. If a comprehension contains either <code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> clauses or

  366. <code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code> expressions or other asynchronous comprehensions it is called

  367. an <em class="dfn">asynchronous comprehension</em>.  An asynchronous comprehension may

  368. suspend the execution of the coroutine function in which it appears.

  369. See also <span class="target" id="index-14"></span><a class="pep reference external" href="https://peps.python.org/pep-0530/"><strong>PEP 530</strong></a>.</p>

  370. <div class="versionadded">

  371. <p><span class="versionmodified added">New in version 3.6: </span>Asynchronous comprehensions were introduced.</p>

  372. </div>

  373. <div class="versionchanged">

  374. <p><span class="versionmodified changed">Changed in version 3.8: </span><code class="docutils literal notranslate"><span class="pre">yield</span></code> and <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> prohibited in the implicitly nested scope.</p>

  375. </div>

  376. <div class="versionchanged">

  377. <p><span class="versionmodified changed">Changed in version 3.11: </span>Asynchronous comprehensions are now allowed inside comprehensions in

  378. asynchronous functions. Outer comprehensions implicitly become

  379. asynchronous.</p>

  380. </div>

  381. </section>

  382. <section id="list-displays">

  383. <span id="lists"></span><h3><span class="section-number">6.2.5. </span>List displays<a class="headerlink" href="#list-displays" title="Link to this heading">¶</a></h3>

  384. <p id="index-15">A list display is a possibly empty series of expressions enclosed in square

  385. brackets:</p>

  386. <pre>

  387. <strong id="grammar-token-python-grammar-list_display">list_display</strong> ::=  &quot;[&quot; [<a class="reference internal" href="#grammar-token-python-grammar-starred_list"><code class="xref docutils literal notranslate"><span class="pre">starred_list</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-comprehension"><code class="xref docutils literal notranslate"><span class="pre">comprehension</span></code></a>] &quot;]&quot;

  388. </pre>

  389. <p>A list display yields a new list object, the contents being specified by either

  390. a list of expressions or a comprehension.  When a comma-separated list of

  391. expressions is supplied, its elements are evaluated from left to right and

  392. placed into the list object in that order.  When a comprehension is supplied,

  393. the list is constructed from the elements resulting from the comprehension.</p>

  394. </section>

  395. <section id="set-displays">

  396. <span id="set"></span><h3><span class="section-number">6.2.6. </span>Set displays<a class="headerlink" href="#set-displays" title="Link to this heading">¶</a></h3>

  397. <p id="index-16">A set display is denoted by curly braces and distinguishable from dictionary

  398. displays by the lack of colons separating keys and values:</p>

  399. <pre>

  400. <strong id="grammar-token-python-grammar-set_display">set_display</strong> ::=  &quot;{&quot; (<a class="reference internal" href="#grammar-token-python-grammar-starred_list"><code class="xref docutils literal notranslate"><span class="pre">starred_list</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-comprehension"><code class="xref docutils literal notranslate"><span class="pre">comprehension</span></code></a>) &quot;}&quot;

  401. </pre>

  402. <p>A set display yields a new mutable set object, the contents being specified by

  403. either a sequence of expressions or a comprehension.  When a comma-separated

  404. list of expressions is supplied, its elements are evaluated from left to right

  405. and added to the set object.  When a comprehension is supplied, the set is

  406. constructed from the elements resulting from the comprehension.</p>

  407. <p>An empty set cannot be constructed with <code class="docutils literal notranslate"><span class="pre">{}</span></code>; this literal constructs an empty

  408. dictionary.</p>

  409. </section>

  410. <section id="dictionary-displays">

  411. <span id="dict"></span><h3><span class="section-number">6.2.7. </span>Dictionary displays<a class="headerlink" href="#dictionary-displays" title="Link to this heading">¶</a></h3>

  412. <p id="index-17">A dictionary display is a possibly empty series of dict items (key/value pairs)

  413. enclosed in curly braces:</p>

  414. <pre>

  415. <strong id="grammar-token-python-grammar-dict_display">dict_display      </strong> ::=  &quot;{&quot; [<a class="reference internal" href="#grammar-token-python-grammar-dict_item_list"><code class="xref docutils literal notranslate"><span class="pre">dict_item_list</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-dict_comprehension"><code class="xref docutils literal notranslate"><span class="pre">dict_comprehension</span></code></a>] &quot;}&quot;

  416. <strong id="grammar-token-python-grammar-dict_item_list">dict_item_list    </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-dict_item"><code class="xref docutils literal notranslate"><span class="pre">dict_item</span></code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-dict_item"><code class="xref docutils literal notranslate"><span class="pre">dict_item</span></code></a>)* [&quot;,&quot;]

  417. <strong id="grammar-token-python-grammar-dict_item">dict_item         </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | &quot;**&quot; <a class="reference internal" href="#grammar-token-python-grammar-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a>

  418. <strong id="grammar-token-python-grammar-dict_comprehension">dict_comprehension</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <a class="reference internal" href="#grammar-token-python-grammar-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a>

  419. </pre>

  420. <p>A dictionary display yields a new dictionary object.</p>

  421. <p>If a comma-separated sequence of dict items is given, they are evaluated

  422. from left to right to define the entries of the dictionary: each key object is

  423. used as a key into the dictionary to store the corresponding value.  This means

  424. that you can specify the same key multiple times in the dict item list, and the

  425. final dictionary’s value for that key will be the last one given.</p>

  426. <p id="index-18">A double asterisk <code class="docutils literal notranslate"><span class="pre">**</span></code> denotes <em class="dfn">dictionary unpacking</em>.

  427. Its operand must be a <a class="reference internal" href="../glossary.html#term-mapping"><span class="xref std std-term">mapping</span></a>.  Each mapping item is added

  428. to the new dictionary.  Later values replace values already set by

  429. earlier dict items and earlier dictionary unpackings.</p>

  430. <div class="versionadded">

  431. <p><span class="versionmodified added">New in version 3.5: </span>Unpacking into dictionary displays, originally proposed by <span class="target" id="index-19"></span><a class="pep reference external" href="https://peps.python.org/pep-0448/"><strong>PEP 448</strong></a>.</p>

  432. </div>

  433. <p>A dict comprehension, in contrast to list and set comprehensions, needs two

  434. expressions separated with a colon followed by the usual “for” and “if” clauses.

  435. When the comprehension is run, the resulting key and value elements are inserted

  436. in the new dictionary in the order they are produced.</p>

  437. <p id="index-20">Restrictions on the types of the key values are listed earlier in section

  438. <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>.  (To summarize, the key type should be <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a>, which excludes

  439. all mutable objects.)  Clashes between duplicate keys are not detected; the last

  440. value (textually rightmost in the display) stored for a given key value

  441. prevails.</p>

  442. <div class="versionchanged">

  443. <p><span class="versionmodified changed">Changed in version 3.8: </span>Prior to Python 3.8, in dict comprehensions, the evaluation order of key

  444. and value was not well-defined.  In CPython, the value was evaluated before

  445. the key.  Starting with 3.8, the key is evaluated before the value, as

  446. proposed by <span class="target" id="index-21"></span><a class="pep reference external" href="https://peps.python.org/pep-0572/"><strong>PEP 572</strong></a>.</p>

  447. </div>

  448. </section>

  449. <section id="generator-expressions">

  450. <span id="genexpr"></span><h3><span class="section-number">6.2.8. </span>Generator expressions<a class="headerlink" href="#generator-expressions" title="Link to this heading">¶</a></h3>

  451. <p id="index-22">A generator expression is a compact generator notation in parentheses:</p>

  452. <pre>

  453. <strong id="grammar-token-python-grammar-generator_expression">generator_expression</strong> ::=  &quot;(&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> <a class="reference internal" href="#grammar-token-python-grammar-comp_for"><code class="xref docutils literal notranslate"><span class="pre">comp_for</span></code></a> &quot;)&quot;

  454. </pre>

  455. <p>A generator expression yields a new generator object.  Its syntax is the same as

  456. for comprehensions, except that it is enclosed in parentheses instead of

  457. brackets or curly braces.</p>

  458. <p>Variables used in the generator expression are evaluated lazily when the

  459. <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method is called for the generator object (in the same

  460. fashion as normal generators).  However, the iterable expression in the

  461. leftmost <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause is immediately evaluated, so that an error

  462. produced by it will be emitted at the point where the generator expression

  463. is defined, rather than at the point where the first value is retrieved.

  464. Subsequent <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clauses and any filter condition in the leftmost

  465. <code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code> clause cannot be evaluated in the enclosing scope as they may

  466. depend on the values obtained from the leftmost iterable. For example:

  467. <code class="docutils literal notranslate"><span class="pre">(x*y</span> <span class="pre">for</span> <span class="pre">x</span> <span class="pre">in</span> <span class="pre">range(10)</span> <span class="pre">for</span> <span class="pre">y</span> <span class="pre">in</span> <span class="pre">range(x,</span> <span class="pre">x+10))</span></code>.</p>

  468. <p>The parentheses can be omitted on calls with only one argument.  See section

  469. <a class="reference internal" href="#calls"><span class="std std-ref">Calls</span></a> for details.</p>

  470. <p>To avoid interfering with the expected operation of the generator expression

  471. itself, <code class="docutils literal notranslate"><span class="pre">yield</span></code> and <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> expressions are prohibited in the

  472. implicitly defined generator.</p>

  473. <p>If a generator expression contains either <code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code>

  474. clauses or <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions it is called an

  475. <em class="dfn">asynchronous generator expression</em>.  An asynchronous generator

  476. expression returns a new asynchronous generator object,

  477. which is an asynchronous iterator (see <a class="reference internal" href="datamodel.html#async-iterators"><span class="std std-ref">Asynchronous Iterators</span></a>).</p>

  478. <div class="versionadded">

  479. <p><span class="versionmodified added">New in version 3.6: </span>Asynchronous generator expressions were introduced.</p>

  480. </div>

  481. <div class="versionchanged">

  482. <p><span class="versionmodified changed">Changed in version 3.7: </span>Prior to Python 3.7, asynchronous generator expressions could

  483. only appear in <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> coroutines.  Starting

  484. with 3.7, any function can use asynchronous generator expressions.</p>

  485. </div>

  486. <div class="versionchanged">

  487. <p><span class="versionmodified changed">Changed in version 3.8: </span><code class="docutils literal notranslate"><span class="pre">yield</span></code> and <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> prohibited in the implicitly nested scope.</p>

  488. </div>

  489. </section>

  490. <section id="yield-expressions">

  491. <span id="yieldexpr"></span><h3><span class="section-number">6.2.9. </span>Yield expressions<a class="headerlink" href="#yield-expressions" title="Link to this heading">¶</a></h3>

  492. <pre id="index-23">

  493. <strong id="grammar-token-python-grammar-yield_atom">yield_atom      </strong> ::=  &quot;(&quot; <a class="reference internal" href="#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a> &quot;)&quot;

  494. <strong id="grammar-token-python-grammar-yield_from">yield_from      </strong> ::=  &quot;yield&quot; &quot;from&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  495. <strong id="grammar-token-python-grammar-yield_expression">yield_expression</strong> ::=  &quot;yield&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-yield_from"><code class="xref docutils literal notranslate"><span class="pre">yield_from</span></code></a>

  496. </pre>

  497. <p>The yield expression is used when defining a <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a> function

  498. or an <a class="reference internal" href="../glossary.html#term-asynchronous-generator"><span class="xref std std-term">asynchronous generator</span></a> function and

  499. thus can only be used in the body of a function definition.  Using a yield

  500. expression in a function’s body causes that function to be a generator function,

  501. and using it in an <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> function’s body causes that

  502. coroutine function to be an asynchronous generator function. For example:</p>

  503. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">gen</span><span class="p">():</span>  <span class="c1"># defines a generator function</span>

  504.     <span class="k">yield</span> <span class="mi">123</span>

  505. 

  506. <span class="k">async</span> <span class="k">def</span> <span class="nf">agen</span><span class="p">():</span> <span class="c1"># defines an asynchronous generator function</span>

  507.     <span class="k">yield</span> <span class="mi">123</span>

  508. </pre></div>

  509. </div>

  510. <p>Due to their side effects on the containing scope, <code class="docutils literal notranslate"><span class="pre">yield</span></code> expressions

  511. are not permitted as part of the implicitly defined scopes used to

  512. implement comprehensions and generator expressions.</p>

  513. <div class="versionchanged">

  514. <p><span class="versionmodified changed">Changed in version 3.8: </span>Yield expressions prohibited in the implicitly nested scopes used to

  515. implement comprehensions and generator expressions.</p>

  516. </div>

  517. <p>Generator functions are described below, while asynchronous generator

  518. functions are described separately in section

  519. <a class="reference internal" href="#asynchronous-generator-functions"><span class="std std-ref">Asynchronous generator functions</span></a>.</p>

  520. <p>When a generator function is called, it returns an iterator known as a

  521. generator.  That generator then controls the execution of the generator

  522. function.  The execution starts when one of the generator’s methods is called.

  523. At that time, the execution proceeds to the first yield expression, where it is

  524. suspended again, returning the value of <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a>

  525. to the generator’s caller,

  526. or <code class="docutils literal notranslate"><span class="pre">None</span></code> if <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> is omitted.

  527. By suspended, we mean that all local state is

  528. retained, including the current bindings of local variables, the instruction

  529. pointer, the internal evaluation stack, and the state of any exception handling.

