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  83. <h3><a href="../contents.html">Table of Contents</a></h3>
  84. <ul>
  85. <li><a class="reference internal" href="#">7. Simple statements</a><ul>
  86. <li><a class="reference internal" href="#expression-statements">7.1. Expression statements</a></li>
  87. <li><a class="reference internal" href="#assignment-statements">7.2. Assignment statements</a><ul>
  88. <li><a class="reference internal" href="#augmented-assignment-statements">7.2.1. Augmented assignment statements</a></li>
  89. <li><a class="reference internal" href="#annotated-assignment-statements">7.2.2. Annotated assignment statements</a></li>
  90. </ul>
  91. </li>
  92. <li><a class="reference internal" href="#the-assert-statement">7.3. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">assert</span></code> statement</a></li>
  93. <li><a class="reference internal" href="#the-pass-statement">7.4. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code> statement</a></li>
  94. <li><a class="reference internal" href="#the-del-statement">7.5. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement</a></li>
  95. <li><a class="reference internal" href="#the-return-statement">7.6. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> statement</a></li>
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  100. <li><a class="reference internal" href="#the-import-statement">7.11. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code> statement</a><ul>
  101. <li><a class="reference internal" href="#future-statements">7.11.1. Future statements</a></li>
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  105. <li><a class="reference internal" href="#the-nonlocal-statement">7.13. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code> statement</a></li>
  106. <li><a class="reference internal" href="#the-type-statement">7.14. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code> statement</a></li>
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  118. <p class="topless"><a href="compound_stmts.html"
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  187. <section id="simple-statements">
  188. <span id="simple"></span><h1><span class="section-number">7. </span>Simple statements<a class="headerlink" href="#simple-statements" title="Link to this heading">¶</a></h1>
  189. <p id="index-0">A simple statement is comprised within a single logical line. Several simple
  190. statements may occur on a single line separated by semicolons. The syntax for
  191. simple statements is:</p>
  192. <pre>
  193. <strong id="grammar-token-python-grammar-simple_stmt">simple_stmt</strong> ::= <a class="reference internal" href="#grammar-token-python-grammar-expression_stmt"><code class="xref docutils literal notranslate"><span class="pre">expression_stmt</span></code></a>
  194. | <a class="reference internal" href="#grammar-token-python-grammar-assert_stmt"><code class="xref docutils literal notranslate"><span class="pre">assert_stmt</span></code></a>
  195. | <a class="reference internal" href="#grammar-token-python-grammar-assignment_stmt"><code class="xref docutils literal notranslate"><span class="pre">assignment_stmt</span></code></a>
  196. | <a class="reference internal" href="#grammar-token-python-grammar-augmented_assignment_stmt"><code class="xref docutils literal notranslate"><span class="pre">augmented_assignment_stmt</span></code></a>
  197. | <a class="reference internal" href="#grammar-token-python-grammar-annotated_assignment_stmt"><code class="xref docutils literal notranslate"><span class="pre">annotated_assignment_stmt</span></code></a>
  198. | <a class="reference internal" href="#grammar-token-python-grammar-pass_stmt"><code class="xref docutils literal notranslate"><span class="pre">pass_stmt</span></code></a>
  199. | <a class="reference internal" href="#grammar-token-python-grammar-del_stmt"><code class="xref docutils literal notranslate"><span class="pre">del_stmt</span></code></a>
  200. | <a class="reference internal" href="#grammar-token-python-grammar-return_stmt"><code class="xref docutils literal notranslate"><span class="pre">return_stmt</span></code></a>
  201. | <a class="reference internal" href="#grammar-token-python-grammar-yield_stmt"><code class="xref docutils literal notranslate"><span class="pre">yield_stmt</span></code></a>
  202. | <a class="reference internal" href="#grammar-token-python-grammar-raise_stmt"><code class="xref docutils literal notranslate"><span class="pre">raise_stmt</span></code></a>
  203. | <a class="reference internal" href="#grammar-token-python-grammar-break_stmt"><code class="xref docutils literal notranslate"><span class="pre">break_stmt</span></code></a>
  204. | <a class="reference internal" href="#grammar-token-python-grammar-continue_stmt"><code class="xref docutils literal notranslate"><span class="pre">continue_stmt</span></code></a>
  205. | <a class="reference internal" href="#grammar-token-python-grammar-import_stmt"><code class="xref docutils literal notranslate"><span class="pre">import_stmt</span></code></a>
  206. | <a class="reference internal" href="#grammar-token-python-grammar-future_stmt"><code class="xref docutils literal notranslate"><span class="pre">future_stmt</span></code></a>
  207. | <a class="reference internal" href="#grammar-token-python-grammar-global_stmt"><code class="xref docutils literal notranslate"><span class="pre">global_stmt</span></code></a>
  208. | <a class="reference internal" href="#grammar-token-python-grammar-nonlocal_stmt"><code class="xref docutils literal notranslate"><span class="pre">nonlocal_stmt</span></code></a>
  209. | <a class="reference internal" href="#grammar-token-python-grammar-type_stmt"><code class="xref docutils literal notranslate"><span class="pre">type_stmt</span></code></a>
  210. </pre>
  211. <section id="expression-statements">
  212. <span id="exprstmts"></span><h2><span class="section-number">7.1. </span>Expression statements<a class="headerlink" href="#expression-statements" title="Link to this heading">¶</a></h2>
  213. <p id="index-2"><span id="index-1"></span>Expression statements are used (mostly interactively) to compute and write a
  214. value, or (usually) to call a procedure (a function that returns no meaningful
  215. result; in Python, procedures return the value <code class="docutils literal notranslate"><span class="pre">None</span></code>). Other uses of
  216. expression statements are allowed and occasionally useful. The syntax for an
  217. expression statement is:</p>
  218. <pre>
  219. <strong id="grammar-token-python-grammar-expression_stmt">expression_stmt</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate"><span class="pre">starred_expression</span></code></a>
  220. </pre>
  221. <p>An expression statement evaluates the expression list (which may be a single
  222. expression).</p>
  223. <p id="index-3">In interactive mode, if the value is not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it is converted to a string
  224. using the built-in <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a> function and the resulting string is written to
  225. standard output on a line by itself (except if the result is <code class="docutils literal notranslate"><span class="pre">None</span></code>, so that
  226. procedure calls do not cause any output.)</p>
  227. </section>
  228. <section id="assignment-statements">
  229. <span id="assignment"></span><h2><span class="section-number">7.2. </span>Assignment statements<a class="headerlink" href="#assignment-statements" title="Link to this heading">¶</a></h2>
  230. <p id="index-4">Assignment statements are used to (re)bind names to values and to modify
  231. attributes or items of mutable objects:</p>
  232. <pre>
  233. <strong id="grammar-token-python-grammar-assignment_stmt">assignment_stmt</strong> ::= (<a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> &quot;=&quot;)+ (<a class="reference internal" href="expressions.html#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate"><span class="pre">starred_expression</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a>)
  234. <strong id="grammar-token-python-grammar-target_list">target_list </strong> ::= <a class="reference internal" href="#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate"><span class="pre">target</span></code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate"><span class="pre">target</span></code></a>)* [&quot;,&quot;]
  235. <strong id="grammar-token-python-grammar-target">target </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>
  236. | &quot;(&quot; [<a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a>] &quot;)&quot;
  237. | &quot;[&quot; [<a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a>] &quot;]&quot;
  238. | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-attributeref"><code class="xref docutils literal notranslate"><span class="pre">attributeref</span></code></a>
  239. | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-subscription"><code class="xref docutils literal notranslate"><span class="pre">subscription</span></code></a>
  240. | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-slicing"><code class="xref docutils literal notranslate"><span class="pre">slicing</span></code></a>
  241. | &quot;*&quot; <a class="reference internal" href="#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate"><span class="pre">target</span></code></a>
  242. </pre>
  243. <p>(See section <a class="reference internal" href="expressions.html#primaries"><span class="std std-ref">Primaries</span></a> for the syntax definitions for <em>attributeref</em>,
  244. <em>subscription</em>, and <em>slicing</em>.)</p>
  245. <p>An assignment statement evaluates the expression list (remember that this can be
  246. a single expression or a comma-separated list, the latter yielding a tuple) and
  247. assigns the single resulting object to each of the target lists, from left to
  248. right.</p>
  249. <p id="index-5">Assignment is defined recursively depending on the form of the target (list).
