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    <ul>
<li><a class="reference internal" href="#">Design and History FAQ</a><ul>
<li><a class="reference internal" href="#why-does-python-use-indentation-for-grouping-of-statements">Why does Python use indentation for grouping of statements?</a></li>
<li><a class="reference internal" href="#why-am-i-getting-strange-results-with-simple-arithmetic-operations">Why am I getting strange results with simple arithmetic operations?</a></li>
<li><a class="reference internal" href="#why-are-floating-point-calculations-so-inaccurate">Why are floating-point calculations so inaccurate?</a></li>
<li><a class="reference internal" href="#why-are-python-strings-immutable">Why are Python strings immutable?</a></li>
<li><a class="reference internal" href="#why-must-self-be-used-explicitly-in-method-definitions-and-calls">Why must ‘self’ be used explicitly in method definitions and calls?</a></li>
<li><a class="reference internal" href="#why-can-t-i-use-an-assignment-in-an-expression">Why can’t I use an assignment in an expression?</a></li>
<li><a class="reference internal" href="#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list">Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?</a></li>
<li><a class="reference internal" href="#why-is-join-a-string-method-instead-of-a-list-or-tuple-method">Why is join() a string method instead of a list or tuple method?</a></li>
<li><a class="reference internal" href="#how-fast-are-exceptions">How fast are exceptions?</a></li>
<li><a class="reference internal" href="#why-isn-t-there-a-switch-or-case-statement-in-python">Why isn’t there a switch or case statement in Python?</a></li>
<li><a class="reference internal" href="#can-t-you-emulate-threads-in-the-interpreter-instead-of-relying-on-an-os-specific-thread-implementation">Can’t you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?</a></li>
<li><a class="reference internal" href="#why-can-t-lambda-expressions-contain-statements">Why can’t lambda expressions contain statements?</a></li>
<li><a class="reference internal" href="#can-python-be-compiled-to-machine-code-c-or-some-other-language">Can Python be compiled to machine code, C or some other language?</a></li>
<li><a class="reference internal" href="#how-does-python-manage-memory">How does Python manage memory?</a></li>
<li><a class="reference internal" href="#why-doesn-t-cpython-use-a-more-traditional-garbage-collection-scheme">Why doesn’t CPython use a more traditional garbage collection scheme?</a></li>
<li><a class="reference internal" href="#why-isn-t-all-memory-freed-when-cpython-exits">Why isn’t all memory freed when CPython exits?</a></li>
<li><a class="reference internal" href="#why-are-there-separate-tuple-and-list-data-types">Why are there separate tuple and list data types?</a></li>
<li><a class="reference internal" href="#how-are-lists-implemented-in-cpython">How are lists implemented in CPython?</a></li>
<li><a class="reference internal" href="#how-are-dictionaries-implemented-in-cpython">How are dictionaries implemented in CPython?</a></li>
<li><a class="reference internal" href="#why-must-dictionary-keys-be-immutable">Why must dictionary keys be immutable?</a></li>
<li><a class="reference internal" href="#why-doesn-t-list-sort-return-the-sorted-list">Why doesn’t list.sort() return the sorted list?</a></li>
<li><a class="reference internal" href="#how-do-you-specify-and-enforce-an-interface-spec-in-python">How do you specify and enforce an interface spec in Python?</a></li>
<li><a class="reference internal" href="#why-is-there-no-goto">Why is there no goto?</a></li>
<li><a class="reference internal" href="#why-can-t-raw-strings-r-strings-end-with-a-backslash">Why can’t raw strings (r-strings) end with a backslash?</a></li>
<li><a class="reference internal" href="#why-doesn-t-python-have-a-with-statement-for-attribute-assignments">Why doesn’t Python have a “with” statement for attribute assignments?</a></li>
<li><a class="reference internal" href="#why-don-t-generators-support-the-with-statement">Why don’t generators support the with statement?</a></li>
<li><a class="reference internal" href="#why-are-colons-required-for-the-if-while-def-class-statements">Why are colons required for the if/while/def/class statements?</a></li>
<li><a class="reference internal" href="#why-does-python-allow-commas-at-the-end-of-lists-and-tuples">Why does Python allow commas at the end of lists and tuples?</a></li>
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  <section id="design-and-history-faq">
<h1><a class="toc-backref" href="#id2" role="doc-backlink">Design and History FAQ</a><a class="headerlink" href="#design-and-history-faq" title="Link to this heading">¶</a></h1>
<nav class="contents" id="contents">
<p class="topic-title">Contents</p>
<ul class="simple">
<li><p><a class="reference internal" href="#design-and-history-faq" id="id2">Design and History FAQ</a></p>
<ul>
<li><p><a class="reference internal" href="#why-does-python-use-indentation-for-grouping-of-statements" id="id3">Why does Python use indentation for grouping of statements?</a></p></li>