  530. When the execution is resumed by calling one of the generator’s methods, the

  531. function can proceed exactly as if the yield expression were just another

  532. external call.  The value of the yield expression after resuming depends on the

  533. method which resumed the execution.  If <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> is used

  534. (typically via either a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> or the <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate"><span class="pre">next()</span></code></a> builtin) then the

  535. result is <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>.  Otherwise, if <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> is used, then

  536. the result will be the value passed in to that method.</p>

  537. <p id="index-24">All of this makes generator functions quite similar to coroutines; they yield

  538. multiple times, they have more than one entry point and their execution can be

  539. suspended.  The only difference is that a generator function cannot control

  540. where the execution should continue after it yields; the control is always

  541. transferred to the generator’s caller.</p>

  542. <p>Yield expressions are allowed anywhere in a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> construct.  If the

  543. generator is not resumed before it is

  544. finalized (by reaching a zero reference count or by being garbage collected),

  545. the generator-iterator’s <a class="reference internal" href="#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> method will be called,

  546. allowing any pending <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clauses to execute.</p>

  547. <p id="index-25">When <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre">&lt;expr&gt;</span></code> is used, the supplied expression must be an

  548. iterable. The values produced by iterating that iterable are passed directly

  549. to the caller of the current generator’s methods. Any values passed in with

  550. <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> and any exceptions passed in with

  551. <a class="reference internal" href="#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate"><span class="pre">throw()</span></code></a> are passed to the underlying iterator if it has the

  552. appropriate methods.  If this is not the case, then <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a>

  553. will raise <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> or <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>, while

  554. <a class="reference internal" href="#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate"><span class="pre">throw()</span></code></a> will just raise the passed in exception immediately.</p>

  555. <p>When the underlying iterator is complete, the <a class="reference internal" href="../library/exceptions.html#StopIteration.value" title="StopIteration.value"><code class="xref py py-attr docutils literal notranslate"><span class="pre">value</span></code></a>

  556. attribute of the raised <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> instance becomes the value of

  557. the yield expression. It can be either set explicitly when raising

  558. <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a>, or automatically when the subiterator is a generator

  559. (by returning a value from the subgenerator).</p>

  560. <div class="versionchanged">

  561. <p><span class="versionmodified changed">Changed in version 3.3: </span>Added <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre">&lt;expr&gt;</span></code> to delegate control flow to a subiterator.</p>

  562. </div>

  563. <p>The parentheses may be omitted when the yield expression is the sole expression

  564. on the right hand side of an assignment statement.</p>

  565. <div class="admonition seealso">

  566. <p class="admonition-title">See also</p>

  567. <dl class="simple">

  568. <dt><span class="target" id="index-26"></span><a class="pep reference external" href="https://peps.python.org/pep-0255/"><strong>PEP 255</strong></a> - Simple Generators</dt><dd><p>The proposal for adding generators and the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement to Python.</p>

  569. </dd>

  570. <dt><span class="target" id="index-27"></span><a class="pep reference external" href="https://peps.python.org/pep-0342/"><strong>PEP 342</strong></a> - Coroutines via Enhanced Generators</dt><dd><p>The proposal to enhance the API and syntax of generators, making them

  571. usable as simple coroutines.</p>

  572. </dd>

  573. <dt><span class="target" id="index-28"></span><a class="pep reference external" href="https://peps.python.org/pep-0380/"><strong>PEP 380</strong></a> - Syntax for Delegating to a Subgenerator</dt><dd><p>The proposal to introduce the <a class="reference internal" href="#grammar-token-python-grammar-yield_from"><code class="xref std std-token docutils literal notranslate"><span class="pre">yield_from</span></code></a> syntax,

  574. making delegation to subgenerators easy.</p>

  575. </dd>

  576. <dt><span class="target" id="index-29"></span><a class="pep reference external" href="https://peps.python.org/pep-0525/"><strong>PEP 525</strong></a> - Asynchronous Generators</dt><dd><p>The proposal that expanded on <span class="target" id="index-30"></span><a class="pep reference external" href="https://peps.python.org/pep-0492/"><strong>PEP 492</strong></a> by adding generator capabilities to

  577. coroutine functions.</p>

  578. </dd>

  579. </dl>

  580. </div>

  581. <section id="generator-iterator-methods">

  582. <span id="generator-methods"></span><span id="index-31"></span><h4><span class="section-number">6.2.9.1. </span>Generator-iterator methods<a class="headerlink" href="#generator-iterator-methods" title="Link to this heading">¶</a></h4>

  583. <p>This subsection describes the methods of a generator iterator.  They can

  584. be used to control the execution of a generator function.</p>

  585. <p>Note that calling any of the generator methods below when the generator

  586. is already executing raises a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> exception.</p>

  587. <dl class="py method" id="index-32">

  588. <dt class="sig sig-object py" id="generator.__next__">

  589. <span class="sig-prename descclassname"><span class="pre">generator.</span></span><span class="sig-name descname"><span class="pre">__next__</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#generator.__next__" title="Link to this definition">¶</a></dt>

  590. <dd><p>Starts the execution of a generator function or resumes it at the last

  591. executed yield expression.  When a generator function is resumed with a

  592. <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method, the current yield expression always

  593. evaluates to <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>.  The execution then continues to the next yield

  594. expression, where the generator is suspended again, and the value of the

  595. <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> is returned to <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a>’s

  596. caller.  If the generator exits without yielding another value, a

  597. <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception is raised.</p>

  598. <p>This method is normally called implicitly, e.g. by a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop, or

  599. by the built-in <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate"><span class="pre">next()</span></code></a> function.</p>

  600. </dd></dl>

  601. 

  602. <dl class="py method">

  603. <dt class="sig sig-object py" id="generator.send">

  604. <span class="sig-prename descclassname"><span class="pre">generator.</span></span><span class="sig-name descname"><span class="pre">send</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#generator.send" title="Link to this definition">¶</a></dt>

  605. <dd><p>Resumes the execution and “sends” a value into the generator function.  The

  606. <em>value</em> argument becomes the result of the current yield expression.  The

  607. <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> method returns the next value yielded by the generator, or

  608. raises <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> if the generator exits without yielding another

  609. value.  When <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> is called to start the generator, it must be called

  610. with <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> as the argument, because there is no yield expression that

  611. could receive the value.</p>

  612. </dd></dl>

  613. 

  614. <dl class="py method">

  615. <dt class="sig sig-object py" id="generator.throw">

  616. <span class="sig-prename descclassname"><span class="pre">generator.</span></span><span class="sig-name descname"><span class="pre">throw</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#generator.throw" title="Link to this definition">¶</a></dt>

  617. <dt class="sig sig-object py">

  618. <span class="sig-prename descclassname"><span class="pre">generator.</span></span><span class="sig-name descname"><span class="pre">throw</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">type</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">traceback</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span></dt>

  619. <dd><p>Raises an exception at the point where the generator was paused,

  620. and returns the next value yielded by the generator function.  If the generator

  621. exits without yielding another value, a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception is

  622. raised.  If the generator function does not catch the passed-in exception, or

  623. raises a different exception, then that exception propagates to the caller.</p>

  624. <p>In typical use, this is called with a single exception instance similar to the

  625. way the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> keyword is used.</p>

  626. <p>For backwards compatibility, however, the second signature is

  627. supported, following a convention from older versions of Python.

  628. The <em>type</em> argument should be an exception class, and <em>value</em>

  629. should be an exception instance. If the <em>value</em> is not provided, the

  630. <em>type</em> constructor is called to get an instance. If <em>traceback</em>

  631. is provided, it is set on the exception, otherwise any existing

  632. <a class="reference internal" href="../library/exceptions.html#BaseException.__traceback__" title="BaseException.__traceback__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__traceback__</span></code></a> attribute stored in <em>value</em> may

  633. be cleared.</p>

  634. <div class="versionchanged">

  635. <p><span class="versionmodified changed">Changed in version 3.12: </span>The second signature (type[, value[, traceback]]) is deprecated and

  636. may be removed in a future version of Python.</p>

  637. </div>

  638. </dd></dl>

  639. 

  640. <dl class="py method" id="index-33">

  641. <dt class="sig sig-object py" id="generator.close">

  642. <span class="sig-prename descclassname"><span class="pre">generator.</span></span><span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#generator.close" title="Link to this definition">¶</a></dt>

  643. <dd><p>Raises a <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> at the point where the generator function was

  644. paused.  If the generator function then exits gracefully, is already closed,

  645. or raises <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> (by not catching the exception), close

  646. returns to its caller.  If the generator yields a value, a

  647. <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> is raised.  If the generator raises any other exception,

  648. it is propagated to the caller.  <a class="reference internal" href="#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> does nothing if the generator

  649. has already exited due to an exception or normal exit.</p>

  650. </dd></dl>

  651. 

  652. </section>

  653. <section id="examples">

  654. <span id="index-34"></span><h4><span class="section-number">6.2.9.2. </span>Examples<a class="headerlink" href="#examples" title="Link to this heading">¶</a></h4>

  655. <p>Here is a simple example that demonstrates the behavior of generators and

  656. generator functions:</p>

  657. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">echo</span><span class="p">(</span><span class="n">value</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>

  658. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Execution starts when &#39;next()&#39; is called for the first time.&quot;</span><span class="p">)</span>

  659. <span class="gp">... </span>    <span class="k">try</span><span class="p">:</span>

  660. <span class="gp">... </span>        <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>

  661. <span class="gp">... </span>            <span class="k">try</span><span class="p">:</span>

  662. <span class="gp">... </span>                <span class="n">value</span> <span class="o">=</span> <span class="p">(</span><span class="k">yield</span> <span class="n">value</span><span class="p">)</span>

  663. <span class="gp">... </span>            <span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>

  664. <span class="gp">... </span>                <span class="n">value</span> <span class="o">=</span> <span class="n">e</span>

  665. <span class="gp">... </span>    <span class="k">finally</span><span class="p">:</span>

  666. <span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Don&#39;t forget to clean up when &#39;close()&#39; is called.&quot;</span><span class="p">)</span>

  667. <span class="gp">...</span>

  668. <span class="gp">&gt;&gt;&gt; </span><span class="n">generator</span> <span class="o">=</span> <span class="n">echo</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>

  669. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">generator</span><span class="p">))</span>

  670. <span class="go">Execution starts when &#39;next()&#39; is called for the first time.</span>

  671. <span class="go">1</span>

  672. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">generator</span><span class="p">))</span>

  673. <span class="go">None</span>

  674. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">generator</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="mi">2</span><span class="p">))</span>

  675. <span class="go">2</span>

  676. <span class="gp">&gt;&gt;&gt; </span><span class="n">generator</span><span class="o">.</span><span class="n">throw</span><span class="p">(</span><span class="ne">TypeError</span><span class="p">,</span> <span class="s2">&quot;spam&quot;</span><span class="p">)</span>

  677. <span class="go">TypeError(&#39;spam&#39;,)</span>

  678. <span class="gp">&gt;&gt;&gt; </span><span class="n">generator</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  679. <span class="go">Don&#39;t forget to clean up when &#39;close()&#39; is called.</span>

  680. </pre></div>

  681. </div>

  682. <p>For examples using <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code>, see <a class="reference internal" href="../whatsnew/3.3.html#pep-380"><span class="std std-ref">PEP 380: Syntax for Delegating to a Subgenerator</span></a> in “What’s New in

  683. Python.”</p>

  684. </section>

  685. <section id="asynchronous-generator-functions">

  686. <span id="id3"></span><h4><span class="section-number">6.2.9.3. </span>Asynchronous generator functions<a class="headerlink" href="#asynchronous-generator-functions" title="Link to this heading">¶</a></h4>

  687. <p>The presence of a yield expression in a function or method defined using

  688. <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> further defines the function as an

  689. <a class="reference internal" href="../glossary.html#term-asynchronous-generator"><span class="xref std std-term">asynchronous generator</span></a> function.</p>

  690. <p>When an asynchronous generator function is called, it returns an

  691. asynchronous iterator known as an asynchronous generator object.

  692. That object then controls the execution of the generator function.

  693. An asynchronous generator object is typically used in an

  694. <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> statement in a coroutine function analogously to

  695. how a generator object would be used in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statement.</p>

  696. <p>Calling one of the asynchronous generator’s methods returns an <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a>

  697. object, and the execution starts when this object is awaited on. At that time,

  698. the execution proceeds to the first yield expression, where it is suspended

  699. again, returning the value of <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> to the

  700. awaiting coroutine. As with a generator, suspension means that all local state

  701. is retained, including the current bindings of local variables, the instruction

  702. pointer, the internal evaluation stack, and the state of any exception handling.

  703. When the execution is resumed by awaiting on the next object returned by the

  704. asynchronous generator’s methods, the function can proceed exactly as if the

  705. yield expression were just another external call. The value of the yield

  706. expression after resuming depends on the method which resumed the execution.  If

  707. <a class="reference internal" href="#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a> is used then the result is <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>. Otherwise, if

  708. <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asend()</span></code></a> is used, then the result will be the value passed in to that

  709. method.</p>

  710. <p>If an asynchronous generator happens to exit early by <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a>, the caller

  711. task being cancelled, or other exceptions, the generator’s async cleanup code

  712. will run and possibly raise exceptions or access context variables in an

  713. unexpected context–perhaps after the lifetime of tasks it depends, or

  714. during the event loop shutdown when the async-generator garbage collection hook

  715. is called.

  716. To prevent this, the caller must explicitly close the async generator by calling

  717. <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> method to finalize the generator and ultimately detach it

  718. from the event loop.</p>

  719. <p>In an asynchronous generator function, yield expressions are allowed anywhere

  720. in a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> construct. However, if an asynchronous generator is not

  721. resumed before it is finalized (by reaching a zero reference count or by

  722. being garbage collected), then a yield expression within a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code>

  723. construct could result in a failure to execute pending <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a>

  724. clauses.  In this case, it is the responsibility of the event loop or

  725. scheduler running the asynchronous generator to call the asynchronous

  726. generator-iterator’s <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> method and run the resulting

  727. coroutine object, thus allowing any pending <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clauses

  728. to execute.</p>

  729. <p>To take care of finalization upon event loop termination, an event loop should

  730. define a <em>finalizer</em> function which takes an asynchronous generator-iterator and

  731. presumably calls <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> and executes the coroutine.

  732. This  <em>finalizer</em> may be registered by calling <a class="reference internal" href="../library/sys.html#sys.set_asyncgen_hooks" title="sys.set_asyncgen_hooks"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.set_asyncgen_hooks()</span></code></a>.

  733. When first iterated over, an asynchronous generator-iterator will store the

  734. registered <em>finalizer</em> to be called upon finalization. For a reference example

  735. of a <em>finalizer</em> method see the implementation of

  736. <code class="docutils literal notranslate"><span class="pre">asyncio.Loop.shutdown_asyncgens</span></code> in <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/asyncio/base_events.py">Lib/asyncio/base_events.py</a>.</p>

  737. <p>The expression <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre">&lt;expr&gt;</span></code> is a syntax error when used in an

  738. asynchronous generator function.</p>

  739. </section>

  740. <section id="asynchronous-generator-iterator-methods">

  741. <span id="asynchronous-generator-methods"></span><span id="index-35"></span><h4><span class="section-number">6.2.9.4. </span>Asynchronous generator-iterator methods<a class="headerlink" href="#asynchronous-generator-iterator-methods" title="Link to this heading">¶</a></h4>

  742. <p>This subsection describes the methods of an asynchronous generator iterator,

  743. which are used to control the execution of a generator function.</p>

  744. <dl class="py method" id="index-36">

  745. <dt class="sig sig-object py" id="agen.__anext__">

  746. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-prename descclassname"><span class="pre">agen.</span></span><span class="sig-name descname"><span class="pre">__anext__</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#agen.__anext__" title="Link to this definition">¶</a></dt>

  747. <dd><p>Returns an awaitable which when run starts to execute the asynchronous

  748. generator or resumes it at the last executed yield expression.  When an

  749. asynchronous generator function is resumed with an <a class="reference internal" href="#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a>

  750. method, the current yield expression always evaluates to <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> in the

  751. returned awaitable, which when run will continue to the next yield

  752. expression. The value of the <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> of the

  753. yield expression is the value of the <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception raised by

  754. the completing coroutine.  If the asynchronous generator exits without

  755. yielding another value, the awaitable instead raises a

  756. <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopAsyncIteration</span></code></a> exception, signalling that the asynchronous

  757. iteration has completed.</p>

  758. <p>This method is normally called implicitly by a <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> loop.</p>

  759. </dd></dl>

  760. 