  250. When a target is part of a mutable object (an attribute reference, subscription
  251. or slicing), the mutable object must ultimately perform the assignment and
  252. decide about its validity, and may raise an exception if the assignment is
  253. unacceptable. The rules observed by various types and the exceptions raised are
  254. given with the definition of the object types (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>).</p>
  255. <p id="index-6">Assignment of an object to a target list, optionally enclosed in parentheses or
  256. square brackets, is recursively defined as follows.</p>
  257. <ul class="simple">
  258. <li><p>If the target list is a single target with no trailing comma,
  259. optionally in parentheses, the object is assigned to that target.</p></li>
  260. <li><p>Else:</p>
  261. <ul>
  262. <li><p>If the target list contains one target prefixed with an asterisk, called a
  263. “starred” target: The object must be an iterable with at least as many items
  264. as there are targets in the target list, minus one. The first items of the
  265. iterable are assigned, from left to right, to the targets before the starred
  266. target. The final items of the iterable are assigned to the targets after
  267. the starred target. A list of the remaining items in the iterable is then
  268. assigned to the starred target (the list can be empty).</p></li>
  269. <li><p>Else: The object must be an iterable with the same number of items as there
  270. are targets in the target list, and the items are assigned, from left to
  271. right, to the corresponding targets.</p></li>
  272. </ul>
  273. </li>
  274. </ul>
  275. <p>Assignment of an object to a single target is recursively defined as follows.</p>
  276. <ul>
  277. <li><p>If the target is an identifier (name):</p>
  278. <ul class="simple">
  279. <li><p>If the name does not occur in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> or <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a>
  280. statement in the current code block: the name is bound to the object in the
  281. current local namespace.</p></li>
  282. <li><p>Otherwise: the name is bound to the object in the global namespace or the
  283. outer namespace determined by <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a>, respectively.</p></li>
  284. </ul>
  285. <p id="index-7">The name is rebound if it was already bound. This may cause the reference
  286. count for the object previously bound to the name to reach zero, causing the
  287. object to be deallocated and its destructor (if it has one) to be called.</p>
  288. </li>
  289. <li id="index-8"><p>If the target is an attribute reference: The primary expression in the
  290. reference is evaluated. It should yield an object with assignable attributes;
  291. if this is not the case, <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> is raised. That object is then
  292. asked to assign the assigned object to the given attribute; if it cannot
  293. perform the assignment, it raises an exception (usually but not necessarily
  294. <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>).</p>
  295. <p id="attr-target-note">Note: If the object is a class instance and the attribute reference occurs on
  296. both sides of the assignment operator, the right-hand side expression, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> can access
  297. either an instance attribute or (if no instance attribute exists) a class
  298. attribute. The left-hand side target <code class="docutils literal notranslate"><span class="pre">a.x</span></code> is always set as an instance attribute,
  299. creating it if necessary. Thus, the two occurrences of <code class="docutils literal notranslate"><span class="pre">a.x</span></code> do not
  300. necessarily refer to the same attribute: if the right-hand side expression refers to a
  301. class attribute, the left-hand side creates a new instance attribute as the target of the
  302. assignment:</p>
  303. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Cls</span><span class="p">:</span>
  304. <span class="n">x</span> <span class="o">=</span> <span class="mi">3</span> <span class="c1"># class variable</span>
  305. <span class="n">inst</span> <span class="o">=</span> <span class="n">Cls</span><span class="p">()</span>
  306. <span class="n">inst</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">inst</span><span class="o">.</span><span class="n">x</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># writes inst.x as 4 leaving Cls.x as 3</span>
  307. </pre></div>
  308. </div>
  309. <p>This description does not necessarily apply to descriptor attributes, such as
  310. properties created with <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a>.</p>
  311. </li>
  312. <li id="index-9"><p>If the target is a subscription: The primary expression in the reference is
  313. evaluated. It should yield either a mutable sequence object (such as a list)
  314. or a mapping object (such as a dictionary). Next, the subscript expression is
  315. evaluated.</p>
  316. <p id="index-10">If the primary is a mutable sequence object (such as a list), the subscript
  317. must yield an integer. If it is negative, the sequence’s length is added to
  318. it. The resulting value must be a nonnegative integer less than the
  319. sequence’s length, and the sequence is asked to assign the assigned object to
  320. its item with that index. If the index is out of range, <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> is
  321. raised (assignment to a subscripted sequence cannot add new items to a list).</p>
  322. <p id="index-11">If the primary is a mapping object (such as a dictionary), the subscript must
  323. have a type compatible with the mapping’s key type, and the mapping is then
  324. asked to create a key/value pair which maps the subscript to the assigned
  325. object. This can either replace an existing key/value pair with the same key
  326. value, or insert a new key/value pair (if no key with the same value existed).</p>
  327. <p>For user-defined objects, the <a class="reference internal" href="datamodel.html#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a> method is called with
  328. appropriate arguments.</p>
  329. </li>
  330. <li id="index-12"><p>If the target is a slicing: The primary expression in the reference is
  331. evaluated. It should yield a mutable sequence object (such as a list). The
  332. assigned object should be a sequence object of the same type. Next, the lower
  333. and upper bound expressions are evaluated, insofar they are present; defaults
  334. are zero and the sequence’s length. The bounds should evaluate to integers.