<li><p><a class="reference internal" href="#why-am-i-getting-strange-results-with-simple-arithmetic-operations" id="id4">Why am I getting strange results with simple arithmetic operations?</a></p></li>
<li><p><a class="reference internal" href="#why-are-floating-point-calculations-so-inaccurate" id="id5">Why are floating-point calculations so inaccurate?</a></p></li>
<li><p><a class="reference internal" href="#why-are-python-strings-immutable" id="id6">Why are Python strings immutable?</a></p></li>
<li><p><a class="reference internal" href="#why-must-self-be-used-explicitly-in-method-definitions-and-calls" id="id7">Why must ‘self’ be used explicitly in method definitions and calls?</a></p></li>
<li><p><a class="reference internal" href="#why-can-t-i-use-an-assignment-in-an-expression" id="id8">Why can’t I use an assignment in an expression?</a></p></li>
<li><p><a class="reference internal" href="#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list" id="id9">Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?</a></p></li>
<li><p><a class="reference internal" href="#why-is-join-a-string-method-instead-of-a-list-or-tuple-method" id="id10">Why is join() a string method instead of a list or tuple method?</a></p></li>
<li><p><a class="reference internal" href="#how-fast-are-exceptions" id="id11">How fast are exceptions?</a></p></li>
<li><p><a class="reference internal" href="#why-isn-t-there-a-switch-or-case-statement-in-python" id="id12">Why isn’t there a switch or case statement in Python?</a></p></li>
<li><p><a class="reference internal" href="#can-t-you-emulate-threads-in-the-interpreter-instead-of-relying-on-an-os-specific-thread-implementation" id="id13">Can’t you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?</a></p></li>
<li><p><a class="reference internal" href="#why-can-t-lambda-expressions-contain-statements" id="id14">Why can’t lambda expressions contain statements?</a></p></li>
<li><p><a class="reference internal" href="#can-python-be-compiled-to-machine-code-c-or-some-other-language" id="id15">Can Python be compiled to machine code, C or some other language?</a></p></li>
<li><p><a class="reference internal" href="#how-does-python-manage-memory" id="id16">How does Python manage memory?</a></p></li>
<li><p><a class="reference internal" href="#why-doesn-t-cpython-use-a-more-traditional-garbage-collection-scheme" id="id17">Why doesn’t CPython use a more traditional garbage collection scheme?</a></p></li>
<li><p><a class="reference internal" href="#why-isn-t-all-memory-freed-when-cpython-exits" id="id18">Why isn’t all memory freed when CPython exits?</a></p></li>
<li><p><a class="reference internal" href="#why-are-there-separate-tuple-and-list-data-types" id="id19">Why are there separate tuple and list data types?</a></p></li>
<li><p><a class="reference internal" href="#how-are-lists-implemented-in-cpython" id="id20">How are lists implemented in CPython?</a></p></li>
<li><p><a class="reference internal" href="#how-are-dictionaries-implemented-in-cpython" id="id21">How are dictionaries implemented in CPython?</a></p></li>
<li><p><a class="reference internal" href="#why-must-dictionary-keys-be-immutable" id="id22">Why must dictionary keys be immutable?</a></p></li>
<li><p><a class="reference internal" href="#why-doesn-t-list-sort-return-the-sorted-list" id="id23">Why doesn’t list.sort() return the sorted list?</a></p></li>
<li><p><a class="reference internal" href="#how-do-you-specify-and-enforce-an-interface-spec-in-python" id="id24">How do you specify and enforce an interface spec in Python?</a></p></li>
<li><p><a class="reference internal" href="#why-is-there-no-goto" id="id25">Why is there no goto?</a></p></li>
<li><p><a class="reference internal" href="#why-can-t-raw-strings-r-strings-end-with-a-backslash" id="id26">Why can’t raw strings (r-strings) end with a backslash?</a></p></li>
<li><p><a class="reference internal" href="#why-doesn-t-python-have-a-with-statement-for-attribute-assignments" id="id27">Why doesn’t Python have a “with” statement for attribute assignments?</a></p></li>
<li><p><a class="reference internal" href="#why-don-t-generators-support-the-with-statement" id="id28">Why don’t generators support the with statement?</a></p></li>
<li><p><a class="reference internal" href="#why-are-colons-required-for-the-if-while-def-class-statements" id="id29">Why are colons required for the if/while/def/class statements?</a></p></li>
<li><p><a class="reference internal" href="#why-does-python-allow-commas-at-the-end-of-lists-and-tuples" id="id30">Why does Python allow commas at the end of lists and tuples?</a></p></li>
</ul>
</li>
</ul>
</nav>
<section id="why-does-python-use-indentation-for-grouping-of-statements">
<h2><a class="toc-backref" href="#id3" role="doc-backlink">Why does Python use indentation for grouping of statements?</a><a class="headerlink" href="#why-does-python-use-indentation-for-grouping-of-statements" title="Link to this heading">¶</a></h2>
<p>Guido van Rossum believes that using indentation for grouping is extremely
elegant and contributes a lot to the clarity of the average Python program.