  761. <dl class="py method">

  762. <dt class="sig sig-object py" id="agen.asend">

  763. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-prename descclassname"><span class="pre">agen.</span></span><span class="sig-name descname"><span class="pre">asend</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#agen.asend" title="Link to this definition">¶</a></dt>

  764. <dd><p>Returns an awaitable which when run resumes the execution of the

  765. asynchronous generator. As with the <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code></a> method for a

  766. generator, this “sends” a value into the asynchronous generator function,

  767. and the <em>value</em> argument becomes the result of the current yield expression.

  768. The awaitable returned by the <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asend()</span></code></a> method will return the next

  769. value yielded by the generator as the value of the raised

  770. <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a>, or raises <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopAsyncIteration</span></code></a> if the

  771. asynchronous generator exits without yielding another value.  When

  772. <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asend()</span></code></a> is called to start the asynchronous

  773. generator, it must be called with <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> as the argument,

  774. because there is no yield expression that could receive the value.</p>

  775. </dd></dl>

  776. 

  777. <dl class="py method">

  778. <dt class="sig sig-object py" id="agen.athrow">

  779. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-prename descclassname"><span class="pre">agen.</span></span><span class="sig-name descname"><span class="pre">athrow</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#agen.athrow" title="Link to this definition">¶</a></dt>

  780. <dt class="sig sig-object py">

  781. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-prename descclassname"><span class="pre">agen.</span></span><span class="sig-name descname"><span class="pre">athrow</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">type</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">traceback</span></span></em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span></dt>

  782. <dd><p>Returns an awaitable that raises an exception of type <code class="docutils literal notranslate"><span class="pre">type</span></code> at the point

  783. where the asynchronous generator was paused, and returns the next value

  784. yielded by the generator function as the value of the raised

  785. <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception.  If the asynchronous generator exits

  786. without yielding another value, a <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopAsyncIteration</span></code></a> exception is

  787. raised by the awaitable.

  788. If the generator function does not catch the passed-in exception, or

  789. raises a different exception, then when the awaitable is run that exception

  790. propagates to the caller of the awaitable.</p>

  791. <div class="versionchanged">

  792. <p><span class="versionmodified changed">Changed in version 3.12: </span>The second signature (type[, value[, traceback]]) is deprecated and

  793. may be removed in a future version of Python.</p>

  794. </div>

  795. </dd></dl>

  796. 

  797. <dl class="py method" id="index-37">

  798. <dt class="sig sig-object py" id="agen.aclose">

  799. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-prename descclassname"><span class="pre">agen.</span></span><span class="sig-name descname"><span class="pre">aclose</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#agen.aclose" title="Link to this definition">¶</a></dt>

  800. <dd><p>Returns an awaitable that when run will throw a <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> into

  801. the asynchronous generator function at the point where it was paused.

  802. If the asynchronous generator function then exits gracefully, is already

  803. closed, or raises <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> (by not catching the exception),

  804. then the returned awaitable will raise a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception.

  805. Any further awaitables returned by subsequent calls to the asynchronous

  806. generator will raise a <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopAsyncIteration</span></code></a> exception.  If the

  807. asynchronous generator yields a value, a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> is raised

  808. by the awaitable.  If the asynchronous generator raises any other exception,

  809. it is propagated to the caller of the awaitable.  If the asynchronous

  810. generator has already exited due to an exception or normal exit, then

  811. further calls to <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> will return an awaitable that does nothing.</p>

  812. </dd></dl>

  813. 

  814. </section>

  815. </section>

  816. </section>

  817. <section id="primaries">

  818. <span id="id4"></span><h2><span class="section-number">6.3. </span>Primaries<a class="headerlink" href="#primaries" title="Link to this heading">¶</a></h2>

  819. <p id="index-38">Primaries represent the most tightly bound operations of the language. Their

  820. syntax is:</p>

  821. <pre>

  822. <strong id="grammar-token-python-grammar-primary">primary</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-atom"><code class="xref docutils literal notranslate"><span class="pre">atom</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-attributeref"><code class="xref docutils literal notranslate"><span class="pre">attributeref</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-subscription"><code class="xref docutils literal notranslate"><span class="pre">subscription</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-slicing"><code class="xref docutils literal notranslate"><span class="pre">slicing</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-call"><code class="xref docutils literal notranslate"><span class="pre">call</span></code></a>

  823. </pre>

  824. <section id="attribute-references">

  825. <span id="id5"></span><h3><span class="section-number">6.3.1. </span>Attribute references<a class="headerlink" href="#attribute-references" title="Link to this heading">¶</a></h3>

  826. <p id="index-39">An attribute reference is a primary followed by a period and a name:</p>

  827. <pre>

  828. <strong id="grammar-token-python-grammar-attributeref">attributeref</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> &quot;.&quot; <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a>

  829. </pre>

  830. <p id="index-40">The primary must evaluate to an object of a type that supports attribute

  831. references, which most objects do.  This object is then asked to produce the

  832. attribute whose name is the identifier. The type and value produced is

  833. determined by the object.  Multiple evaluations of the same attribute

  834. reference may yield different objects.</p>

  835. <p>This production can be customized by overriding the

  836. <a class="reference internal" href="datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a> method or the <a class="reference internal" href="datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a>

  837. method.  The <code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code> method is called first and either

  838. returns a value or raises <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> if the attribute is not

  839. available.</p>

  840. <p>If an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> is raised and the object has a <code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code>

  841. method, that method is called as a fallback.</p>

  842. </section>

  843. <section id="subscriptions">

  844. <span id="id6"></span><h3><span class="section-number">6.3.2. </span>Subscriptions<a class="headerlink" href="#subscriptions" title="Link to this heading">¶</a></h3>

  845. <p id="index-42"><span id="index-41"></span>The subscription of an instance of a <a class="reference internal" href="datamodel.html#sequence-types"><span class="std std-ref">container class</span></a>

  846. will generally select an element from the container. The subscription of a

  847. <a class="reference internal" href="../glossary.html#term-generic-type"><span class="xref std std-term">generic class</span></a> will generally return a

  848. <a class="reference internal" href="../library/stdtypes.html#types-genericalias"><span class="std std-ref">GenericAlias</span></a> object.</p>

  849. <pre>

  850. <strong id="grammar-token-python-grammar-subscription">subscription</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> &quot;[&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> &quot;]&quot;

  851. </pre>

  852. <p>When an object is subscripted, the interpreter will evaluate the primary and

  853. the expression list.</p>

  854. <p>The primary must evaluate to an object that supports subscription. An object

  855. may support subscription through defining one or both of

  856. <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> and <a class="reference internal" href="datamodel.html#object.__class_getitem__" title="object.__class_getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__class_getitem__()</span></code></a>. When the

  857. primary is subscripted, the evaluated result of the expression list will be

  858. passed to one of these methods. For more details on when <code class="docutils literal notranslate"><span class="pre">__class_getitem__</span></code>

  859. is called instead of <code class="docutils literal notranslate"><span class="pre">__getitem__</span></code>, see <a class="reference internal" href="datamodel.html#classgetitem-versus-getitem"><span class="std std-ref">__class_getitem__ versus __getitem__</span></a>.</p>

  860. <p>If the expression list contains at least one comma, it will evaluate to a

  861. <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a> containing the items of the expression list. Otherwise, the

  862. expression list will evaluate to the value of the list’s sole member.</p>

  863. <p>For built-in objects, there are two types of objects that support subscription

  864. via <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>:</p>

  865. <ol class="arabic simple">

  866. <li><p>Mappings. If the primary is a <a class="reference internal" href="../glossary.html#term-mapping"><span class="xref std std-term">mapping</span></a>, the expression list must

  867. evaluate to an object whose value is one of the keys of the mapping, and the

  868. subscription selects the value in the mapping that corresponds to that key.

  869. An example of a builtin mapping class is the <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> class.</p></li>

  870. <li><p>Sequences. If the primary is a <a class="reference internal" href="../glossary.html#term-sequence"><span class="xref std std-term">sequence</span></a>, the expression list must

  871. evaluate to an <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> or a <a class="reference internal" href="../library/functions.html#slice" title="slice"><code class="xref py py-class docutils literal notranslate"><span class="pre">slice</span></code></a> (as discussed in the

  872. following section). Examples of builtin sequence classes include the

  873. <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a>, <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a> and <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a> classes.</p></li>

  874. </ol>

  875. <p>The formal syntax makes no special provision for negative indices in

  876. <a class="reference internal" href="../glossary.html#term-sequence"><span class="xref std std-term">sequences</span></a>. However, built-in sequences all provide a <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>

  877. method that interprets negative indices by adding the length of the sequence

  878. to the index so that, for example, <code class="docutils literal notranslate"><span class="pre">x[-1]</span></code> selects the last item of <code class="docutils literal notranslate"><span class="pre">x</span></code>. The

  879. resulting value must be a nonnegative integer less than the number of items in

  880. the sequence, and the subscription selects the item whose index is that value

  881. (counting from zero). Since the support for negative indices and slicing

  882. occurs in the object’s <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method, subclasses overriding

  883. this method will need to explicitly add that support.</p>

  884. <p id="index-43">A <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">string</span></code></a> is a special kind of sequence whose items are

  885. <em>characters</em>. A character is not a separate data type but a

  886. string of exactly one character.</p>

  887. </section>

  888. <section id="slicings">

  889. <span id="id7"></span><h3><span class="section-number">6.3.3. </span>Slicings<a class="headerlink" href="#slicings" title="Link to this heading">¶</a></h3>

  890. <p id="index-45"><span id="index-44"></span>A slicing selects a range of items in a sequence object (e.g., a string, tuple

  891. or list).  Slicings may be used as expressions or as targets in assignment or

  892. <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statements.  The syntax for a slicing:</p>

  893. <pre>

  894. <strong id="grammar-token-python-grammar-slicing">slicing     </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> &quot;[&quot; <a class="reference internal" href="#grammar-token-python-grammar-slice_list"><code class="xref docutils literal notranslate"><span class="pre">slice_list</span></code></a> &quot;]&quot;

  895. <strong id="grammar-token-python-grammar-slice_list">slice_list  </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-slice_item"><code class="xref docutils literal notranslate"><span class="pre">slice_item</span></code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-slice_item"><code class="xref docutils literal notranslate"><span class="pre">slice_item</span></code></a>)* [&quot;,&quot;]

  896. <strong id="grammar-token-python-grammar-slice_item">slice_item  </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-proper_slice"><code class="xref docutils literal notranslate"><span class="pre">proper_slice</span></code></a>

  897. <strong id="grammar-token-python-grammar-proper_slice">proper_slice</strong> ::=  [<a class="reference internal" href="#grammar-token-python-grammar-lower_bound"><code class="xref docutils literal notranslate"><span class="pre">lower_bound</span></code></a>] &quot;:&quot; [<a class="reference internal" href="#grammar-token-python-grammar-upper_bound"><code class="xref docutils literal notranslate"><span class="pre">upper_bound</span></code></a>] [ &quot;:&quot; [<a class="reference internal" href="#grammar-token-python-grammar-stride"><code class="xref docutils literal notranslate"><span class="pre">stride</span></code></a>] ]

  898. <strong id="grammar-token-python-grammar-lower_bound">lower_bound </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  899. <strong id="grammar-token-python-grammar-upper_bound">upper_bound </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  900. <strong id="grammar-token-python-grammar-stride">stride      </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  901. </pre>

  902. <p>There is ambiguity in the formal syntax here: anything that looks like an

  903. expression list also looks like a slice list, so any subscription can be

  904. interpreted as a slicing.  Rather than further complicating the syntax, this is

  905. disambiguated by defining that in this case the interpretation as a subscription

  906. takes priority over the interpretation as a slicing (this is the case if the

  907. slice list contains no proper slice).</p>

  908. <p id="index-46">The semantics for a slicing are as follows.  The primary is indexed (using the

  909. same <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a> method as

  910. normal subscription) with a key that is constructed from the slice list, as

  911. follows.  If the slice list contains at least one comma, the key is a tuple

  912. containing the conversion of the slice items; otherwise, the conversion of the

  913. lone slice item is the key.  The conversion of a slice item that is an

  914. expression is that expression.  The conversion of a proper slice is a slice

  915. object (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>) whose <a class="reference internal" href="../library/functions.html#slice.start" title="slice.start"><code class="xref py py-attr docutils literal notranslate"><span class="pre">start</span></code></a>,

  916. <a class="reference internal" href="../library/functions.html#slice.stop" title="slice.stop"><code class="xref py py-attr docutils literal notranslate"><span class="pre">stop</span></code></a> and <a class="reference internal" href="../library/functions.html#slice.step" title="slice.step"><code class="xref py py-attr docutils literal notranslate"><span class="pre">step</span></code></a> attributes are the values of the

  917. expressions given as lower bound, upper bound and stride, respectively,

  918. substituting <code class="docutils literal notranslate"><span class="pre">None</span></code> for missing expressions.</p>

  919. </section>

  920. <section id="calls">

  921. <span id="index-47"></span><span id="id8"></span><h3><span class="section-number">6.3.4. </span>Calls<a class="headerlink" href="#calls" title="Link to this heading">¶</a></h3>

  922. <p>A call calls a callable object (e.g., a <a class="reference internal" href="../glossary.html#term-function"><span class="xref std std-term">function</span></a>) with a possibly empty

  923. series of <a class="reference internal" href="../glossary.html#term-argument"><span class="xref std std-term">arguments</span></a>:</p>

  924. <pre>

  925. <strong id="grammar-token-python-grammar-call">call                </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a> &quot;(&quot; [<a class="reference internal" href="#grammar-token-python-grammar-argument_list"><code class="xref docutils literal notranslate"><span class="pre">argument_list</span></code></a> [&quot;,&quot;] | <a class="reference internal" href="#grammar-token-python-grammar-comprehension"><code class="xref docutils literal notranslate"><span class="pre">comprehension</span></code></a>] &quot;)&quot;

  926. <strong id="grammar-token-python-grammar-argument_list">argument_list       </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-positional_arguments"><code class="xref docutils literal notranslate"><span class="pre">positional_arguments</span></code></a> [&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-starred_and_keywords"><code class="xref docutils literal notranslate"><span class="pre">starred_and_keywords</span></code></a>]

  927.                             [&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-keywords_arguments"><code class="xref docutils literal notranslate"><span class="pre">keywords_arguments</span></code></a>]

  928.                           | <a class="reference internal" href="#grammar-token-python-grammar-starred_and_keywords"><code class="xref docutils literal notranslate"><span class="pre">starred_and_keywords</span></code></a> [&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-keywords_arguments"><code class="xref docutils literal notranslate"><span class="pre">keywords_arguments</span></code></a>]

  929.                           | <a class="reference internal" href="#grammar-token-python-grammar-keywords_arguments"><code class="xref docutils literal notranslate"><span class="pre">keywords_arguments</span></code></a>

  930. <strong id="grammar-token-python-grammar-positional_arguments">positional_arguments</strong> ::=  positional_item (&quot;,&quot; positional_item)*