  335. If either bound is negative, the sequence’s length is added to it. The
  336. resulting bounds are clipped to lie between zero and the sequence’s length,
  337. inclusive. Finally, the sequence object is asked to replace the slice with
  338. the items of the assigned sequence. The length of the slice may be different
  339. from the length of the assigned sequence, thus changing the length of the
  340. target sequence, if the target sequence allows it.</p></li>
  341. </ul>
  342. <div class="impl-detail compound">
  343. <p><strong>CPython implementation detail:</strong> In the current implementation, the syntax for targets is taken to be the same
  344. as for expressions, and invalid syntax is rejected during the code generation
  345. phase, causing less detailed error messages.</p>
  346. </div>
  347. <p>Although the definition of assignment implies that overlaps between the
  348. left-hand side and the right-hand side are ‘simultaneous’ (for example <code class="docutils literal notranslate"><span class="pre">a,</span> <span class="pre">b</span> <span class="pre">=</span>
  349. <span class="pre">b,</span> <span class="pre">a</span></code> swaps two variables), overlaps <em>within</em> the collection of assigned-to
  350. variables occur left-to-right, sometimes resulting in confusion. For instance,
  351. the following program prints <code class="docutils literal notranslate"><span class="pre">[0,</span> <span class="pre">2]</span></code>:</p>
  352. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span>
  353. <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span>
  354. <span class="n">i</span><span class="p">,</span> <span class="n">x</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span> <span class="c1"># i is updated, then x[i] is updated</span>
  355. <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
  356. </pre></div>
  357. </div>
  358. <div class="admonition seealso">
  359. <p class="admonition-title">See also</p>
  360. <dl class="simple">
  361. <dt><span class="target" id="index-13"></span><a class="pep reference external" href="https://peps.python.org/pep-3132/"><strong>PEP 3132</strong></a> - Extended Iterable Unpacking</dt><dd><p>The specification for the <code class="docutils literal notranslate"><span class="pre">*target</span></code> feature.</p>
  362. </dd>
  363. </dl>
  364. </div>
  365. <section id="augmented-assignment-statements">
  366. <span id="augassign"></span><h3><span class="section-number">7.2.1. </span>Augmented assignment statements<a class="headerlink" href="#augmented-assignment-statements" title="Link to this heading">¶</a></h3>
  367. <p id="index-14">Augmented assignment is the combination, in a single statement, of a binary
  368. operation and an assignment statement:</p>
  369. <pre>
  370. <strong id="grammar-token-python-grammar-augmented_assignment_stmt">augmented_assignment_stmt</strong> ::= <a class="reference internal" href="#grammar-token-python-grammar-augtarget"><code class="xref docutils literal notranslate"><span class="pre">augtarget</span></code></a> <a class="reference internal" href="#grammar-token-python-grammar-augop"><code class="xref docutils literal notranslate"><span class="pre">augop</span></code></a> (<a class="reference internal" href="expressions.html#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="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a>)
  371. <strong id="grammar-token-python-grammar-augtarget">augtarget </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="expressions.html#grammar-token-python-grammar-attributeref"><code class="xref docutils literal notranslate"><span class="pre">attributeref</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-subscription"><code class="xref docutils literal notranslate"><span class="pre">subscription</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-slicing"><code class="xref docutils literal notranslate"><span class="pre">slicing</span></code></a>
  372. <strong id="grammar-token-python-grammar-augop">augop </strong> ::= &quot;+=&quot; | &quot;-=&quot; | &quot;*=&quot; | &quot;&#64;=&quot; | &quot;/=&quot; | &quot;//=&quot; | &quot;%=&quot; | &quot;**=&quot;
  373. | &quot;&gt;&gt;=&quot; | &quot;&lt;&lt;=&quot; | &quot;&amp;=&quot; | &quot;^=&quot; | &quot;|=&quot;
  374. </pre>
  375. <p>(See section <a class="reference internal" href="expressions.html#primaries"><span class="std std-ref">Primaries</span></a> for the syntax definitions of the last three
  376. symbols.)</p>
  377. <p>An augmented assignment evaluates the target (which, unlike normal assignment
  378. statements, cannot be an unpacking) and the expression list, performs the binary
  379. operation specific to the type of assignment on the two operands, and assigns
  380. the result to the original target. The target is only evaluated once.</p>
  381. <p>An augmented assignment expression like <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+=</span> <span class="pre">1</span></code> can be rewritten as <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">x</span> <span class="pre">+</span>
  382. <span class="pre">1</span></code> to achieve a similar, but not exactly equal effect. In the augmented
  383. version, <code class="docutils literal notranslate"><span class="pre">x</span></code> is only evaluated once. Also, when possible, the actual operation
  384. is performed <em>in-place</em>, meaning that rather than creating a new object and
  385. assigning that to the target, the old object is modified instead.</p>
  386. <p>Unlike normal assignments, augmented assignments evaluate the left-hand side
  387. <em>before</em> evaluating the right-hand side. For example, <code class="docutils literal notranslate"><span class="pre">a[i]</span> <span class="pre">+=</span> <span class="pre">f(x)</span></code> first
  388. looks-up <code class="docutils literal notranslate"><span class="pre">a[i]</span></code>, then it evaluates <code class="docutils literal notranslate"><span class="pre">f(x)</span></code> and performs the addition, and
  389. lastly, it writes the result back to <code class="docutils literal notranslate"><span class="pre">a[i]</span></code>.</p>
  390. <p>With the exception of assigning to tuples and multiple targets in a single
  391. statement, the assignment done by augmented assignment statements is handled the
  392. same way as normal assignments. Similarly, with the exception of the possible
  393. <em>in-place</em> behavior, the binary operation performed by augmented assignment is
  394. the same as the normal binary operations.</p>
  395. <p>For targets which are attribute references, the same <a class="reference internal" href="#attr-target-note"><span class="std std-ref">caveat about class
  396. and instance attributes</span></a> applies as for regular assignments.</p>
  397. </section>
  398. <section id="annotated-assignment-statements">
  399. <span id="annassign"></span><h3><span class="section-number">7.2.2. </span>Annotated assignment statements<a class="headerlink" href="#annotated-assignment-statements" title="Link to this heading">¶</a></h3>
  400. <p id="index-15"><a class="reference internal" href="../glossary.html#term-variable-annotation"><span class="xref std std-term">Annotation</span></a> assignment is the combination, in a single
  401. statement, of a variable or attribute annotation and an optional assignment statement:</p>
  402. <pre>
  403. <strong id="grammar-token-python-grammar-annotated_assignment_stmt">annotated_assignment_stmt</strong> ::= <a class="reference internal" href="#grammar-token-python-grammar-augtarget"><code class="xref docutils literal notranslate"><span class="pre">augtarget</span></code></a> &quot;:&quot; <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>
  404. [&quot;=&quot; (<a class="reference internal" href="expressions.html#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate"><span class="pre">starred_expression</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a>)]
  405. </pre>
  406. <p>The difference from normal <a class="reference internal" href="#assignment"><span class="std std-ref">Assignment statements</span></a> is that only a single target is allowed.</p>
  407. <p>For simple names as assignment targets, if in class or module scope,
  408. the annotations are evaluated and stored in a special class or module
  409. attribute <code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code>
  410. that is a dictionary mapping from variable names (mangled if private) to
  411. evaluated annotations. This attribute is writable and is automatically
  412. created at the start of class or module body execution, if annotations
  413. are found statically.</p>
  414. <p>For expressions as assignment targets, the annotations are evaluated if
  415. in class or module scope, but not stored.</p>
  416. <p>If a name is annotated in a function scope, then this name is local for
  417. that scope. Annotations are never evaluated and stored in function scopes.</p>
  418. <p>If the right hand side is present, an annotated
  419. assignment performs the actual assignment before evaluating annotations
  420. (where applicable). If the right hand side is not present for an expression
  421. target, then the interpreter evaluates the target except for the last
  422. <a class="reference internal" href="datamodel.html#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a> or <a class="reference internal" href="datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> call.</p>
  423. <div class="admonition seealso">
  424. <p class="admonition-title">See also</p>
  425. <dl class="simple">
  426. <dt><span class="target" id="index-16"></span><a class="pep reference external" href="https://peps.python.org/pep-0526/"><strong>PEP 526</strong></a> - Syntax for Variable Annotations</dt><dd><p>The proposal that added syntax for annotating the types of variables
  427. (including class variables and instance variables), instead of expressing
  428. them through comments.</p>
  429. </dd>
  430. <dt><span class="target" id="index-17"></span><a class="pep reference external" href="https://peps.python.org/pep-0484/"><strong>PEP 484</strong></a> - Type hints</dt><dd><p>The proposal that added the <a class="reference internal" href="../library/typing.html#module-typing" title="typing: Support for type hints (see :pep:`484`)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">typing</span></code></a> module to provide a standard
  431. syntax for type annotations that can be used in static analysis tools and
  432. IDEs.</p>
  433. </dd>
  434. </dl>
  435. </div>
  436. <div class="versionchanged">
  437. <p><span class="versionmodified changed">Changed in version 3.8: </span>Now annotated assignments allow the same expressions in the right hand side as
  438. regular assignments. Previously, some expressions (like un-parenthesized
  439. tuple expressions) caused a syntax error.</p>
  440. </div>
  441. </section>
  442. </section>
  443. <section id="the-assert-statement">
  444. <span id="assert"></span><h2><span class="section-number">7.3. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">assert</span></code> statement<a class="headerlink" href="#the-assert-statement" title="Link to this heading">¶</a></h2>
  445. <p id="index-18">Assert statements are a convenient way to insert debugging assertions into a
  446. program:</p>
  447. <pre>
  448. <strong id="grammar-token-python-grammar-assert_stmt">assert_stmt</strong> ::= &quot;assert&quot; <a class="reference internal" href="expressions.html#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="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>]
  449. </pre>
  450. <p>The simple form, <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">expression</span></code>, is equivalent to</p>
  451. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">__debug__</span><span class="p">:</span>
  452. <span class="k">if</span> <span class="ow">not</span> <span class="n">expression</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">AssertionError</span>
  453. </pre></div>
  454. </div>
  455. <p>The extended form, <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">expression1,</span> <span class="pre">expression2</span></code>, is equivalent to</p>
  456. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">__debug__</span><span class="p">:</span>
  457. <span class="k">if</span> <span class="ow">not</span> <span class="n">expression1</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">AssertionError</span><span class="p">(</span><span class="n">expression2</span><span class="p">)</span>
  458. </pre></div>
  459. </div>
  460. <p id="index-19">These equivalences assume that <a class="reference internal" href="../library/constants.html#debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a> and <a class="reference internal" href="../library/exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AssertionError</span></code></a> refer to
  461. the built-in variables with those names. In the current implementation, the
  462. built-in variable <a class="reference internal" href="../library/constants.html#debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a> is <code class="docutils literal notranslate"><span class="pre">True</span></code> under normal circumstances,
  463. <code class="docutils literal notranslate"><span class="pre">False</span></code> when optimization is requested (command line option <a class="reference internal" href="../using/cmdline.html#cmdoption-O"><code class="xref std std-option docutils literal notranslate"><span class="pre">-O</span></code></a>). The current
  464. code generator emits no code for an assert statement when optimization is
  465. requested at compile time. Note that it is unnecessary to include the source
  466. code for the expression that failed in the error message; it will be displayed
  467. as part of the stack trace.</p>
  468. <p>Assignments to <a class="reference internal" href="../library/constants.html#debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a> are illegal. The value for the built-in variable
  469. is determined when the interpreter starts.</p>
  470. </section>
  471. <section id="the-pass-statement">
  472. <span id="pass"></span><h2><span class="section-number">7.4. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code> statement<a class="headerlink" href="#the-pass-statement" title="Link to this heading">¶</a></h2>
  473. <pre id="index-20">
  474. <strong id="grammar-token-python-grammar-pass_stmt">pass_stmt</strong> ::= &quot;pass&quot;
  475. </pre>
  476. <p><a class="reference internal" href="#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> is a null operation — when it is executed, nothing happens.