Most people learn to love this feature after a while.</p>
<p>Since there are no begin/end brackets there cannot be a disagreement between
grouping perceived by the parser and the human reader.  Occasionally C
programmers will encounter a fragment of code like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="p">(</span><span class="n">x</span> <span class="o">&lt;=</span> <span class="n">y</span><span class="p">)</span>
        <span class="n">x</span><span class="o">++</span><span class="p">;</span>
        <span class="n">y</span><span class="o">--</span><span class="p">;</span>
<span class="n">z</span><span class="o">++</span><span class="p">;</span>
</pre></div>
</div>
<p>Only the <code class="docutils literal notranslate"><span class="pre">x++</span></code> statement is executed if the condition is true, but the
indentation leads many to believe otherwise.  Even experienced C programmers will
sometimes stare at it a long time wondering as to why <code class="docutils literal notranslate"><span class="pre">y</span></code> is being decremented even
for <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">&gt;</span> <span class="pre">y</span></code>.</p>
<p>Because there are no begin/end brackets, Python is much less prone to
coding-style conflicts.  In C there are many different ways to place the braces.
After becoming used to reading and writing code using a particular style,
it is normal to feel somewhat uneasy when reading (or being required to write)
in a different one.</p>
<p>Many coding styles place begin/end brackets on a line by themselves.  This makes
programs considerably longer and wastes valuable screen space, making it harder
to get a good overview of a program.  Ideally, a function should fit on one
screen (say, 20–30 lines).  20 lines of Python can do a lot more work than 20
lines of C.  This is not solely due to the lack of begin/end brackets – the
lack of declarations and the high-level data types are also responsible – but
the indentation-based syntax certainly helps.</p>
</section>
<section id="why-am-i-getting-strange-results-with-simple-arithmetic-operations">
<h2><a class="toc-backref" href="#id4" role="doc-backlink">Why am I getting strange results with simple arithmetic operations?</a><a class="headerlink" href="#why-am-i-getting-strange-results-with-simple-arithmetic-operations" title="Link to this heading">¶</a></h2>
<p>See the next question.</p>
</section>
<section id="why-are-floating-point-calculations-so-inaccurate">
<h2><a class="toc-backref" href="#id5" role="doc-backlink">Why are floating-point calculations so inaccurate?</a><a class="headerlink" href="#why-are-floating-point-calculations-so-inaccurate" title="Link to this heading">¶</a></h2>
<p>Users are often surprised by results like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="mf">1.2</span> <span class="o">-</span> <span class="mf">1.0</span>
<span class="go">0.19999999999999996</span>
</pre></div>
</div>
<p>and think it is a bug in Python.  It’s not.  This has little to do with Python,
and much more to do with how the underlying platform handles floating-point
numbers.</p>
<p>The <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a> type in CPython uses a C <code class="docutils literal notranslate"><span class="pre">double</span></code> for storage.  A
<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a> object’s value is stored in binary floating-point with a fixed
precision (typically 53 bits) and Python uses C operations, which in turn rely
on the hardware implementation in the processor, to perform floating-point
operations. This means that as far as floating-point operations are concerned,
Python behaves like many popular languages including C and Java.</p>
<p>Many numbers that can be written easily in decimal notation cannot be expressed
exactly in binary floating-point.  For example, after:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">x</span> <span class="o">=</span> <span class="mf">1.2</span>
</pre></div>
</div>
<p>the value stored for <code class="docutils literal notranslate"><span class="pre">x</span></code> is a (very good) approximation to the decimal value
<code class="docutils literal notranslate"><span class="pre">1.2</span></code>, but is not exactly equal to it.  On a typical machine, the actual
stored value is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mf">1.0011001100110011001100110011001100110011001100110011</span> <span class="p">(</span><span class="n">binary</span><span class="p">)</span>
</pre></div>
</div>