  931. <strong id="grammar-token-python-grammar-positional_item">positional_item     </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate"><span class="pre">assignment_expression</span></code></a> | &quot;*&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  932. <strong id="grammar-token-python-grammar-starred_and_keywords">starred_and_keywords</strong> ::=  (&quot;*&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a>)

  933.                           (&quot;,&quot; &quot;*&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | &quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a>)*

  934. <strong id="grammar-token-python-grammar-keywords_arguments">keywords_arguments  </strong> ::=  (<a class="reference internal" href="#grammar-token-python-grammar-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a> | &quot;**&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)

  935.                           (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-keyword_item"><code class="xref docutils literal notranslate"><span class="pre">keyword_item</span></code></a> | &quot;,&quot; &quot;**&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)*

  936. <strong id="grammar-token-python-grammar-keyword_item">keyword_item        </strong> ::=  <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> &quot;=&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  937. </pre>

  938. <p>An optional trailing comma may be present after the positional and keyword arguments

  939. but does not affect the semantics.</p>

  940. <p id="index-48">The primary must evaluate to a callable object (user-defined functions, built-in

  941. functions, methods of built-in objects, class objects, methods of class

  942. instances, and all objects having a <a class="reference internal" href="datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method are callable).  All

  943. argument expressions are evaluated before the call is attempted.  Please refer

  944. to section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a> for the syntax of formal <a class="reference internal" href="../glossary.html#term-parameter"><span class="xref std std-term">parameter</span></a> lists.</p>

  945. <p>If keyword arguments are present, they are first converted to positional

  946. arguments, as follows.  First, a list of unfilled slots is created for the

  947. formal parameters.  If there are N positional arguments, they are placed in the

  948. first N slots.  Next, for each keyword argument, the identifier is used to

  949. determine the corresponding slot (if the identifier is the same as the first

  950. formal parameter name, the first slot is used, and so on).  If the slot is

  951. already filled, a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised. Otherwise, the

  952. argument is placed in the slot, filling it (even if the expression is

  953. <code class="docutils literal notranslate"><span class="pre">None</span></code>, it fills the slot).  When all arguments have been processed, the slots

  954. that are still unfilled are filled with the corresponding default value from the

  955. function definition.  (Default values are calculated, once, when the function is

  956. defined; thus, a mutable object such as a list or dictionary used as default

  957. value will be shared by all calls that don’t specify an argument value for the

  958. corresponding slot; this should usually be avoided.)  If there are any unfilled

  959. slots for which no default value is specified, a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is

  960. raised.  Otherwise, the list of filled slots is used as the argument list for

  961. the call.</p>

  962. <div class="impl-detail compound">

  963. <p><strong>CPython implementation detail:</strong> An implementation may provide built-in functions whose positional parameters

  964. do not have names, even if they are ‘named’ for the purpose of documentation,

  965. and which therefore cannot be supplied by keyword.  In CPython, this is the

  966. case for functions implemented in C that use <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> to

  967. parse their arguments.</p>

  968. </div>

  969. <p>If there are more positional arguments than there are formal parameter slots, a

  970. <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised, unless a formal parameter using the syntax

  971. <code class="docutils literal notranslate"><span class="pre">*identifier</span></code> is present; in this case, that formal parameter receives a tuple

  972. containing the excess positional arguments (or an empty tuple if there were no

  973. excess positional arguments).</p>

  974. <p>If any keyword argument does not correspond to a formal parameter name, a

  975. <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised, unless a formal parameter using the syntax

  976. <code class="docutils literal notranslate"><span class="pre">**identifier</span></code> is present; in this case, that formal parameter receives a

  977. dictionary containing the excess keyword arguments (using the keywords as keys

  978. and the argument values as corresponding values), or a (new) empty dictionary if

  979. there were no excess keyword arguments.</p>

  980. <p id="index-49">If the syntax <code class="docutils literal notranslate"><span class="pre">*expression</span></code> appears in the function call, <code class="docutils literal notranslate"><span class="pre">expression</span></code> must

  981. evaluate to an <a class="reference internal" href="../glossary.html#term-iterable"><span class="xref std std-term">iterable</span></a>.  Elements from these iterables are

  982. treated as if they were additional positional arguments.  For the call

  983. <code class="docutils literal notranslate"><span class="pre">f(x1,</span> <span class="pre">x2,</span> <span class="pre">*y,</span> <span class="pre">x3,</span> <span class="pre">x4)</span></code>, if <em>y</em> evaluates to a sequence <em>y1</em>, …, <em>yM</em>,

  984. this is equivalent to a call with M+4 positional arguments <em>x1</em>, <em>x2</em>,

  985. <em>y1</em>, …, <em>yM</em>, <em>x3</em>, <em>x4</em>.</p>

  986. <p>A consequence of this is that although the <code class="docutils literal notranslate"><span class="pre">*expression</span></code> syntax may appear

  987. <em>after</em> explicit keyword arguments, it is processed <em>before</em> the

  988. keyword arguments (and any <code class="docutils literal notranslate"><span class="pre">**expression</span></code> arguments – see below).  So:</p>

  989. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span>

  990. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">)</span>

  991. <span class="gp">...</span>

  992. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="p">(</span><span class="n">b</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">2</span><span class="p">,))</span>

  993. <span class="go">2 1</span>

  994. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="p">(</span><span class="n">a</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">2</span><span class="p">,))</span>

  995. <span class="gt">Traceback (most recent call last):</span>

  996.   File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">1</span>, in <span class="n">&lt;module&gt;</span>

  997. <span class="gr">TypeError</span>: <span class="n">f() got multiple values for keyword argument &#39;a&#39;</span>

  998. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="o">*</span><span class="p">(</span><span class="mi">2</span><span class="p">,))</span>

  999. <span class="go">1 2</span>

  1000. </pre></div>

  1001. </div>

  1002. <p>It is unusual for both keyword arguments and the <code class="docutils literal notranslate"><span class="pre">*expression</span></code> syntax to be

  1003. used in the same call, so in practice this confusion does not often arise.</p>

  1004. <p id="index-50">If the syntax <code class="docutils literal notranslate"><span class="pre">**expression</span></code> appears in the function call, <code class="docutils literal notranslate"><span class="pre">expression</span></code> must

  1005. evaluate to a <a class="reference internal" href="../glossary.html#term-mapping"><span class="xref std std-term">mapping</span></a>, the contents of which are treated as

  1006. additional keyword arguments. If a parameter matching a key has already been

  1007. given a value (by an explicit keyword argument, or from another unpacking),

  1008. a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised.</p>

  1009. <p>When <code class="docutils literal notranslate"><span class="pre">**expression</span></code> is used, each key in this mapping must be

  1010. a string.

  1011. Each value from the mapping is assigned to the first formal parameter

  1012. eligible for keyword assignment whose name is equal to the key.

  1013. A key need not be a Python identifier (e.g. <code class="docutils literal notranslate"><span class="pre">&quot;max-temp</span> <span class="pre">°F&quot;</span></code> is acceptable,

  1014. although it will not match any formal parameter that could be declared).

  1015. If there is no match to a formal parameter

  1016. the key-value pair is collected by the <code class="docutils literal notranslate"><span class="pre">**</span></code> parameter, if there is one,

  1017. or if there is not, a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised.</p>

  1018. <p>Formal parameters using the syntax <code class="docutils literal notranslate"><span class="pre">*identifier</span></code> or <code class="docutils literal notranslate"><span class="pre">**identifier</span></code> cannot be

  1019. used as positional argument slots or as keyword argument names.</p>

  1020. <div class="versionchanged">

  1021. <p><span class="versionmodified changed">Changed in version 3.5: </span>Function calls accept any number of <code class="docutils literal notranslate"><span class="pre">*</span></code> and <code class="docutils literal notranslate"><span class="pre">**</span></code> unpackings,

  1022. positional arguments may follow iterable unpackings (<code class="docutils literal notranslate"><span class="pre">*</span></code>),

  1023. and keyword arguments may follow dictionary unpackings (<code class="docutils literal notranslate"><span class="pre">**</span></code>).

  1024. Originally proposed by <span class="target" id="index-51"></span><a class="pep reference external" href="https://peps.python.org/pep-0448/"><strong>PEP 448</strong></a>.</p>

  1025. </div>

  1026. <p>A call always returns some value, possibly <code class="docutils literal notranslate"><span class="pre">None</span></code>, unless it raises an

  1027. exception.  How this value is computed depends on the type of the callable

  1028. object.</p>

  1029. <p>If it is—</p>

  1030. <dl>

  1031. <dt>a user-defined function:</dt><dd><p id="index-52">The code block for the function is executed, passing it the argument list.  The

  1032. first thing the code block will do is bind the formal parameters to the

  1033. arguments; this is described in section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a>.  When the code block

  1034. executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement, this specifies the return value of the

  1035. function call.</p>

  1036. </dd>

  1037. <dt>a built-in function or method:</dt><dd><p id="index-53">The result is up to the interpreter; see <a class="reference internal" href="../library/functions.html#built-in-funcs"><span class="std std-ref">Built-in Functions</span></a> for the

  1038. descriptions of built-in functions and methods.</p>

  1039. </dd>

  1040. <dt>a class object:</dt><dd><p id="index-54">A new instance of that class is returned.</p>

  1041. </dd>

  1042. <dt>a class instance method:</dt><dd><p id="index-55">The corresponding user-defined function is called, with an argument list that is

  1043. one longer than the argument list of the call: the instance becomes the first

  1044. argument.</p>

  1045. </dd>

  1046. <dt>a class instance:</dt><dd><p id="index-56">The class must define a <a class="reference internal" href="datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__call__()</span></code></a> method; the effect is then the same as

  1047. if that method was called.</p>

  1048. </dd>

  1049. </dl>

  1050. </section>

  1051. </section>

  1052. <section id="await-expression">

  1053. <span id="await"></span><span id="index-57"></span><h2><span class="section-number">6.4. </span>Await expression<a class="headerlink" href="#await-expression" title="Link to this heading">¶</a></h2>

  1054. <p>Suspend the execution of <a class="reference internal" href="../glossary.html#term-coroutine"><span class="xref std std-term">coroutine</span></a> on an <a class="reference internal" href="../glossary.html#term-awaitable"><span class="xref std std-term">awaitable</span></a> object.

  1055. Can only be used inside a <a class="reference internal" href="../glossary.html#term-coroutine-function"><span class="xref std std-term">coroutine function</span></a>.</p>

  1056. <pre>

  1057. <strong id="grammar-token-python-grammar-await_expr">await_expr</strong> ::=  &quot;await&quot; <a class="reference internal" href="#grammar-token-python-grammar-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a>

  1058. </pre>

  1059. <div class="versionadded">

  1060. <p><span class="versionmodified added">New in version 3.5.</span></p>

  1061. </div>

  1062. </section>

  1063. <section id="the-power-operator">

  1064. <span id="power"></span><h2><span class="section-number">6.5. </span>The power operator<a class="headerlink" href="#the-power-operator" title="Link to this heading">¶</a></h2>

  1065. <p id="index-58">The power operator binds more tightly than unary operators on its left; it binds

  1066. less tightly than unary operators on its right.  The syntax is:</p>

  1067. <pre>

  1068. <strong id="grammar-token-python-grammar-power">power</strong> ::=  (<a class="reference internal" href="#grammar-token-python-grammar-await_expr"><code class="xref docutils literal notranslate"><span class="pre">await_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-primary"><code class="xref docutils literal notranslate"><span class="pre">primary</span></code></a>) [&quot;**&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a>]

  1069. </pre>

  1070. <p>Thus, in an unparenthesized sequence of power and unary operators, the operators

  1071. are evaluated from right to left (this does not constrain the evaluation order

  1072. for the operands): <code class="docutils literal notranslate"><span class="pre">-1**2</span></code> results in <code class="docutils literal notranslate"><span class="pre">-1</span></code>.</p>

  1073. <p>The power operator has the same semantics as the built-in <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate"><span class="pre">pow()</span></code></a> function,

  1074. when called with two arguments: it yields its left argument raised to the power

  1075. of its right argument.  The numeric arguments are first converted to a common

  1076. type, and the result is of that type.</p>

  1077. <p>For int operands, the result has the same type as the operands unless the second

  1078. argument is negative; in that case, all arguments are converted to float and a

  1079. float result is delivered. For example, <code class="docutils literal notranslate"><span class="pre">10**2</span></code> returns <code class="docutils literal notranslate"><span class="pre">100</span></code>, but

  1080. <code class="docutils literal notranslate"><span class="pre">10**-2</span></code> returns <code class="docutils literal notranslate"><span class="pre">0.01</span></code>.</p>

  1081. <p>Raising <code class="docutils literal notranslate"><span class="pre">0.0</span></code> to a negative power results in a <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a>.

  1082. Raising a negative number to a fractional power results in a <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate"><span class="pre">complex</span></code></a>

  1083. number. (In earlier versions it raised a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>.)</p>

  1084. <p>This operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__pow__" title="object.__pow__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__pow__()</span></code></a> method.</p>

  1085. </section>

  1086. <section id="unary-arithmetic-and-bitwise-operations">

  1087. <span id="unary"></span><h2><span class="section-number">6.6. </span>Unary arithmetic and bitwise operations<a class="headerlink" href="#unary-arithmetic-and-bitwise-operations" title="Link to this heading">¶</a></h2>

  1088. <p id="index-59">All unary arithmetic and bitwise operations have the same priority:</p>

  1089. <pre>

  1090. <strong id="grammar-token-python-grammar-u_expr">u_expr</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-power"><code class="xref docutils literal notranslate"><span class="pre">power</span></code></a> | &quot;-&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | &quot;+&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | &quot;~&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a>

  1091. </pre>

  1092. <p id="index-60">The unary <code class="docutils literal notranslate"><span class="pre">-</span></code> (minus) operator yields the negation of its numeric argument; the

  1093. operation can be overridden with the <a class="reference internal" href="datamodel.html#object.__neg__" title="object.__neg__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__neg__()</span></code></a> special method.</p>

  1094. <p id="index-61">The unary <code class="docutils literal notranslate"><span class="pre">+</span></code> (plus) operator yields its numeric argument unchanged; the

  1095. operation can be overridden with the <a class="reference internal" href="datamodel.html#object.__pos__" title="object.__pos__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__pos__()</span></code></a> special method.</p>

  1096. <p id="index-62">The unary <code class="docutils literal notranslate"><span class="pre">~</span></code> (invert) operator yields the bitwise inversion of its integer

  1097. argument.  The bitwise inversion of <code class="docutils literal notranslate"><span class="pre">x</span></code> is defined as <code class="docutils literal notranslate"><span class="pre">-(x+1)</span></code>.  It only

  1098. applies to integral numbers or to custom objects that override the

  1099. <a class="reference internal" href="datamodel.html#object.__invert__" title="object.__invert__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__invert__()</span></code></a> special method.</p>

  1100. <p id="index-63">In all three cases, if the argument does not have the proper type, a