  477. It is useful as a placeholder when a statement is required syntactically, but no
  478. code needs to be executed, for example:</p>
  479. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">arg</span><span class="p">):</span> <span class="k">pass</span> <span class="c1"># a function that does nothing (yet)</span>
  480. <span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="k">pass</span> <span class="c1"># a class with no methods (yet)</span>
  481. </pre></div>
  482. </div>
  483. </section>
  484. <section id="the-del-statement">
  485. <span id="del"></span><h2><span class="section-number">7.5. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement<a class="headerlink" href="#the-del-statement" title="Link to this heading">¶</a></h2>
  486. <pre id="index-21">
  487. <strong id="grammar-token-python-grammar-del_stmt">del_stmt</strong> ::= &quot;del&quot; <a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a>
  488. </pre>
  489. <p>Deletion is recursively defined very similar to the way assignment is defined.
  490. Rather than spelling it out in full details, here are some hints.</p>
  491. <p>Deletion of a target list recursively deletes each target, from left to right.</p>
  492. <p id="index-22">Deletion of a name removes the binding of that name from the local or global
  493. namespace, depending on whether the name occurs in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement
  494. in the same code block. If the name is unbound, 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> exception
  495. will be raised.</p>
  496. <p id="index-23">Deletion of attribute references, subscriptions and slicings is passed to the
  497. primary object involved; deletion of a slicing is in general equivalent to
  498. assignment of an empty slice of the right type (but even this is determined by
  499. the sliced object).</p>
  500. <div class="versionchanged">
  501. <p><span class="versionmodified changed">Changed in version 3.2: </span>Previously it was illegal to delete a name from the local namespace if it
  502. occurs as a free variable in a nested block.</p>
  503. </div>
  504. </section>
  505. <section id="the-return-statement">
  506. <span id="return"></span><h2><span class="section-number">7.6. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> statement<a class="headerlink" href="#the-return-statement" title="Link to this heading">¶</a></h2>
  507. <pre id="index-24">
  508. <strong id="grammar-token-python-grammar-return_stmt">return_stmt</strong> ::= &quot;return&quot; [<a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a>]
  509. </pre>
  510. <p><a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> may only occur syntactically nested in a function definition,
  511. not within a nested class definition.</p>
  512. <p>If an expression list is present, it is evaluated, else <code class="docutils literal notranslate"><span class="pre">None</span></code> is substituted.</p>
  513. <p><a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> leaves the current function call with the expression list (or
  514. <code class="docutils literal notranslate"><span class="pre">None</span></code>) as return value.</p>
  515. <p id="index-25">When <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> passes control out of 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> statement with a
  516. <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> clause, that <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause is executed before
  517. really leaving the function.</p>
  518. <p>In a generator function, the <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement indicates that the
  519. generator is done and will cause <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> to be raised. The returned
  520. value (if any) is used as an argument to construct <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> and
  521. becomes 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">StopIteration.value</span></code></a> attribute.</p>
  522. <p>In an asynchronous generator function, an empty <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement
  523. indicates that the asynchronous generator is done and will cause
  524. <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> to be raised. A non-empty <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code>
  525. statement is a syntax error in an asynchronous generator function.</p>
  526. </section>
  527. <section id="the-yield-statement">
  528. <span id="yield"></span><h2><span class="section-number">7.7. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code> statement<a class="headerlink" href="#the-yield-statement" title="Link to this heading">¶</a></h2>
  529. <pre id="index-26">
  530. <strong id="grammar-token-python-grammar-yield_stmt">yield_stmt</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a>
  531. </pre>
  532. <p>A <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement is semantically equivalent to a <a class="reference internal" href="expressions.html#yieldexpr"><span class="std std-ref">yield
  533. expression</span></a>. The yield statement can be used to omit the parentheses
  534. that would otherwise be required in the equivalent yield expression
  535. statement. For example, the yield statements</p>
  536. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">yield</span> <span class="o">&lt;</span><span class="n">expr</span><span class="o">&gt;</span>
  537. <span class="k">yield from</span> <span class="o">&lt;</span><span class="n">expr</span><span class="o">&gt;</span>
  538. </pre></div>
  539. </div>
  540. <p>are equivalent to the yield expression statements</p>
  541. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="k">yield</span> <span class="o">&lt;</span><span class="n">expr</span><span class="o">&gt;</span><span class="p">)</span>
  542. <span class="p">(</span><span class="k">yield from</span> <span class="o">&lt;</span><span class="n">expr</span><span class="o">&gt;</span><span class="p">)</span>
  543. </pre></div>
  544. </div>
  545. <p>Yield expressions and statements are only used when defining a <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a>
  546. function, and are only used in the body of the generator function. Using yield
  547. in a function definition is sufficient to cause that definition to create a
  548. generator function instead of a normal function.</p>
  549. <p>For full details of <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> semantics, refer to the
  550. <a class="reference internal" href="expressions.html#yieldexpr"><span class="std std-ref">Yield expressions</span></a> section.</p>
  551. </section>
  552. <section id="the-raise-statement">
  553. <span id="raise"></span><h2><span class="section-number">7.8. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code> statement<a class="headerlink" href="#the-raise-statement" title="Link to this heading">¶</a></h2>
  554. <pre id="index-27">
  555. <strong id="grammar-token-python-grammar-raise_stmt">raise_stmt</strong> ::= &quot;raise&quot; [<a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> [&quot;from&quot; <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>]]
  556. </pre>
  557. <p>If no expressions are present, <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> re-raises the
  558. exception that is currently being handled, which is also known as the <em>active exception</em>.
  559. If there isn’t currently an active exception, 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> exception is raised
  560. indicating that this is an error.</p>
  561. <p>Otherwise, <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> evaluates the first expression as the exception
  562. object. It must be either a subclass or an instance of <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseException</span></code></a>.