  1101. <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised.</p>

  1102. </section>

  1103. <section id="binary-arithmetic-operations">

  1104. <span id="binary"></span><h2><span class="section-number">6.7. </span>Binary arithmetic operations<a class="headerlink" href="#binary-arithmetic-operations" title="Link to this heading">¶</a></h2>

  1105. <p id="index-64">The binary arithmetic operations have the conventional priority levels.  Note

  1106. that some of these operations also apply to certain non-numeric types.  Apart

  1107. from the power operator, there are only two levels, one for multiplicative

  1108. operators and one for additive operators:</p>

  1109. <pre>

  1110. <strong id="grammar-token-python-grammar-m_expr">m_expr</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> &quot;*&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> &quot;&#64;&quot; <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> |

  1111.             <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> &quot;//&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> &quot;/&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a> |

  1112.             <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> &quot;%&quot; <a class="reference internal" href="#grammar-token-python-grammar-u_expr"><code class="xref docutils literal notranslate"><span class="pre">u_expr</span></code></a>

  1113. <strong id="grammar-token-python-grammar-a_expr">a_expr</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> &quot;+&quot; <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> &quot;-&quot; <a class="reference internal" href="#grammar-token-python-grammar-m_expr"><code class="xref docutils literal notranslate"><span class="pre">m_expr</span></code></a>

  1114. </pre>

  1115. <p id="index-65">The <code class="docutils literal notranslate"><span class="pre">*</span></code> (multiplication) operator yields the product of its arguments.  The

  1116. arguments must either both be numbers, or one argument must be an integer and

  1117. the other must be a sequence. In the former case, the numbers are converted to a

  1118. common type and then multiplied together.  In the latter case, sequence

  1119. repetition is performed; a negative repetition factor yields an empty sequence.</p>

  1120. <p>This operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__mul__" title="object.__mul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__mul__()</span></code></a> and

  1121. <a class="reference internal" href="datamodel.html#object.__rmul__" title="object.__rmul__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rmul__()</span></code></a> methods.</p>

  1122. <p id="index-66">The <code class="docutils literal notranslate"><span class="pre">&#64;</span></code> (at) operator is intended to be used for matrix multiplication.  No

  1123. builtin Python types implement this operator.</p>

  1124. <div class="versionadded">

  1125. <p><span class="versionmodified added">New in version 3.5.</span></p>

  1126. </div>

  1127. <p id="index-67">The <code class="docutils literal notranslate"><span class="pre">/</span></code> (division) and <code class="docutils literal notranslate"><span class="pre">//</span></code> (floor division) operators yield the quotient of

  1128. their arguments.  The numeric arguments are first converted to a common type.

  1129. Division of integers yields a float, while floor division of integers results in an

  1130. integer; the result is that of mathematical division with the ‘floor’ function

  1131. applied to the result.  Division by zero raises the <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a>

  1132. exception.</p>

  1133. <p>This operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__truediv__" title="object.__truediv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__truediv__()</span></code></a> and

  1134. <a class="reference internal" href="datamodel.html#object.__floordiv__" title="object.__floordiv__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__floordiv__()</span></code></a> methods.</p>

  1135. <p id="index-68">The <code class="docutils literal notranslate"><span class="pre">%</span></code> (modulo) operator yields the remainder from the division of the first

  1136. argument by the second.  The numeric arguments are first converted to a common

  1137. type.  A zero right argument raises the <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a> exception.  The

  1138. arguments may be floating point numbers, e.g., <code class="docutils literal notranslate"><span class="pre">3.14%0.7</span></code> equals <code class="docutils literal notranslate"><span class="pre">0.34</span></code>

  1139. (since <code class="docutils literal notranslate"><span class="pre">3.14</span></code> equals <code class="docutils literal notranslate"><span class="pre">4*0.7</span> <span class="pre">+</span> <span class="pre">0.34</span></code>.)  The modulo operator always yields a

  1140. result with the same sign as its second operand (or zero); the absolute value of

  1141. the result is strictly smaller than the absolute value of the second operand

  1142. <a class="footnote-reference brackets" href="#id17" id="id9" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a>.</p>

  1143. <p>The floor division and modulo operators are connected by the following

  1144. identity: <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">(x//y)*y</span> <span class="pre">+</span> <span class="pre">(x%y)</span></code>.  Floor division and modulo are also

  1145. connected with the built-in function <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">divmod()</span></code></a>: <code class="docutils literal notranslate"><span class="pre">divmod(x,</span> <span class="pre">y)</span> <span class="pre">==</span> <span class="pre">(x//y,</span>

  1146. <span class="pre">x%y)</span></code>. <a class="footnote-reference brackets" href="#id18" id="id10" role="doc-noteref"><span class="fn-bracket">[</span>2<span class="fn-bracket">]</span></a>.</p>

  1147. <p>In addition to performing the modulo operation on numbers, the <code class="docutils literal notranslate"><span class="pre">%</span></code> operator is

  1148. also overloaded by string objects to perform old-style string formatting (also

  1149. known as interpolation).  The syntax for string formatting is described in the

  1150. Python Library Reference, section <a class="reference internal" href="../library/stdtypes.html#old-string-formatting"><span class="std std-ref">printf-style String Formatting</span></a>.</p>

  1151. <p>The <em>modulo</em> operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__mod__" title="object.__mod__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__mod__()</span></code></a> method.</p>

  1152. <p>The floor division operator, the modulo operator, and the <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">divmod()</span></code></a>

  1153. function are not defined for complex numbers.  Instead, convert to a floating

  1154. point number using the <a class="reference internal" href="../library/functions.html#abs" title="abs"><code class="xref py py-func docutils literal notranslate"><span class="pre">abs()</span></code></a> function if appropriate.</p>

  1155. <p id="index-69">The <code class="docutils literal notranslate"><span class="pre">+</span></code> (addition) operator yields the sum of its arguments.  The arguments

  1156. must either both be numbers or both be sequences of the same type.  In the

  1157. former case, the numbers are converted to a common type and then added together.

  1158. In the latter case, the sequences are concatenated.</p>

  1159. <p>This operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__add__()</span></code></a> and

  1160. <a class="reference internal" href="datamodel.html#object.__radd__" title="object.__radd__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__radd__()</span></code></a> methods.</p>

  1161. <p id="index-70">The <code class="docutils literal notranslate"><span class="pre">-</span></code> (subtraction) operator yields the difference of its arguments.  The

  1162. numeric arguments are first converted to a common type.</p>

  1163. <p>This operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__sub__" title="object.__sub__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__sub__()</span></code></a> method.</p>

  1164. </section>

  1165. <section id="shifting-operations">

  1166. <span id="shifting"></span><h2><span class="section-number">6.8. </span>Shifting operations<a class="headerlink" href="#shifting-operations" title="Link to this heading">¶</a></h2>

  1167. <p id="index-71">The shifting operations have lower priority than the arithmetic operations:</p>

  1168. <pre>

  1169. <strong id="grammar-token-python-grammar-shift_expr">shift_expr</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-shift_expr"><code class="xref docutils literal notranslate"><span class="pre">shift_expr</span></code></a> (&quot;&lt;&lt;&quot; | &quot;&gt;&gt;&quot;) <a class="reference internal" href="#grammar-token-python-grammar-a_expr"><code class="xref docutils literal notranslate"><span class="pre">a_expr</span></code></a>

  1170. </pre>

  1171. <p>These operators accept integers as arguments.  They shift the first argument to

  1172. the left or right by the number of bits given by the second argument.</p>

  1173. <p>This operation can be customized using the special <a class="reference internal" href="datamodel.html#object.__lshift__" title="object.__lshift__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lshift__()</span></code></a> and

  1174. <a class="reference internal" href="datamodel.html#object.__rshift__" title="object.__rshift__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rshift__()</span></code></a> methods.</p>

  1175. <p id="index-72">A right shift by <em>n</em> bits is defined as floor division by <code class="docutils literal notranslate"><span class="pre">pow(2,n)</span></code>.  A left

  1176. shift by <em>n</em> bits is defined as multiplication with <code class="docutils literal notranslate"><span class="pre">pow(2,n)</span></code>.</p>

  1177. </section>

  1178. <section id="binary-bitwise-operations">

  1179. <span id="bitwise"></span><h2><span class="section-number">6.9. </span>Binary bitwise operations<a class="headerlink" href="#binary-bitwise-operations" title="Link to this heading">¶</a></h2>

  1180. <p id="index-73">Each of the three bitwise operations has a different priority level:</p>

  1181. <pre>

  1182. <strong id="grammar-token-python-grammar-and_expr">and_expr</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-shift_expr"><code class="xref docutils literal notranslate"><span class="pre">shift_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-and_expr"><code class="xref docutils literal notranslate"><span class="pre">and_expr</span></code></a> &quot;&amp;&quot; <a class="reference internal" href="#grammar-token-python-grammar-shift_expr"><code class="xref docutils literal notranslate"><span class="pre">shift_expr</span></code></a>

  1183. <strong id="grammar-token-python-grammar-xor_expr">xor_expr</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-and_expr"><code class="xref docutils literal notranslate"><span class="pre">and_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-xor_expr"><code class="xref docutils literal notranslate"><span class="pre">xor_expr</span></code></a> &quot;^&quot; <a class="reference internal" href="#grammar-token-python-grammar-and_expr"><code class="xref docutils literal notranslate"><span class="pre">and_expr</span></code></a>

  1184. <strong id="grammar-token-python-grammar-or_expr">or_expr </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-xor_expr"><code class="xref docutils literal notranslate"><span class="pre">xor_expr</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> &quot;|&quot; <a class="reference internal" href="#grammar-token-python-grammar-xor_expr"><code class="xref docutils literal notranslate"><span class="pre">xor_expr</span></code></a>

  1185. </pre>

  1186. <p id="index-74">The <code class="docutils literal notranslate"><span class="pre">&amp;</span></code> operator yields the bitwise AND of its arguments, which must be

  1187. integers or one of them must be a custom object overriding <a class="reference internal" href="datamodel.html#object.__and__" title="object.__and__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__and__()</span></code></a> or

  1188. <a class="reference internal" href="datamodel.html#object.__rand__" title="object.__rand__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rand__()</span></code></a> special methods.</p>

  1189. <p id="index-75">The <code class="docutils literal notranslate"><span class="pre">^</span></code> operator yields the bitwise XOR (exclusive OR) of its arguments, which

  1190. must be integers or one of them must be a custom object overriding <a class="reference internal" href="datamodel.html#object.__xor__" title="object.__xor__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__xor__()</span></code></a> or

  1191. <a class="reference internal" href="datamodel.html#object.__rxor__" title="object.__rxor__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rxor__()</span></code></a> special methods.</p>

  1192. <p id="index-76">The <code class="docutils literal notranslate"><span class="pre">|</span></code> operator yields the bitwise (inclusive) OR of its arguments, which

  1193. must be integers or one of them must be a custom object overriding <a class="reference internal" href="datamodel.html#object.__or__" title="object.__or__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__or__()</span></code></a> or

  1194. <a class="reference internal" href="datamodel.html#object.__ror__" title="object.__ror__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ror__()</span></code></a> special methods.</p>

  1195. </section>

  1196. <section id="comparisons">

  1197. <span id="id11"></span><h2><span class="section-number">6.10. </span>Comparisons<a class="headerlink" href="#comparisons" title="Link to this heading">¶</a></h2>

  1198. <p id="index-77">Unlike C, all comparison operations in Python have the same priority, which is

  1199. lower than that of any arithmetic, shifting or bitwise operation.  Also unlike

  1200. C, expressions like <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">&lt;</span> <span class="pre">b</span> <span class="pre">&lt;</span> <span class="pre">c</span></code> have the interpretation that is conventional

  1201. in mathematics:</p>

  1202. <pre>

  1203. <strong id="grammar-token-python-grammar-comparison">comparison   </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> (<a class="reference internal" href="#grammar-token-python-grammar-comp_operator"><code class="xref docutils literal notranslate"><span class="pre">comp_operator</span></code></a> <a class="reference internal" href="#grammar-token-python-grammar-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a>)*

  1204. <strong id="grammar-token-python-grammar-comp_operator">comp_operator</strong> ::=  &quot;&lt;&quot; | &quot;&gt;&quot; | &quot;==&quot; | &quot;&gt;=&quot; | &quot;&lt;=&quot; | &quot;!=&quot;

  1205.                    | &quot;is&quot; [&quot;not&quot;] | [&quot;not&quot;] &quot;in&quot;

  1206. </pre>

  1207. <p>Comparisons yield boolean values: <code class="docutils literal notranslate"><span class="pre">True</span></code> or <code class="docutils literal notranslate"><span class="pre">False</span></code>. Custom

  1208. <em class="dfn">rich comparison methods</em> may return non-boolean values. In this case

  1209. Python will call <a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-func docutils literal notranslate"><span class="pre">bool()</span></code></a> on such value in boolean contexts.</p>

  1210. <p id="index-78">Comparisons can be chained arbitrarily, e.g., <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span> <span class="pre">&lt;=</span> <span class="pre">z</span></code> is equivalent to

  1211. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span> <span class="pre">and</span> <span class="pre">y</span> <span class="pre">&lt;=</span> <span class="pre">z</span></code>, except that <code class="docutils literal notranslate"><span class="pre">y</span></code> is evaluated only once (but in both

  1212. cases <code class="docutils literal notranslate"><span class="pre">z</span></code> is not evaluated at all when <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span></code> is found to be false).</p>

  1213. <p>Formally, if <em>a</em>, <em>b</em>, <em>c</em>, …, <em>y</em>, <em>z</em> are expressions and <em>op1</em>, <em>op2</em>, …,

  1214. <em>opN</em> are comparison operators, then <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">op1</span> <span class="pre">b</span> <span class="pre">op2</span> <span class="pre">c</span> <span class="pre">...</span> <span class="pre">y</span> <span class="pre">opN</span> <span class="pre">z</span></code> is equivalent

  1215. to <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">op1</span> <span class="pre">b</span> <span class="pre">and</span> <span class="pre">b</span> <span class="pre">op2</span> <span class="pre">c</span> <span class="pre">and</span> <span class="pre">...</span> <span class="pre">y</span> <span class="pre">opN</span> <span class="pre">z</span></code>, except that each expression is

  1216. evaluated at most once.</p>

  1217. <p>Note that <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">op1</span> <span class="pre">b</span> <span class="pre">op2</span> <span class="pre">c</span></code> doesn’t imply any kind of comparison between <em>a</em> and

  1218. <em>c</em>, so that, e.g., <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span> <span class="pre">&gt;</span> <span class="pre">z</span></code> is perfectly legal (though perhaps not

  1219. pretty).</p>

  1220. <section id="value-comparisons">

  1221. <span id="expressions-value-comparisons"></span><h3><span class="section-number">6.10.1. </span>Value comparisons<a class="headerlink" href="#value-comparisons" title="Link to this heading">¶</a></h3>

  1222. <p>The operators <code class="docutils literal notranslate"><span class="pre">&lt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;</span></code>, <code class="docutils literal notranslate"><span class="pre">==</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;=</span></code>, <code class="docutils literal notranslate"><span class="pre">&lt;=</span></code>, and <code class="docutils literal notranslate"><span class="pre">!=</span></code> compare the

  1223. values of two objects.  The objects do not need to have the same type.</p>

  1224. <p>Chapter <a class="reference internal" href="datamodel.html#objects"><span class="std std-ref">Objects, values and types</span></a> states that objects have a value (in addition to type

  1225. and identity).  The value of an object is a rather abstract notion in Python:

  1226. For example, there is no canonical access method for an object’s value.  Also,

  1227. there is no requirement that the value of an object should be constructed in a

  1228. particular way, e.g. comprised of all its data attributes. Comparison operators

  1229. implement a particular notion of what the value of an object is.  One can think

  1230. of them as defining the value of an object indirectly, by means of their

  1231. comparison implementation.</p>

  1232. <p>Because all types are (direct or indirect) subtypes of <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>, they

  1233. inherit the default comparison behavior from <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>.  Types can

  1234. customize their comparison behavior by implementing

  1235. <em class="dfn">rich comparison methods</em> like <a class="reference internal" href="datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>, described in

  1236. <a class="reference internal" href="datamodel.html#customization"><span class="std std-ref">Basic customization</span></a>.</p>

  1237. <p>The default behavior for equality comparison (<code class="docutils literal notranslate"><span class="pre">==</span></code> and <code class="docutils literal notranslate"><span class="pre">!=</span></code>) is based on

  1238. the identity of the objects.  Hence, equality comparison of instances with the

  1239. same identity results in equality, and equality comparison of instances with

  1240. different identities results in inequality.  A motivation for this default

  1241. behavior is the desire that all objects should be reflexive (i.e. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y</span></code>

  1242. implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code>).</p>

  1243. <p>A default order comparison (<code class="docutils literal notranslate"><span class="pre">&lt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&lt;=</span></code>, and <code class="docutils literal notranslate"><span class="pre">&gt;=</span></code>) is not provided;

  1244. an attempt raises <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.  A motivation for this default behavior is

  1245. the lack of a similar invariant as for equality.</p>

  1246. <p>The behavior of the default equality comparison, that instances with different

  1247. identities are always unequal, may be in contrast to what types will need that

  1248. have a sensible definition of object value and value-based equality.  Such

  1249. types will need to customize their comparison behavior, and in fact, a number

  1250. of built-in types have done that.</p>

  1251. <p>The following list describes the comparison behavior of the most important

  1252. built-in types.</p>

  1253. <ul>

  1254. <li><p>Numbers of built-in numeric types (<a class="reference internal" href="../library/stdtypes.html#typesnumeric"><span class="std std-ref">Numeric Types — int, float, complex</span></a>) and of the standard

  1255. library types <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate"><span class="pre">fractions.Fraction</span></code></a> and <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate"><span class="pre">decimal.Decimal</span></code></a> can be

  1256. compared within and across their types, with the restriction that complex

  1257. numbers do not support order comparison.  Within the limits of the types

  1258. involved, they compare mathematically (algorithmically) correct without loss

  1259. of precision.</p>

  1260. <p>The not-a-number values <code class="docutils literal notranslate"><span class="pre">float('NaN')</span></code> and <code class="docutils literal notranslate"><span class="pre">decimal.Decimal('NaN')</span></code> are

  1261. special.  Any ordered comparison of a number to a not-a-number value is false.

  1262. A counter-intuitive implication is that not-a-number values are not equal to

  1263. themselves.  For example, if <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">float('NaN')</span></code>, <code class="docutils literal notranslate"><span class="pre">3</span> <span class="pre">&lt;</span> <span class="pre">x</span></code>, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">3</span></code> and

  1264. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">x</span></code> are all false, while <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">!=</span> <span class="pre">x</span></code> is true.  This behavior is

  1265. compliant with IEEE 754.</p>

  1266. </li>

  1267. <li><p><code class="docutils literal notranslate"><span class="pre">None</span></code> and <a class="reference internal" href="../library/constants.html#NotImplemented" title="NotImplemented"><code class="xref py py-data docutils literal notranslate"><span class="pre">NotImplemented</span></code></a> are singletons.  <span class="target" id="index-79"></span><a class="pep reference external" href="https://peps.python.org/pep-0008/"><strong>PEP 8</strong></a> advises that

  1268. comparisons for singletons should always be done with <code class="docutils literal notranslate"><span class="pre">is</span></code> or <code class="docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code>,

  1269. never the equality operators.</p></li>

  1270. <li><p>Binary sequences (instances of <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> or <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytearray</span></code></a>) can be

  1271. compared within and across their types.  They compare lexicographically using

  1272. the numeric values of their elements.</p></li>

  1273. <li><p>Strings (instances of <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a>) compare lexicographically using the

  1274. numerical Unicode code points (the result of the built-in function

  1275. <a class="reference internal" href="../library/functions.html#ord" title="ord"><code class="xref py py-func docutils literal notranslate"><span class="pre">ord()</span></code></a>) of their characters. <a class="footnote-reference brackets" href="#id19" id="id12" role="doc-noteref"><span class="fn-bracket">[</span>3<span class="fn-bracket">]</span></a></p>

  1276. <p>Strings and binary sequences cannot be directly compared.</p>

  1277. </li>

  1278. <li><p>Sequences (instances of <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a>, <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a>, or <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-class docutils literal notranslate"><span class="pre">range</span></code></a>) can

  1279. be compared only within each of their types, with the restriction that ranges

  1280. do not support order comparison.  Equality comparison across these types

  1281. results in inequality, and ordering comparison across these types raises

  1282. <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>

  1283. <p>Sequences compare lexicographically using comparison of corresponding

  1284. elements.  The built-in containers typically assume identical objects are

  1285. equal to themselves.  That lets them bypass equality tests for identical

  1286. objects to improve performance and to maintain their internal invariants.</p>

  1287. <p>Lexicographical comparison between built-in collections works as follows:</p>

  1288. <ul class="simple">

  1289. <li><p>For two collections to compare equal, they must be of the same type, have

  1290. the same length, and each pair of corresponding elements must compare

  1291. equal (for example, <code class="docutils literal notranslate"><span class="pre">[1,2]</span> <span class="pre">==</span> <span class="pre">(1,2)</span></code> is false because the type is not the

  1292. same).</p></li>

  1293. <li><p>Collections that support order comparison are ordered the same as their

  1294. first unequal elements (for example, <code class="docutils literal notranslate"><span class="pre">[1,2,x]</span> <span class="pre">&lt;=</span> <span class="pre">[1,2,y]</span></code> has the same

  1295. value as <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;=</span> <span class="pre">y</span></code>).  If a corresponding element does not exist, the

  1296. shorter collection is ordered first (for example, <code class="docutils literal notranslate"><span class="pre">[1,2]</span> <span class="pre">&lt;</span> <span class="pre">[1,2,3]</span></code> is

  1297. true).</p></li>

  1298. </ul>

  1299. </li>

  1300. <li><p>Mappings (instances of <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>) compare equal if and only if they have

  1301. equal <code class="docutils literal notranslate"><span class="pre">(key,</span> <span class="pre">value)</span></code> pairs. Equality comparison of the keys and values

  1302. enforces reflexivity.</p>

  1303. <p>Order comparisons (<code class="docutils literal notranslate"><span class="pre">&lt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&lt;=</span></code>, and <code class="docutils literal notranslate"><span class="pre">&gt;=</span></code>) raise <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>

  1304. </li>

  1305. <li><p>Sets (instances of <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a> or <a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate"><span class="pre">frozenset</span></code></a>) can be compared within

  1306. and across their types.</p>

  1307. <p>They define order

  1308. comparison operators to mean subset and superset tests.  Those relations do

  1309. not define total orderings (for example, the two sets <code class="docutils literal notranslate"><span class="pre">{1,2}</span></code> and <code class="docutils literal notranslate"><span class="pre">{2,3}</span></code>

  1310. are not equal, nor subsets of one another, nor supersets of one

  1311. another).  Accordingly, sets are not appropriate arguments for functions

  1312. which depend on total ordering (for example, <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a>, <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a>, and

  1313. <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> produce undefined results given a list of sets as inputs).</p>

  1314. <p>Comparison of sets enforces reflexivity of its elements.</p>

  1315. </li>

  1316. <li><p>Most other built-in types have no comparison methods implemented, so they

  1317. inherit the default comparison behavior.</p></li>

  1318. </ul>

  1319. <p>User-defined classes that customize their comparison behavior should follow

  1320. some consistency rules, if possible:</p>

  1321. <ul>

  1322. <li><p>Equality comparison should be reflexive.

  1323. In other words, identical objects should compare equal:</p>

  1324. <blockquote>

  1325. <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code></p>

  1326. </div></blockquote>

  1327. </li>

  1328. <li><p>Comparison should be symmetric.

  1329. In other words, the following expressions should have the same result:</p>

  1330. <blockquote>

  1331. <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span></code></p>

  1332. <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">!=</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">!=</span> <span class="pre">x</span></code></p>

  1333. <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">&gt;</span> <span class="pre">x</span></code></p>

  1334. <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;=</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">&gt;=</span> <span class="pre">x</span></code></p>

  1335. </div></blockquote>

  1336. </li>

  1337. <li><p>Comparison should be transitive.

  1338. The following (non-exhaustive) examples illustrate that:</p>

  1339. <blockquote>

  1340. <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&gt;</span> <span class="pre">y</span> <span class="pre">and</span> <span class="pre">y</span> <span class="pre">&gt;</span> <span class="pre">z</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&gt;</span> <span class="pre">z</span></code></p>

  1341. <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span> <span class="pre">and</span> <span class="pre">y</span> <span class="pre">&lt;=</span> <span class="pre">z</span></code> implies <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">z</span></code></p>

  1342. </div></blockquote>

  1343. </li>

  1344. <li><p>Inverse comparison should result in the boolean negation.

  1345. In other words, the following expressions should have the same result:</p>

  1346. <blockquote>

  1347. <div><p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span> <span class="pre">!=</span> <span class="pre">y</span></code></p>

  1348. <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&lt;</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span> <span class="pre">&gt;=</span> <span class="pre">y</span></code> (for total ordering)</p>

  1349. <p><code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&gt;</span> <span class="pre">y</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span> <span class="pre">&lt;=</span> <span class="pre">y</span></code> (for total ordering)</p>

  1350. </div></blockquote>

  1351. <p>The last two expressions apply to totally ordered collections (e.g. to

  1352. sequences, but not to sets or mappings). See also the

  1353. <a class="reference internal" href="../library/functools.html#functools.total_ordering" title="functools.total_ordering"><code class="xref py py-func docutils literal notranslate"><span class="pre">total_ordering()</span></code></a> decorator.</p>

  1354. </li>

  1355. <li><p>The <a class="reference internal" href="../library/functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate"><span class="pre">hash()</span></code></a> result should be consistent with equality.

  1356. Objects that are equal should either have the same hash value,

  1357. or be marked as unhashable.</p></li>

  1358. </ul>

  1359. <p>Python does not enforce these consistency rules. In fact, the not-a-number

  1360. values are an example for not following these rules.</p>

  1361. </section>

  1362. <section id="membership-test-operations">

  1363. <span id="membership-test-details"></span><span id="not-in"></span><span id="in"></span><h3><span class="section-number">6.10.2. </span>Membership test operations<a class="headerlink" href="#membership-test-operations" title="Link to this heading">¶</a></h3>

  1364. <p>The operators <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> and <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a> test for membership.  <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span>

  1365. <span class="pre">s</span></code> evaluates to <code class="docutils literal notranslate"><span class="pre">True</span></code> if <em>x</em> is a member of <em>s</em>, and <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.

  1366. <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">not</span> <span class="pre">in</span> <span class="pre">s</span></code> returns the negation of <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">s</span></code>.  All built-in sequences and

  1367. set types support this as well as dictionary, for which <code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code> tests

  1368. whether the dictionary has a given key. For container types such as list, tuple,

  1369. set, frozenset, dict, or collections.deque, the expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is equivalent

  1370. to <code class="docutils literal notranslate"><span class="pre">any(x</span> <span class="pre">is</span> <span class="pre">e</span> <span class="pre">or</span> <span class="pre">x</span> <span class="pre">==</span> <span class="pre">e</span> <span class="pre">for</span> <span class="pre">e</span> <span class="pre">in</span> <span class="pre">y)</span></code>.</p>

  1371. <p>For the string and bytes types, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> if and only if <em>x</em> is a

  1372. substring of <em>y</em>.  An equivalent test is <code class="docutils literal notranslate"><span class="pre">y.find(x)</span> <span class="pre">!=</span> <span class="pre">-1</span></code>.  Empty strings are

  1373. always considered to be a substring of any other string, so <code class="docutils literal notranslate"><span class="pre">&quot;&quot;</span> <span class="pre">in</span> <span class="pre">&quot;abc&quot;</span></code> will

  1374. return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  1375. <p>For user-defined classes which define the <a class="reference internal" href="datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> method, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span>

  1376. <span class="pre">y</span></code> returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if <code class="docutils literal notranslate"><span class="pre">y.__contains__(x)</span></code> returns a true value, and

  1377. <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p>

  1378. <p>For user-defined classes which do not define <a class="reference internal" href="datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__contains__()</span></code></a> but do define

  1379. <a class="reference internal" href="datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> if some value <code class="docutils literal notranslate"><span class="pre">z</span></code>, for which the

  1380. expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">z</span> <span class="pre">or</span> <span class="pre">x</span> <span class="pre">==</span> <span class="pre">z</span></code> is true, is produced while iterating over <code class="docutils literal notranslate"><span class="pre">y</span></code>.