  563. If it is a class, the exception instance will be obtained when needed by
  564. instantiating the class with no arguments.</p>
  565. <p>The <em class="dfn">type</em> of the exception is the exception instance’s class, the
  566. <em class="dfn">value</em> is the instance itself.</p>
  567. <p id="index-28">A traceback object is normally created automatically when an exception is raised
  568. and attached to it as the <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.
  569. You can create an exception and set your own traceback in one step using the
  570. <a class="reference internal" href="../library/exceptions.html#BaseException.with_traceback" title="BaseException.with_traceback"><code class="xref py py-meth docutils literal notranslate"><span class="pre">with_traceback()</span></code></a> exception method (which returns the
  571. same exception instance, with its traceback set to its argument), like so:</p>
  572. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">&quot;foo occurred&quot;</span><span class="p">)</span><span class="o">.</span><span class="n">with_traceback</span><span class="p">(</span><span class="n">tracebackobj</span><span class="p">)</span>
  573. </pre></div>
  574. </div>
  575. <p id="index-29">The <code class="docutils literal notranslate"><span class="pre">from</span></code> clause is used for exception chaining: if given, the second
  576. <em>expression</em> must be another exception class or instance. If the second
  577. expression is an exception instance, it will be attached to the raised
  578. exception as the <a class="reference internal" href="../library/exceptions.html#BaseException.__cause__" title="BaseException.__cause__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__cause__</span></code></a> attribute (which is writable). If the
  579. expression is an exception class, the class will be instantiated and the
  580. resulting exception instance will be attached to the raised exception as the
  581. <code class="xref py py-attr docutils literal notranslate"><span class="pre">__cause__</span></code> attribute. If the raised exception is not handled, both
  582. exceptions will be printed:</p>
  583. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">try</span><span class="p">:</span>
  584. <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
  585. <span class="gp">... </span><span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">exc</span><span class="p">:</span>
  586. <span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">&quot;Something bad happened&quot;</span><span class="p">)</span> <span class="kn">from</span> <span class="nn">exc</span>
  587. <span class="gp">...</span>
  588. <span class="gt">Traceback (most recent call last):</span>
  589. File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">2</span>, in <span class="n">&lt;module&gt;</span>
  590. <span class="w"> </span><span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
  591. <span class="w"> </span><span class="pm">~~^~~</span>
  592. <span class="gr">ZeroDivisionError</span>: <span class="n">division by zero</span>
  593. <span class="gt">The above exception was the direct cause of the following exception:</span>
  594. <span class="gt">Traceback (most recent call last):</span>
  595. File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">4</span>, in <span class="n">&lt;module&gt;</span>
  596. <span class="w"> </span><span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">&quot;Something bad happened&quot;</span><span class="p">)</span> <span class="kn">from</span> <span class="nn">exc</span>
  597. <span class="gr">RuntimeError</span>: <span class="n">Something bad happened</span>
  598. </pre></div>
  599. </div>
  600. <p>A similar mechanism works implicitly if a new exception is raised when
  601. an exception is already being handled. An exception may be handled
  602. when an <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> or <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> clause, or a
  603. <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement, is used. The previous exception is then
  604. attached as the new exception’s <a class="reference internal" href="../library/exceptions.html#BaseException.__context__" title="BaseException.__context__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__context__</span></code></a> attribute:</p>
  605. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">try</span><span class="p">:</span>
  606. <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
  607. <span class="gp">... </span><span class="k">except</span><span class="p">:</span>
  608. <span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">&quot;Something bad happened&quot;</span><span class="p">)</span>
  609. <span class="gp">...</span>
  610. <span class="gt">Traceback (most recent call last):</span>
  611. File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">2</span>, in <span class="n">&lt;module&gt;</span>
  612. <span class="w"> </span><span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
  613. <span class="w"> </span><span class="pm">~~^~~</span>
  614. <span class="gr">ZeroDivisionError</span>: <span class="n">division by zero</span>
  615. <span class="gt">During handling of the above exception, another exception occurred:</span>
  616. <span class="gt">Traceback (most recent call last):</span>
  617. File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">4</span>, in <span class="n">&lt;module&gt;</span>
  618. <span class="w"> </span><span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">&quot;Something bad happened&quot;</span><span class="p">)</span>
  619. <span class="gr">RuntimeError</span>: <span class="n">Something bad happened</span>
  620. </pre></div>
  621. </div>
  622. <p>Exception chaining can be explicitly suppressed by specifying <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
  623. the <code class="docutils literal notranslate"><span class="pre">from</span></code> clause:</p>
  624. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">try</span><span class="p">:</span>
  625. <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
  626. <span class="gp">... </span><span class="k">except</span><span class="p">:</span>
  627. <span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">&quot;Something bad happened&quot;</span><span class="p">)</span> <span class="kn">from</span> <span class="kc">None</span>
  628. <span class="gp">...</span>
  629. <span class="gt">Traceback (most recent call last):</span>
  630. File <span class="nb">&quot;&lt;stdin&gt;&quot;</span>, line <span class="m">4</span>, in <span class="n">&lt;module&gt;</span>
  631. <span class="gr">RuntimeError</span>: <span class="n">Something bad happened</span>
  632. </pre></div>
  633. </div>
  634. <p>Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions"><span class="std std-ref">Exceptions</span></a>,
  635. and information about handling exceptions is in section <a class="reference internal" href="compound_stmts.html#try"><span class="std std-ref">The try statement</span></a>.</p>
  636. <div class="versionchanged">
  637. <p><span class="versionmodified changed">Changed in version 3.3: </span><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> is now permitted as <code class="docutils literal notranslate"><span class="pre">Y</span></code> in <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">X</span> <span class="pre">from</span> <span class="pre">Y</span></code>.</p>
  638. <p>Added the <a class="reference internal" href="../library/exceptions.html#BaseException.__suppress_context__" title="BaseException.__suppress_context__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__suppress_context__</span></code></a> attribute to suppress
  639. automatic display of the exception context.</p>
  640. </div>
  641. <div class="versionchanged">
  642. <p><span class="versionmodified changed">Changed in version 3.11: </span>If the traceback of the active exception is modified in an <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>
  643. clause, a subsequent <code class="docutils literal notranslate"><span class="pre">raise</span></code> statement re-raises the exception with the
  644. modified traceback. Previously, the exception was re-raised with the
  645. traceback it had when it was caught.</p>
  646. </div>
  647. </section>
  648. <section id="the-break-statement">
  649. <span id="break"></span><h2><span class="section-number">7.9. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code> statement<a class="headerlink" href="#the-break-statement" title="Link to this heading">¶</a></h2>
  650. <pre id="index-30">
  651. <strong id="grammar-token-python-grammar-break_stmt">break_stmt</strong> ::= &quot;break&quot;
  652. </pre>
  653. <p><a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> may only occur syntactically nested 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> or
  654. <a class="reference internal" href="compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop, but not nested in a function or class definition within
  655. that loop.</p>
  656. <p id="index-31">It terminates the nearest enclosing loop, skipping the optional <code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code>
  657. clause if the loop has one.</p>
  658. <p>If 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 is terminated by <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a>, the loop control
  659. target keeps its current value.</p>
  660. <p id="index-32">When <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> passes control out of 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> statement with a
  661. <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> clause, that <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause is executed before
  662. really leaving the loop.</p>
  663. </section>
  664. <section id="the-continue-statement">
  665. <span id="continue"></span><h2><span class="section-number">7.10. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code> statement<a class="headerlink" href="#the-continue-statement" title="Link to this heading">¶</a></h2>
  666. <pre id="index-33">
  667. <strong id="grammar-token-python-grammar-continue_stmt">continue_stmt</strong> ::= &quot;continue&quot;
  668. </pre>
  669. <p><a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> may only occur syntactically nested 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> or