  1381. If an exception is raised during the iteration, it is as if <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> raised

  1382. that exception.</p>

  1383. <p>Lastly, the old-style iteration protocol is tried: if a class defines

  1384. <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getitem__()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">y</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code> if and only if there is a non-negative

  1385. integer index <em>i</em> such that <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">y[i]</span> <span class="pre">or</span> <span class="pre">x</span> <span class="pre">==</span> <span class="pre">y[i]</span></code>, and no lower integer index

  1386. raises the <a class="reference internal" href="../library/exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> exception.  (If any other exception is raised, it is as

  1387. if <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> raised that exception).</p>

  1388. <p id="index-80">The operator <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a> is defined to have the inverse truth value of

  1389. <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a>.</p>

  1390. </section>

  1391. <section id="is-not">

  1392. <span id="is"></span><span id="index-81"></span><span id="identity-comparisons"></span><h3><span class="section-number">6.10.3. </span>Identity comparisons<a class="headerlink" href="#is-not" title="Link to this heading">¶</a></h3>

  1393. <p>The operators <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a> and <a class="reference internal" href="#is-not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code></a> test for an object’s identity: <code class="docutils literal notranslate"><span class="pre">x</span>

  1394. <span class="pre">is</span> <span class="pre">y</span></code> is true if and only if <em>x</em> and <em>y</em> are the same object.  An Object’s identity

  1395. is determined using the <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-meth docutils literal notranslate"><span class="pre">id()</span></code></a> function.  <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">is</span> <span class="pre">not</span> <span class="pre">y</span></code> yields the inverse

  1396. truth value. <a class="footnote-reference brackets" href="#id20" id="id13" role="doc-noteref"><span class="fn-bracket">[</span>4<span class="fn-bracket">]</span></a></p>

  1397. </section>

  1398. </section>

  1399. <section id="boolean-operations">

  1400. <span id="not"></span><span id="or"></span><span id="and"></span><span id="booleans"></span><h2><span class="section-number">6.11. </span>Boolean operations<a class="headerlink" href="#boolean-operations" title="Link to this heading">¶</a></h2>

  1401. <pre id="index-82">

  1402. <strong id="grammar-token-python-grammar-or_test">or_test </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-and_test"><code class="xref docutils literal notranslate"><span class="pre">and_test</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> &quot;or&quot; <a class="reference internal" href="#grammar-token-python-grammar-and_test"><code class="xref docutils literal notranslate"><span class="pre">and_test</span></code></a>

  1403. <strong id="grammar-token-python-grammar-and_test">and_test</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-not_test"><code class="xref docutils literal notranslate"><span class="pre">not_test</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-and_test"><code class="xref docutils literal notranslate"><span class="pre">and_test</span></code></a> &quot;and&quot; <a class="reference internal" href="#grammar-token-python-grammar-not_test"><code class="xref docutils literal notranslate"><span class="pre">not_test</span></code></a>

  1404. <strong id="grammar-token-python-grammar-not_test">not_test</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-comparison"><code class="xref docutils literal notranslate"><span class="pre">comparison</span></code></a> | &quot;not&quot; <a class="reference internal" href="#grammar-token-python-grammar-not_test"><code class="xref docutils literal notranslate"><span class="pre">not_test</span></code></a>

  1405. </pre>

  1406. <p>In the context of Boolean operations, and also when expressions are used by

  1407. control flow statements, the following values are interpreted as false:

  1408. <code class="docutils literal notranslate"><span class="pre">False</span></code>, <code class="docutils literal notranslate"><span class="pre">None</span></code>, numeric zero of all types, and empty strings and containers

  1409. (including strings, tuples, lists, dictionaries, sets and frozensets).  All

  1410. other values are interpreted as true.  User-defined objects can customize their

  1411. truth value by providing a <a class="reference internal" href="datamodel.html#object.__bool__" title="object.__bool__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__bool__()</span></code></a> method.</p>

  1412. <p id="index-83">The operator <a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span></code></a> yields <code class="docutils literal notranslate"><span class="pre">True</span></code> if its argument is false, <code class="docutils literal notranslate"><span class="pre">False</span></code>

  1413. otherwise.</p>

  1414. <p id="index-84">The expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">and</span> <span class="pre">y</span></code> first evaluates <em>x</em>; if <em>x</em> is false, its value is

  1415. returned; otherwise, <em>y</em> is evaluated and the resulting value is returned.</p>

  1416. <p id="index-85">The expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">or</span> <span class="pre">y</span></code> first evaluates <em>x</em>; if <em>x</em> is true, its value is

  1417. returned; otherwise, <em>y</em> is evaluated and the resulting value is returned.</p>

  1418. <p>Note that neither <a class="reference internal" href="#and"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">and</span></code></a> nor <a class="reference internal" href="#or"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">or</span></code></a> restrict the value and type

  1419. they return to <code class="docutils literal notranslate"><span class="pre">False</span></code> and <code class="docutils literal notranslate"><span class="pre">True</span></code>, but rather return the last evaluated

  1420. argument.  This is sometimes useful, e.g., if <code class="docutils literal notranslate"><span class="pre">s</span></code> is a string that should be

  1421. replaced by a default value if it is empty, the expression <code class="docutils literal notranslate"><span class="pre">s</span> <span class="pre">or</span> <span class="pre">'foo'</span></code> yields

  1422. the desired value.  Because <a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span></code></a> has to create a new value, it

  1423. returns a boolean value regardless of the type of its argument

  1424. (for example, <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">'foo'</span></code> produces <code class="docutils literal notranslate"><span class="pre">False</span></code> rather than <code class="docutils literal notranslate"><span class="pre">''</span></code>.)</p>

  1425. </section>

  1426. <section id="assignment-expressions">

  1427. <span id="index-86"></span><h2><span class="section-number">6.12. </span>Assignment expressions<a class="headerlink" href="#assignment-expressions" title="Link to this heading">¶</a></h2>

  1428. <pre>

  1429. <strong id="grammar-token-python-grammar-assignment_expression">assignment_expression</strong> ::=  [<a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> &quot;:=&quot;] <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  1430. </pre>

  1431. <p>An assignment expression (sometimes also called a “named expression” or

  1432. “walrus”) assigns an <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression</span></code></a> to an

  1433. <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref std std-token docutils literal notranslate"><span class="pre">identifier</span></code></a>, while also returning the value of the

  1434. <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression</span></code></a>.</p>

  1435. <p>One common use case is when handling matched regular expressions:</p>

  1436. <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">matching</span> <span class="o">:=</span> <span class="n">pattern</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">data</span><span class="p">):</span>

  1437.     <span class="n">do_something</span><span class="p">(</span><span class="n">matching</span><span class="p">)</span>

  1438. </pre></div>

  1439. </div>

  1440. <p>Or, when processing a file stream in chunks:</p>

  1441. <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="n">chunk</span> <span class="o">:=</span> <span class="n">file</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">9000</span><span class="p">):</span>

  1442.     <span class="n">process</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span>

  1443. </pre></div>

  1444. </div>

  1445. <p>Assignment expressions must be surrounded by parentheses when

  1446. used as expression statements and when used as sub-expressions in

  1447. slicing, conditional, lambda,

  1448. keyword-argument, and comprehension-if expressions and

  1449. in <code class="docutils literal notranslate"><span class="pre">assert</span></code>, <code class="docutils literal notranslate"><span class="pre">with</span></code>, and <code class="docutils literal notranslate"><span class="pre">assignment</span></code> statements.

  1450. In all other places where they can be used, parentheses are not required,

  1451. including in <code class="docutils literal notranslate"><span class="pre">if</span></code> and <code class="docutils literal notranslate"><span class="pre">while</span></code> statements.</p>

  1452. <div class="versionadded">

  1453. <p><span class="versionmodified added">New in version 3.8: </span>See <span class="target" id="index-87"></span><a class="pep reference external" href="https://peps.python.org/pep-0572/"><strong>PEP 572</strong></a> for more details about assignment expressions.</p>

  1454. </div>

  1455. </section>

  1456. <section id="conditional-expressions">

  1457. <span id="if-expr"></span><h2><span class="section-number">6.13. </span>Conditional expressions<a class="headerlink" href="#conditional-expressions" title="Link to this heading">¶</a></h2>

  1458. <pre id="index-88">

  1459. <strong id="grammar-token-python-grammar-conditional_expression">conditional_expression</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> [&quot;if&quot; <a class="reference internal" href="#grammar-token-python-grammar-or_test"><code class="xref docutils literal notranslate"><span class="pre">or_test</span></code></a> &quot;else&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>]

  1460. <strong id="grammar-token-python-grammar-expression">expression            </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-conditional_expression"><code class="xref docutils literal notranslate"><span class="pre">conditional_expression</span></code></a> | <a class="reference internal" href="#grammar-token-python-grammar-lambda_expr"><code class="xref docutils literal notranslate"><span class="pre">lambda_expr</span></code></a>

  1461. </pre>

  1462. <p>Conditional expressions (sometimes called a “ternary operator”) have the lowest

  1463. priority of all Python operations.</p>

  1464. <p>The expression <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">if</span> <span class="pre">C</span> <span class="pre">else</span> <span class="pre">y</span></code> first evaluates the condition, <em>C</em> rather than <em>x</em>.

  1465. If <em>C</em> is true, <em>x</em> is evaluated and its value is returned; otherwise, <em>y</em> is

  1466. evaluated and its value is returned.</p>

  1467. <p>See <span class="target" id="index-89"></span><a class="pep reference external" href="https://peps.python.org/pep-0308/"><strong>PEP 308</strong></a> for more details about conditional expressions.</p>

  1468. </section>

  1469. <section id="lambda">

  1470. <span id="lambdas"></span><span id="id14"></span><h2><span class="section-number">6.14. </span>Lambdas<a class="headerlink" href="#lambda" title="Link to this heading">¶</a></h2>

  1471. <pre id="index-90">

  1472. <strong id="grammar-token-python-grammar-lambda_expr">lambda_expr</strong> ::=  &quot;lambda&quot; [<a class="reference internal" href="compound_stmts.html#grammar-token-python-grammar-parameter_list"><code class="xref docutils literal notranslate"><span class="pre">parameter_list</span></code></a>] &quot;:&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>

  1473. </pre>

  1474. <p>Lambda expressions (sometimes called lambda forms) are used to create anonymous

  1475. functions. The expression <code class="docutils literal notranslate"><span class="pre">lambda</span> <span class="pre">parameters:</span> <span class="pre">expression</span></code> yields a function

  1476. object.  The unnamed object behaves like a function object defined with:</p>

  1477. <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>def &lt;lambda&gt;(parameters):

  1478.     return expression

  1479. </pre></div>

  1480. </div>

  1481. <p>See section <a class="reference internal" href="compound_stmts.html#function"><span class="std std-ref">Function definitions</span></a> for the syntax of parameter lists.  Note that

  1482. functions created with lambda expressions cannot contain statements or

  1483. annotations.</p>

  1484. </section>

  1485. <section id="expression-lists">

  1486. <span id="exprlists"></span><h2><span class="section-number">6.15. </span>Expression lists<a class="headerlink" href="#expression-lists" title="Link to this heading">¶</a></h2>

  1487. <pre id="index-91">

  1488. <strong id="grammar-token-python-grammar-expression_list">expression_list   </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)* [&quot;,&quot;]

  1489. <strong id="grammar-token-python-grammar-starred_list">starred_list      </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a>)* [&quot;,&quot;]

  1490. <strong id="grammar-token-python-grammar-starred_expression">starred_expression</strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> | (<a class="reference internal" href="#grammar-token-python-grammar-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a> &quot;,&quot;)* [<a class="reference internal" href="#grammar-token-python-grammar-starred_item"><code class="xref docutils literal notranslate"><span class="pre">starred_item</span></code></a>]

  1491. <strong id="grammar-token-python-grammar-starred_item">starred_item      </strong> ::=  <a class="reference internal" href="#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate"><span class="pre">assignment_expression</span></code></a> | &quot;*&quot; <a class="reference internal" href="#grammar-token-python-grammar-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a>

  1492. </pre>

  1493. <p id="index-92">Except when part of a list or set display, an expression list

  1494. containing at least one comma yields a tuple.  The length of

  1495. the tuple is the number of expressions in the list.  The expressions are

  1496. evaluated from left to right.</p>

  1497. <p id="index-93">An asterisk <code class="docutils literal notranslate"><span class="pre">*</span></code> denotes <em class="dfn">iterable unpacking</em>.  Its operand must be

  1498. an <a class="reference internal" href="../glossary.html#term-iterable"><span class="xref std std-term">iterable</span></a>.  The iterable is expanded into a sequence of items,

  1499. which are included in the new tuple, list, or set, at the site of

  1500. the unpacking.</p>

  1501. <div class="versionadded">

  1502. <p><span class="versionmodified added">New in version 3.5: </span>Iterable unpacking in expression lists, originally proposed by <span class="target" id="index-94"></span><a class="pep reference external" href="https://peps.python.org/pep-0448/"><strong>PEP 448</strong></a>.</p>

  1503. </div>

  1504. <p id="index-95">A trailing comma is required only to create a one-item tuple,

  1505. such as <code class="docutils literal notranslate"><span class="pre">1,</span></code>; it is optional in all other cases.

  1506. A single expression without a

  1507. trailing comma doesn’t create a tuple, but rather yields the value of that

  1508. expression. (To create an empty tuple, use an empty pair of parentheses:

  1509. <code class="docutils literal notranslate"><span class="pre">()</span></code>.)</p>

  1510. </section>

  1511. <section id="evaluation-order">

  1512. <span id="evalorder"></span><h2><span class="section-number">6.16. </span>Evaluation order<a class="headerlink" href="#evaluation-order" title="Link to this heading">¶</a></h2>

  1513. <p id="index-96">Python evaluates expressions from left to right.  Notice that while evaluating

  1514. an assignment, the right-hand side is evaluated before the left-hand side.</p>

  1515. <p>In the following lines, expressions will be evaluated in the arithmetic order of

  1516. their suffixes:</p>

  1517. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">expr1</span><span class="p">,</span> <span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">,</span> <span class="n">expr4</span>

  1518. <span class="p">(</span><span class="n">expr1</span><span class="p">,</span> <span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">,</span> <span class="n">expr4</span><span class="p">)</span>

  1519. <span class="p">{</span><span class="n">expr1</span><span class="p">:</span> <span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">:</span> <span class="n">expr4</span><span class="p">}</span>

  1520. <span class="n">expr1</span> <span class="o">+</span> <span class="n">expr2</span> <span class="o">*</span> <span class="p">(</span><span class="n">expr3</span> <span class="o">-</span> <span class="n">expr4</span><span class="p">)</span>

  1521. <span class="n">expr1</span><span class="p">(</span><span class="n">expr2</span><span class="p">,</span> <span class="n">expr3</span><span class="p">,</span> <span class="o">*</span><span class="n">expr4</span><span class="p">,</span> <span class="o">**</span><span class="n">expr5</span><span class="p">)</span>

  1522. <span class="n">expr3</span><span class="p">,</span> <span class="n">expr4</span> <span class="o">=</span> <span class="n">expr1</span><span class="p">,</span> <span class="n">expr2</span>

  1523. </pre></div>

  1524. </div>

  1525. </section>

  1526. <section id="operator-precedence">

  1527. <span id="operator-summary"></span><h2><span class="section-number">6.17. </span>Operator precedence<a class="headerlink" href="#operator-precedence" title="Link to this heading">¶</a></h2>

  1528. <p id="index-97">The following table summarizes the operator precedence in Python, from highest

  1529. precedence (most binding) to lowest precedence (least binding).  Operators in

  1530. the same box have the same precedence.  Unless the syntax is explicitly given,

  1531. operators are binary.  Operators in the same box group left to right (except for

  1532. exponentiation and conditional expressions, which group from right to left).</p>

  1533. <p>Note that comparisons, membership tests, and identity tests, all have the same

  1534. precedence and have a left-to-right chaining feature as described in the

  1535. <a class="reference internal" href="#comparisons"><span class="std std-ref">Comparisons</span></a> section.</p>

  1536. <table class="docutils align-default">

  1537. <thead>

  1538. <tr class="row-odd"><th class="head"><p>Operator</p></th>

  1539. <th class="head"><p>Description</p></th>

  1540. </tr>

  1541. </thead>

  1542. <tbody>

  1543. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">(expressions...)</span></code>,</p>

  1544. <p><code class="docutils literal notranslate"><span class="pre">[expressions...]</span></code>,

  1545. <code class="docutils literal notranslate"><span class="pre">{key:</span> <span class="pre">value...}</span></code>,

  1546. <code class="docutils literal notranslate"><span class="pre">{expressions...}</span></code></p>

  1547. </td>

  1548. <td><p>Binding or parenthesized

  1549. expression,

  1550. list display,

  1551. dictionary display,

  1552. set display</p></td>

  1553. </tr>

  1554. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">x[index]</span></code>, <code class="docutils literal notranslate"><span class="pre">x[index:index]</span></code>,

  1555. <code class="docutils literal notranslate"><span class="pre">x(arguments...)</span></code>, <code class="docutils literal notranslate"><span class="pre">x.attribute</span></code></p></td>

  1556. <td><p>Subscription, slicing,

  1557. call, attribute reference</p></td>

  1558. </tr>

  1559. <tr class="row-even"><td><p><a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span> <span class="pre">x</span></code></a></p></td>

  1560. <td><p>Await expression</p></td>

  1561. </tr>

  1562. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">**</span></code></p></td>

  1563. <td><p>Exponentiation <a class="footnote-reference brackets" href="#id21" id="id15" role="doc-noteref"><span class="fn-bracket">[</span>5<span class="fn-bracket">]</span></a></p></td>

  1564. </tr>

  1565. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">+x</span></code>, <code class="docutils literal notranslate"><span class="pre">-x</span></code>, <code class="docutils literal notranslate"><span class="pre">~x</span></code></p></td>

  1566. <td><p>Positive, negative, bitwise NOT</p></td>

  1567. </tr>

  1568. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">*</span></code>, <code class="docutils literal notranslate"><span class="pre">&#64;</span></code>, <code class="docutils literal notranslate"><span class="pre">/</span></code>, <code class="docutils literal notranslate"><span class="pre">//</span></code>, <code class="docutils literal notranslate"><span class="pre">%</span></code></p></td>

  1569. <td><p>Multiplication, matrix

  1570. multiplication, division, floor

  1571. division, remainder <a class="footnote-reference brackets" href="#id22" id="id16" role="doc-noteref"><span class="fn-bracket">[</span>6<span class="fn-bracket">]</span></a></p></td>

  1572. </tr>

  1573. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code></p></td>

  1574. <td><p>Addition and subtraction</p></td>

  1575. </tr>

  1576. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">&lt;&lt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;&gt;</span></code></p></td>

  1577. <td><p>Shifts</p></td>

  1578. </tr>

  1579. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">&amp;</span></code></p></td>

  1580. <td><p>Bitwise AND</p></td>

  1581. </tr>

  1582. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">^</span></code></p></td>

  1583. <td><p>Bitwise XOR</p></td>

  1584. </tr>

  1585. <tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">|</span></code></p></td>

  1586. <td><p>Bitwise OR</p></td>

  1587. </tr>

  1588. <tr class="row-odd"><td><p><a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a>, <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code></a>,

  1589. <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a>, <a class="reference internal" href="#is-not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code></a>, <code class="docutils literal notranslate"><span class="pre">&lt;</span></code>,

  1590. <code class="docutils literal notranslate"><span class="pre">&lt;=</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;</span></code>, <code class="docutils literal notranslate"><span class="pre">&gt;=</span></code>, <code class="docutils literal notranslate"><span class="pre">!=</span></code>, <code class="docutils literal notranslate"><span class="pre">==</span></code></p></td>

  1591. <td><p>Comparisons, including membership

  1592. tests and identity tests</p></td>

  1593. </tr>

  1594. <tr class="row-even"><td><p><a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">not</span> <span class="pre">x</span></code></a></p></td>

  1595. <td><p>Boolean NOT</p></td>

  1596. </tr>

  1597. <tr class="row-odd"><td><p><a class="reference internal" href="#and"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">and</span></code></a></p></td>

  1598. <td><p>Boolean AND</p></td>

  1599. </tr>

  1600. <tr class="row-even"><td><p><a class="reference internal" href="#or"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">or</span></code></a></p></td>

  1601. <td><p>Boolean OR</p></td>

  1602. </tr>

  1603. <tr class="row-odd"><td><p><a class="reference internal" href="#if-expr"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> – <code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></p></td>

  1604. <td><p>Conditional expression</p></td>

  1605. </tr>

  1606. <tr class="row-even"><td><p><a class="reference internal" href="#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a></p></td>

  1607. <td><p>Lambda expression</p></td>

  1608. </tr>

  1609. <tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">:=</span></code></p></td>

  1610. <td><p>Assignment expression</p></td>

  1611. </tr>

  1612. </tbody>

  1613. </table>

  1614. <p class="rubric">Footnotes</p>

  1615. <aside class="footnote-list brackets">

  1616. <aside class="footnote brackets" id="id17" role="doc-footnote">

  1617. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id9">1</a><span class="fn-bracket">]</span></span>

  1618. <p>While <code class="docutils literal notranslate"><span class="pre">abs(x%y)</span> <span class="pre">&lt;</span> <span class="pre">abs(y)</span></code> is true mathematically, for floats it may not be

  1619. true numerically due to roundoff.  For example, and assuming a platform on which

  1620. a Python float is an IEEE 754 double-precision number, in order that <code class="docutils literal notranslate"><span class="pre">-1e-100</span> <span class="pre">%</span>

  1621. <span class="pre">1e100</span></code> have the same sign as <code class="docutils literal notranslate"><span class="pre">1e100</span></code>, the computed result is <code class="docutils literal notranslate"><span class="pre">-1e-100</span> <span class="pre">+</span>

  1622. <span class="pre">1e100</span></code>, which is numerically exactly equal to <code class="docutils literal notranslate"><span class="pre">1e100</span></code>.  The function

  1623. <a class="reference internal" href="../library/math.html#math.fmod" title="math.fmod"><code class="xref py py-func docutils literal notranslate"><span class="pre">math.fmod()</span></code></a> returns a result whose sign matches the sign of the

  1624. first argument instead, and so returns <code class="docutils literal notranslate"><span class="pre">-1e-100</span></code> in this case. Which approach

  1625. is more appropriate depends on the application.</p>

  1626. </aside>

  1627. <aside class="footnote brackets" id="id18" role="doc-footnote">

  1628. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id10">2</a><span class="fn-bracket">]</span></span>

  1629. <p>If x is very close to an exact integer multiple of y, it’s possible for

  1630. <code class="docutils literal notranslate"><span class="pre">x//y</span></code> to be one larger than <code class="docutils literal notranslate"><span class="pre">(x-x%y)//y</span></code> due to rounding.  In such

  1631. cases, Python returns the latter result, in order to preserve that

  1632. <code class="docutils literal notranslate"><span class="pre">divmod(x,y)[0]</span> <span class="pre">*</span> <span class="pre">y</span> <span class="pre">+</span> <span class="pre">x</span> <span class="pre">%</span> <span class="pre">y</span></code> be very close to <code class="docutils literal notranslate"><span class="pre">x</span></code>.</p>

  1633. </aside>

  1634. <aside class="footnote brackets" id="id19" role="doc-footnote">

  1635. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id12">3</a><span class="fn-bracket">]</span></span>

  1636. <p>The Unicode standard distinguishes between <em class="dfn">code points</em>

  1637. (e.g. U+0041) and <em class="dfn">abstract characters</em> (e.g. “LATIN CAPITAL LETTER A”).

  1638. While most abstract characters in Unicode are only represented using one

  1639. code point, there is a number of abstract characters that can in addition be

  1640. represented using a sequence of more than one code point.  For example, the

  1641. abstract character “LATIN CAPITAL LETTER C WITH CEDILLA” can be represented

  1642. as a single <em class="dfn">precomposed character</em> at code position U+00C7, or as a

  1643. sequence of a <em class="dfn">base character</em> at code position U+0043 (LATIN CAPITAL

  1644. LETTER C), followed by a <em class="dfn">combining character</em> at code position U+0327

  1645. (COMBINING CEDILLA).</p>

  1646. <p>The comparison operators on strings compare at the level of Unicode code

  1647. points. This may be counter-intuitive to humans.  For example,

  1648. <code class="docutils literal notranslate"><span class="pre">&quot;\u00C7&quot;</span> <span class="pre">==</span> <span class="pre">&quot;\u0043\u0327&quot;</span></code> is <code class="docutils literal notranslate"><span class="pre">False</span></code>, even though both strings

  1649. represent the same abstract character “LATIN CAPITAL LETTER C WITH CEDILLA”.</p>

  1650. <p>To compare strings at the level of abstract characters (that is, in a way

  1651. intuitive to humans), use <a class="reference internal" href="../library/unicodedata.html#unicodedata.normalize" title="unicodedata.normalize"><code class="xref py py-func docutils literal notranslate"><span class="pre">unicodedata.normalize()</span></code></a>.</p>

  1652. </aside>

  1653. <aside class="footnote brackets" id="id20" role="doc-footnote">

  1654. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id13">4</a><span class="fn-bracket">]</span></span>

  1655. <p>Due to automatic garbage-collection, free lists, and the dynamic nature of

  1656. descriptors, you may notice seemingly unusual behaviour in certain uses of

  1657. the <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a> operator, like those involving comparisons between instance

  1658. methods, or constants.  Check their documentation for more info.</p>

  1659. </aside>

  1660. <aside class="footnote brackets" id="id21" role="doc-footnote">

  1661. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id15">5</a><span class="fn-bracket">]</span></span>

  1662. <p>The power operator <code class="docutils literal notranslate"><span class="pre">**</span></code> binds less tightly than an arithmetic or

  1663. bitwise unary operator on its right, that is, <code class="docutils literal notranslate"><span class="pre">2**-1</span></code> is <code class="docutils literal notranslate"><span class="pre">0.5</span></code>.</p>

  1664. </aside>

  1665. <aside class="footnote brackets" id="id22" role="doc-footnote">

  1666. <span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id16">6</a><span class="fn-bracket">]</span></span>

  1667. <p>The <code class="docutils literal notranslate"><span class="pre">%</span></code> operator is also used for string formatting; the same

  1668. precedence applies.</p>

  1669. </aside>

  1670. </aside>

  1671. </section>

  1672. </section>

  1673. 

  1674. 

  1675.             <div class="clearer"></div>

  1676.           </div>

  1677.         </div>

  1678.       </div>

  1679.       <div class="sphinxsidebar" role="navigation" aria-label="main navigation">

  1680.         <div class="sphinxsidebarwrapper">

  1681.   <div>

  1682.     <h3><a href="../contents.html">Table of Contents</a></h3>

  1683.     <ul>

  1684. <li><a class="reference internal" href="#">6. Expressions</a><ul>

  1685. <li><a class="reference internal" href="#arithmetic-conversions">6.1. Arithmetic conversions</a></li>

  1686. <li><a class="reference internal" href="#atoms">6.2. Atoms</a><ul>

  1687. <li><a class="reference internal" href="#atom-identifiers">6.2.1. Identifiers (Names)</a></li>

  1688. <li><a class="reference internal" href="#literals">6.2.2. Literals</a></li>

  1689. <li><a class="reference internal" href="#parenthesized-forms">6.2.3. Parenthesized forms</a></li>

  1690. <li><a class="reference internal" href="#displays-for-lists-sets-and-dictionaries">6.2.4. Displays for lists, sets and dictionaries</a></li>

  1691. <li><a class="reference internal" href="#list-displays">6.2.5. List displays</a></li>

  1692. <li><a class="reference internal" href="#set-displays">6.2.6. Set displays</a></li>

  1693. <li><a class="reference internal" href="#dictionary-displays">6.2.7. Dictionary displays</a></li>

  1694. <li><a class="reference internal" href="#generator-expressions">6.2.8. Generator expressions</a></li>

  1695. <li><a class="reference internal" href="#yield-expressions">6.2.9. Yield expressions</a><ul>

  1696. <li><a class="reference internal" href="#generator-iterator-methods">6.2.9.1. Generator-iterator methods</a></li>

  1697. <li><a class="reference internal" href="#examples">6.2.9.2. Examples</a></li>

  1698. <li><a class="reference internal" href="#asynchronous-generator-functions">6.2.9.3. Asynchronous generator functions</a></li>

  1699. <li><a class="reference internal" href="#asynchronous-generator-iterator-methods">6.2.9.4. Asynchronous generator-iterator methods</a></li>

  1700. </ul>

  1701. </li>

  1702. </ul>

  1703. </li>

  1704. <li><a class="reference internal" href="#primaries">6.3. Primaries</a><ul>

  1705. <li><a class="reference internal" href="#attribute-references">6.3.1. Attribute references</a></li>

  1706. <li><a class="reference internal" href="#subscriptions">6.3.2. Subscriptions</a></li>

  1707. <li><a class="reference internal" href="#slicings">6.3.3. Slicings</a></li>

  1708. <li><a class="reference internal" href="#calls">6.3.4. Calls</a></li>

  1709. </ul>

  1710. </li>

  1711. <li><a class="reference internal" href="#await-expression">6.4. Await expression</a></li>

  1712. <li><a class="reference internal" href="#the-power-operator">6.5. The power operator</a></li>

  1713. <li><a class="reference internal" href="#unary-arithmetic-and-bitwise-operations">6.6. Unary arithmetic and bitwise operations</a></li>

  1714. <li><a class="reference internal" href="#binary-arithmetic-operations">6.7. Binary arithmetic operations</a></li>

  1715. <li><a class="reference internal" href="#shifting-operations">6.8. Shifting operations</a></li>

  1716. <li><a class="reference internal" href="#binary-bitwise-operations">6.9. Binary bitwise operations</a></li>

  1717. <li><a class="reference internal" href="#comparisons">6.10. Comparisons</a><ul>

  1718. <li><a class="reference internal" href="#value-comparisons">6.10.1. Value comparisons</a></li>

  1719. <li><a class="reference internal" href="#membership-test-operations">6.10.2. Membership test operations</a></li>

  1720. <li><a class="reference internal" href="#is-not">6.10.3. Identity comparisons</a></li>

  1721. </ul>

  1722. </li>

  1723. <li><a class="reference internal" href="#boolean-operations">6.11. Boolean operations</a></li>

  1724. <li><a class="reference internal" href="#assignment-expressions">6.12. Assignment expressions</a></li>

  1725. <li><a class="reference internal" href="#conditional-expressions">6.13. Conditional expressions</a></li>

  1726. <li><a class="reference internal" href="#lambda">6.14. Lambdas</a></li>

  1727. <li><a class="reference internal" href="#expression-lists">6.15. Expression lists</a></li>

  1728. <li><a class="reference internal" href="#evaluation-order">6.16. Evaluation order</a></li>

  1729. <li><a class="reference internal" href="#operator-precedence">6.17. Operator precedence</a></li>

  1730. </ul>

  1731. </li>

  1732. </ul>

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