  670. <a class="reference internal" href="compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop, but not nested in a function or class definition within
  671. that loop. It continues with the next cycle of the nearest enclosing loop.</p>
  672. <p>When <a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> passes control out of 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> statement with a
  673. <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> clause, that <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause is executed before
  674. really starting the next loop cycle.</p>
  675. </section>
  676. <section id="the-import-statement">
  677. <span id="from"></span><span id="import"></span><h2><span class="section-number">7.11. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code> statement<a class="headerlink" href="#the-import-statement" title="Link to this heading">¶</a></h2>
  678. <pre id="index-34">
  679. <strong id="grammar-token-python-grammar-import_stmt">import_stmt </strong> ::= &quot;import&quot; <a class="reference internal" href="#grammar-token-python-grammar-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> [&quot;as&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>] (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> [&quot;as&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>])*
  680. | &quot;from&quot; <a class="reference internal" href="#grammar-token-python-grammar-relative_module"><code class="xref docutils literal notranslate"><span class="pre">relative_module</span></code></a> &quot;import&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> [&quot;as&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>]
  681. (&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> [&quot;as&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>])*
  682. | &quot;from&quot; <a class="reference internal" href="#grammar-token-python-grammar-relative_module"><code class="xref docutils literal notranslate"><span class="pre">relative_module</span></code></a> &quot;import&quot; &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> [&quot;as&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>]
  683. (&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> [&quot;as&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>])* [&quot;,&quot;] &quot;)&quot;
  684. | &quot;from&quot; <a class="reference internal" href="#grammar-token-python-grammar-relative_module"><code class="xref docutils literal notranslate"><span class="pre">relative_module</span></code></a> &quot;import&quot; &quot;*&quot;
  685. <strong id="grammar-token-python-grammar-module">module </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="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a>
  686. <strong id="grammar-token-python-grammar-relative_module">relative_module</strong> ::= &quot;.&quot;* <a class="reference internal" href="#grammar-token-python-grammar-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> | &quot;.&quot;+
  687. </pre>
  688. <p>The basic import statement (no <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> clause) is executed in two
  689. steps:</p>
  690. <ol class="arabic simple">
  691. <li><p>find a module, loading and initializing it if necessary</p></li>
  692. <li><p>define a name or names in the local namespace for the scope where
  693. the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement occurs.</p></li>
  694. </ol>
  695. <p>When the statement contains multiple clauses (separated by
  696. commas) the two steps are carried out separately for each clause, just
  697. as though the clauses had been separated out into individual import
  698. statements.</p>
  699. <p>The details of the first step, finding and loading modules, are described in
  700. greater detail in the section on the <a class="reference internal" href="import.html#importsystem"><span class="std std-ref">import system</span></a>,
  701. which also describes the various types of packages and modules that can
  702. be imported, as well as all the hooks that can be used to customize
  703. the import system. Note that failures in this step may indicate either
  704. that the module could not be located, <em>or</em> that an error occurred while
  705. initializing the module, which includes execution of the module’s code.</p>
  706. <p>If the requested module is retrieved successfully, it will be made
  707. available in the local namespace in one of three ways:</p>
  708. <ul class="simple" id="index-35">
  709. <li><p>If the module name is followed by <code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code>, then the name
  710. following <code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> is bound directly to the imported module.</p></li>
  711. <li><p>If no other name is specified, and the module being imported is a top
  712. level module, the module’s name is bound in the local namespace as a
  713. reference to the imported module</p></li>
  714. <li><p>If the module being imported is <em>not</em> a top level module, then the name
  715. of the top level package that contains the module is bound in the local
  716. namespace as a reference to the top level package. The imported module
  717. must be accessed using its full qualified name rather than directly</p></li>
  718. </ul>
  719. <p id="index-36">The <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> form uses a slightly more complex process:</p>
  720. <ol class="arabic simple">
  721. <li><p>find the module specified in the <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> clause, loading and
  722. initializing it if necessary;</p></li>
  723. <li><p>for each of the identifiers specified in the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> clauses:</p>
  724. <ol class="arabic simple">
  725. <li><p>check if the imported module has an attribute by that name</p></li>
  726. <li><p>if not, attempt to import a submodule with that name and then
  727. check the imported module again for that attribute</p></li>
  728. <li><p>if the attribute is not found, <a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> is raised.</p></li>
  729. <li><p>otherwise, a reference to that value is stored in the local namespace,
  730. using the name in the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> clause if it is present,
  731. otherwise using the attribute name</p></li>
  732. </ol>
  733. </li>
  734. </ol>
  735. <p>Examples:</p>
  736. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">foo</span> <span class="c1"># foo imported and bound locally</span>
  737. <span class="kn">import</span> <span class="nn">foo.bar.baz</span> <span class="c1"># foo, foo.bar, and foo.bar.baz imported, foo bound locally</span>
  738. <span class="kn">import</span> <span class="nn">foo.bar.baz</span> <span class="k">as</span> <span class="nn">fbb</span> <span class="c1"># foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as fbb</span>
  739. <span class="kn">from</span> <span class="nn">foo.bar</span> <span class="kn">import</span> <span class="n">baz</span> <span class="c1"># foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as baz</span>
  740. <span class="kn">from</span> <span class="nn">foo</span> <span class="kn">import</span> <span class="n">attr</span> <span class="c1"># foo imported and foo.attr bound as attr</span>
  741. </pre></div>
  742. </div>
  743. <p id="index-37">If the list of identifiers is replaced by a star (<code class="docutils literal notranslate"><span class="pre">'*'</span></code>), all public
  744. names defined in the module are bound in the local namespace for the scope
  745. where the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement occurs.</p>
  746. <p id="index-38">The <em>public names</em> defined by a module are determined by checking the module’s
  747. namespace for a variable named <code class="docutils literal notranslate"><span class="pre">__all__</span></code>; if defined, it must be a sequence
  748. of strings which are names defined or imported by that module. The names
  749. given in <code class="docutils literal notranslate"><span class="pre">__all__</span></code> are all considered public and are required to exist. If
  750. <code class="docutils literal notranslate"><span class="pre">__all__</span></code> is not defined, the set of public names includes all names found
  751. in the module’s namespace which do not begin with an underscore character
  752. (<code class="docutils literal notranslate"><span class="pre">'_'</span></code>). <code class="docutils literal notranslate"><span class="pre">__all__</span></code> should contain the entire public API. It is intended
  753. to avoid accidentally exporting items that are not part of the API (such as
  754. library modules which were imported and used within the module).</p>
  755. <p>The wild card form of import — <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">*</span></code> — is only allowed at
  756. the module level. Attempting to use it in class or function definitions will
  757. raise a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>.</p>
  758. <p id="index-39">When specifying what module to import you do not have to specify the absolute
  759. name of the module. When a module or package is contained within another
  760. package it is possible to make a relative import within the same top package
  761. without having to mention the package name. By using leading dots in the
  762. specified module or package after <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> you can specify how high to
  763. traverse up the current package hierarchy without specifying exact names. One
  764. leading dot means the current package where the module making the import
  765. exists. Two dots means up one package level. Three dots is up two levels, etc.
  766. So if you execute <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">.</span> <span class="pre">import</span> <span class="pre">mod</span></code> from a module in the <code class="docutils literal notranslate"><span class="pre">pkg</span></code> package
  767. then you will end up importing <code class="docutils literal notranslate"><span class="pre">pkg.mod</span></code>. If you execute <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">..subpkg2</span>
  768. <span class="pre">import</span> <span class="pre">mod</span></code> from within <code class="docutils literal notranslate"><span class="pre">pkg.subpkg1</span></code> you will import <code class="docutils literal notranslate"><span class="pre">pkg.subpkg2.mod</span></code>.
  769. The specification for relative imports is contained in
  770. the <a class="reference internal" href="import.html#relativeimports"><span class="std std-ref">Package Relative Imports</span></a> section.</p>
  771. <p><a class="reference internal" href="../library/importlib.html#importlib.import_module" title="importlib.import_module"><code class="xref py py-func docutils literal notranslate"><span class="pre">importlib.import_module()</span></code></a> is provided to support applications that
  772. determine dynamically the modules to be loaded.</p>
  773. <p class="audit-hook">Raises an <a class="reference internal" href="../library/sys.html#auditing"><span class="std std-ref">auditing event</span></a> <code class="docutils literal notranslate"><span class="pre">import</span></code> with arguments <code class="docutils literal notranslate"><span class="pre">module</span></code>, <code class="docutils literal notranslate"><span class="pre">filename</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.path</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.meta_path</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.path_hooks</span></code>.</p>
  774. <section id="future-statements">
  775. <span id="future"></span><h3><span class="section-number">7.11.1. </span>Future statements<a class="headerlink" href="#future-statements" title="Link to this heading">¶</a></h3>
  776. <p id="index-40">A <em class="dfn">future statement</em> is a directive to the compiler that a particular
  777. module should be compiled using syntax or semantics that will be available in a
  778. specified future release of Python where the feature becomes standard.</p>
  779. <p>The future statement is intended to ease migration to future versions of Python
  780. that introduce incompatible changes to the language. It allows use of the new
  781. features on a per-module basis before the release in which the feature becomes
  782. standard.</p>
  783. <pre>
  784. <strong id="grammar-token-python-grammar-future_stmt">future_stmt</strong> ::= &quot;from&quot; &quot;__future__&quot; &quot;import&quot; <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate"><span class="pre">feature</span></code></a> [&quot;as&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>]
  785. (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate"><span class="pre">feature</span></code></a> [&quot;as&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>])*
  786. | &quot;from&quot; &quot;__future__&quot; &quot;import&quot; &quot;(&quot; <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate"><span class="pre">feature</span></code></a> [&quot;as&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>]
  787. (&quot;,&quot; <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate"><span class="pre">feature</span></code></a> [&quot;as&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>])* [&quot;,&quot;] &quot;)&quot;
  788. <strong id="grammar-token-python-grammar-feature">feature </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>
  789. </pre>
  790. <p>A future statement must appear near the top of the module. The only lines that
  791. can appear before a future statement are:</p>
  792. <ul class="simple">
  793. <li><p>the module docstring (if any),</p></li>
  794. <li><p>comments,</p></li>
  795. <li><p>blank lines, and</p></li>
  796. <li><p>other future statements.</p></li>
  797. </ul>
  798. <p>The only feature that requires using the future statement is
  799. <code class="docutils literal notranslate"><span class="pre">annotations</span></code> (see <span class="target" id="index-41"></span><a class="pep reference external" href="https://peps.python.org/pep-0563/"><strong>PEP 563</strong></a>).</p>
  800. <p>All historical features enabled by the future statement are still recognized
  801. by Python 3. The list includes <code class="docutils literal notranslate"><span class="pre">absolute_import</span></code>, <code class="docutils literal notranslate"><span class="pre">division</span></code>,
  802. <code class="docutils literal notranslate"><span class="pre">generators</span></code>, <code class="docutils literal notranslate"><span class="pre">generator_stop</span></code>, <code class="docutils literal notranslate"><span class="pre">unicode_literals</span></code>,
  803. <code class="docutils literal notranslate"><span class="pre">print_function</span></code>, <code class="docutils literal notranslate"><span class="pre">nested_scopes</span></code> and <code class="docutils literal notranslate"><span class="pre">with_statement</span></code>. They are
  804. all redundant because they are always enabled, and only kept for
  805. backwards compatibility.</p>
  806. <p>A future statement is recognized and treated specially at compile time: Changes
  807. to the semantics of core constructs are often implemented by generating
  808. different code. It may even be the case that a new feature introduces new
  809. incompatible syntax (such as a new reserved word), in which case the compiler
  810. may need to parse the module differently. Such decisions cannot be pushed off
  811. until runtime.</p>
  812. <p>For any given release, the compiler knows which feature names have been defined,
  813. and raises a compile-time error if a future statement contains a feature not
  814. known to it.</p>
  815. <p>The direct runtime semantics are the same as for any import statement: there is
  816. a standard module <a class="reference internal" href="../library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a>, described later, and it will be imported in
  817. the usual way at the time the future statement is executed.</p>
  818. <p>The interesting runtime semantics depend on the specific feature enabled by the
  819. future statement.</p>
  820. <p>Note that there is nothing special about the statement:</p>
  821. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">__future__</span> <span class="p">[</span><span class="k">as</span> <span class="n">name</span><span class="p">]</span>
  822. </pre></div>
  823. </div>
  824. <p>That is not a future statement; it’s an ordinary import statement with no
  825. special semantics or syntax restrictions.</p>
  826. <p>Code compiled by calls to the built-in functions <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> and <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a>
  827. that occur in a module <code class="xref py py-mod docutils literal notranslate"><span class="pre">M</span></code> containing a future statement will, by default,
  828. use the new syntax or semantics associated with the future statement. This can
  829. be controlled by optional arguments to <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> — see the documentation
  830. of that function for details.</p>
  831. <p>A future statement typed at an interactive interpreter prompt will take effect
  832. for the rest of the interpreter session. If an interpreter is started with the
  833. <a class="reference internal" href="../using/cmdline.html#cmdoption-i"><code class="xref std std-option docutils literal notranslate"><span class="pre">-i</span></code></a> option, is passed a script name to execute, and the script includes
  834. a future statement, it will be in effect in the interactive session started
  835. after the script is executed.</p>
  836. <div class="admonition seealso">
  837. <p class="admonition-title">See also</p>
  838. <dl class="simple">
  839. <dt><span class="target" id="index-42"></span><a class="pep reference external" href="https://peps.python.org/pep-0236/"><strong>PEP 236</strong></a> - Back to the __future__</dt><dd><p>The original proposal for the __future__ mechanism.</p>
  840. </dd>
  841. </dl>
  842. </div>
  843. </section>
  844. </section>
  845. <section id="the-global-statement">
  846. <span id="global"></span><h2><span class="section-number">7.12. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statement<a class="headerlink" href="#the-global-statement" title="Link to this heading">¶</a></h2>
  847. <pre id="index-43">
  848. <strong id="grammar-token-python-grammar-global_stmt">global_stmt</strong> ::= &quot;global&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> (&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>)*
  849. </pre>
  850. <p>The <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement is a declaration which holds for the entire
  851. current code block. It means that the listed identifiers are to be interpreted
  852. as globals. It would be impossible to assign to a global variable without
  853. <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code>, although free variables may refer to globals without being
  854. declared global.</p>
  855. <p>Names listed in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement must not be used in the same code
  856. block textually preceding that <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statement.</p>
  857. <p>Names listed in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement must not be defined as formal
  858. parameters, or as targets in <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements or <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clauses, or 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> target list, <a class="reference internal" href="compound_stmts.html#class"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">class</span></code></a>
  859. definition, function definition, <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement, or variable
  860. annotation.</p>
  861. <div class="impl-detail compound">
  862. <p><strong>CPython implementation detail:</strong> The current implementation does not enforce some of these restrictions, but
  863. programs should not abuse this freedom, as future implementations may enforce
  864. them or silently change the meaning of the program.</p>
  865. </div>
  866. <p id="index-44"><strong>Programmer’s note:</strong> <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> is a directive to the parser. It
  867. applies only to code parsed at the same time as the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statement.
  868. In particular, a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statement contained in a string or code
  869. object supplied to the built-in <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> function does not affect the code
  870. block <em>containing</em> the function call, and code contained in such a string is
  871. unaffected by <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statements in the code containing the function
  872. call. The same applies to the <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> and <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> functions.</p>
  873. </section>
  874. <section id="the-nonlocal-statement">
  875. <span id="nonlocal"></span><h2><span class="section-number">7.13. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code> statement<a class="headerlink" href="#the-nonlocal-statement" title="Link to this heading">¶</a></h2>
  876. <pre id="index-45">
  877. <strong id="grammar-token-python-grammar-nonlocal_stmt">nonlocal_stmt</strong> ::= &quot;nonlocal&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> (&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>)*
  878. </pre>
  879. <p>When the definition of a function or class is nested (enclosed) within
  880. the definitions of other functions, its nonlocal scopes are the local
  881. scopes of the enclosing functions. The <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> statement
  882. causes the listed identifiers to refer to names previously bound in
  883. nonlocal scopes. It allows encapsulated code to rebind such nonlocal
  884. identifiers. If a name is bound in more than one nonlocal scope, the
  885. nearest binding is used. If a name is not bound in any nonlocal scope,
  886. or if there is no nonlocal scope, a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> is raised.</p>
  887. <p>The nonlocal statement applies to the entire scope of a function or
  888. class body. A <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> is raised if a variable is used or
  889. assigned to prior to its nonlocal declaration in the scope.</p>
  890. <div class="admonition seealso">
  891. <p class="admonition-title">See also</p>
  892. <dl class="simple">
  893. <dt><span class="target" id="index-46"></span><a class="pep reference external" href="https://peps.python.org/pep-3104/"><strong>PEP 3104</strong></a> - Access to Names in Outer Scopes</dt><dd><p>The specification for the <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> statement.</p>
  894. </dd>
  895. </dl>
  896. </div>
  897. <p><strong>Programmer’s note:</strong> <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> is a directive to the parser
  898. and applies only to code parsed along with it. See the note for the
  899. <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement.</p>
  900. </section>
  901. <section id="the-type-statement">
  902. <span id="type"></span><h2><span class="section-number">7.14. </span>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code> statement<a class="headerlink" href="#the-type-statement" title="Link to this heading">¶</a></h2>
  903. <pre id="index-47">
  904. <strong id="grammar-token-python-grammar-type_stmt">type_stmt</strong> ::= 'type' <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="compound_stmts.html#grammar-token-python-grammar-type_params"><code class="xref docutils literal notranslate"><span class="pre">type_params</span></code></a>] &quot;=&quot; <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>
  905. </pre>
  906. <p>The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code> statement declares a type alias, which is an instance
  907. of <a class="reference internal" href="../library/typing.html#typing.TypeAliasType" title="typing.TypeAliasType"><code class="xref py py-class docutils literal notranslate"><span class="pre">typing.TypeAliasType</span></code></a>.</p>
  908. <p>For example, the following statement creates a type alias:</p>
  909. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">type</span> <span class="n">Point</span> <span class="o">=</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">float</span><span class="p">,</span> <span class="nb">float</span><span class="p">]</span>
  910. </pre></div>
  911. </div>
  912. <p>This code is roughly equivalent to:</p>
  913. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">annotation</span><span class="o">-</span><span class="k">def</span> <span class="nf">VALUE_OF_Point</span><span class="p">():</span>
  914. <span class="k">return</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">float</span><span class="p">,</span> <span class="nb">float</span><span class="p">]</span>
  915. <span class="n">Point</span> <span class="o">=</span> <span class="n">typing</span><span class="o">.</span><span class="n">TypeAliasType</span><span class="p">(</span><span class="s2">&quot;Point&quot;</span><span class="p">,</span> <span class="n">VALUE_OF_Point</span><span class="p">())</span>
  916. </pre></div>
  917. </div>
  918. <p><code class="docutils literal notranslate"><span class="pre">annotation-def</span></code> indicates an <a class="reference internal" href="executionmodel.html#annotation-scopes"><span class="std std-ref">annotation scope</span></a>, which behaves
  919. mostly like a function, but with several small differences.</p>
  920. <p>The value of the
  921. type alias is evaluated in the annotation scope. It is not evaluated when the
  922. type alias is created, but only when the value is accessed through the type alias’s
  923. <code class="xref py py-attr docutils literal notranslate"><span class="pre">__value__</span></code> attribute (see <a class="reference internal" href="executionmodel.html#lazy-evaluation"><span class="std std-ref">Lazy evaluation</span></a>).
  924. This allows the type alias to refer to names that are not yet defined.</p>
  925. <p>Type aliases may be made generic by adding a <a class="reference internal" href="compound_stmts.html#type-params"><span class="std std-ref">type parameter list</span></a>
  926. after the name. See <a class="reference internal" href="compound_stmts.html#generic-type-aliases"><span class="std std-ref">Generic type aliases</span></a> for more.</p>
  927. <p><code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code> is a <a class="reference internal" href="lexical_analysis.html#soft-keywords"><span class="std std-ref">soft keyword</span></a>.</p>
  928. <div class="versionadded">
  929. <p><span class="versionmodified added">New in version 3.12.</span></p>
  930. </div>
  931. <div class="admonition seealso">
  932. <p class="admonition-title">See also</p>
  933. <dl class="simple">
  934. <dt><span class="target" id="index-48"></span><a class="pep reference external" href="https://peps.python.org/pep-0695/"><strong>PEP 695</strong></a> - Type Parameter Syntax</dt><dd><p>Introduced the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code> statement and syntax for
  935. generic classes and functions.</p>
  936. </dd>
  937. </dl>
  938. </div>
  939. </section>
  940. </section>
  941. <div class="clearer"></div>
  942. </div>
  943. </div>
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  945. <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
  946. <div class="sphinxsidebarwrapper">
  947. <div>
  948. <h3><a href="../contents.html">Table of Contents</a></h3>
  949. <ul>
  950. <li><a class="reference internal" href="#">7. Simple statements</a><ul>
  951. <li><a class="reference internal" href="#expression-statements">7.1. Expression statements</a></li>
  952. <li><a class="reference internal" href="#assignment-statements">7.2. Assignment statements</a><ul>
  953. <li><a class="reference internal" href="#augmented-assignment-statements">7.2.1. Augmented assignment statements</a></li>
  954. <li><a class="reference internal" href="#annotated-assignment-statements">7.2.2. Annotated assignment statements</a></li>
  955. </ul>
  956. </li>
  957. <li><a class="reference internal" href="#the-assert-statement">7.3. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">assert</span></code> statement</a></li>
  958. <li><a class="reference internal" href="#the-pass-statement">7.4. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code> statement</a></li>
  959. <li><a class="reference internal" href="#the-del-statement">7.5. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code> statement</a></li>
  960. <li><a class="reference internal" href="#the-return-statement">7.6. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> statement</a></li>
  961. <li><a class="reference internal" href="#the-yield-statement">7.7. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code> statement</a></li>
  962. <li><a class="reference internal" href="#the-raise-statement">7.8. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code> statement</a></li>
  963. <li><a class="reference internal" href="#the-break-statement">7.9. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code> statement</a></li>
  964. <li><a class="reference internal" href="#the-continue-statement">7.10. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code> statement</a></li>
  965. <li><a class="reference internal" href="#the-import-statement">7.11. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code> statement</a><ul>
  966. <li><a class="reference internal" href="#future-statements">7.11.1. Future statements</a></li>
  967. </ul>
  968. </li>
  969. <li><a class="reference internal" href="#the-global-statement">7.12. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statement</a></li>
  970. <li><a class="reference internal" href="#the-nonlocal-statement">7.13. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code> statement</a></li>
  971. <li><a class="reference internal" href="#the-type-statement">7.14. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">type</span></code> statement</a></li>
  972. </ul>
  973. </li>
  974. </ul>
  975. </div>
  976. <div>
  977. <h4>Previous topic</h4>
  978. <p class="topless"><a href="expressions.html"
  979. title="previous chapter"><span class="section-number">6. </span>Expressions</a></p>
  980. </div>
  981. <div>
  982. <h4>Next topic</h4>
  983. <p class="topless"><a href="compound_stmts.html"
  984. title="next chapter"><span class="section-number">8. </span>Compound statements</a></p>
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