<p>which is exactly:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mf">1.1999999999999999555910790149937383830547332763671875</span> <span class="p">(</span><span class="n">decimal</span><span class="p">)</span>
</pre></div>
</div>
<p>The typical precision of 53 bits provides Python floats with 15–16
decimal digits of accuracy.</p>
<p>For a fuller explanation, please see the <a class="reference internal" href="../tutorial/floatingpoint.html#tut-fp-issues"><span class="std std-ref">floating point arithmetic</span></a> chapter in the Python tutorial.</p>
</section>
<section id="why-are-python-strings-immutable">
<h2><a class="toc-backref" href="#id6" role="doc-backlink">Why are Python strings immutable?</a><a class="headerlink" href="#why-are-python-strings-immutable" title="Link to this heading">¶</a></h2>
<p>There are several advantages.</p>
<p>One is performance: knowing that a string is immutable means we can allocate
space for it at creation time, and the storage requirements are fixed and
unchanging.  This is also one of the reasons for the distinction between tuples
and lists.</p>
<p>Another advantage is that strings in Python are considered as “elemental” as
numbers.  No amount of activity will change the value 8 to anything else, and in
Python, no amount of activity will change the string “eight” to anything else.</p>
</section>
<section id="why-must-self-be-used-explicitly-in-method-definitions-and-calls">
<span id="why-self"></span><h2><a class="toc-backref" href="#id7" role="doc-backlink">Why must ‘self’ be used explicitly in method definitions and calls?</a><a class="headerlink" href="#why-must-self-be-used-explicitly-in-method-definitions-and-calls" title="Link to this heading">¶</a></h2>
<p>The idea was borrowed from Modula-3.  It turns out to be very useful, for a
variety of reasons.</p>
<p>First, it’s more obvious that you are using a method or instance attribute
instead of a local variable.  Reading <code class="docutils literal notranslate"><span class="pre">self.x</span></code> or <code class="docutils literal notranslate"><span class="pre">self.meth()</span></code> makes it
absolutely clear that an instance variable or method is used even if you don’t
know the class definition by heart.  In C++, you can sort of tell by the lack of
a local variable declaration (assuming globals are rare or easily recognizable)
– but in Python, there are no local variable declarations, so you’d have to
look up the class definition to be sure.  Some C++ and Java coding standards
call for instance attributes to have an <code class="docutils literal notranslate"><span class="pre">m_</span></code> prefix, so this explicitness is
still useful in those languages, too.</p>
<p>Second, it means that no special syntax is necessary if you want to explicitly
reference or call the method from a particular class.  In C++, if you want to
use a method from a base class which is overridden in a derived class, you have
to use the <code class="docutils literal notranslate"><span class="pre">::</span></code> operator – in Python you can write
<code class="docutils literal notranslate"><span class="pre">baseclass.methodname(self,</span> <span class="pre">&lt;argument</span> <span class="pre">list&gt;)</span></code>.  This is particularly useful
for <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> methods, and in general in cases where a derived class
method wants to extend the base class method of the same name and thus has to
call the base class method somehow.</p>
<p>Finally, for instance variables it solves a syntactic problem with assignment:
since local variables in Python are (by definition!) those variables to which a
value is assigned in a function body (and that aren’t explicitly declared
global), there has to be some way to tell the interpreter that an assignment was
meant to assign to an instance variable instead of to a local variable, and it
should preferably be syntactic (for efficiency reasons).  C++ does this through
declarations, but Python doesn’t have declarations and it would be a pity having
to introduce them just for this purpose.  Using the explicit <code class="docutils literal notranslate"><span class="pre">self.var</span></code> solves
this nicely.  Similarly, for using instance variables, having to write
<code class="docutils literal notranslate"><span class="pre">self.var</span></code> means that references to unqualified names inside a method don’t
have to search the instance’s directories.  To put it another way, local
variables and instance variables live in two different namespaces, and you need
to tell Python which namespace to use.</p>
</section>
<section id="why-can-t-i-use-an-assignment-in-an-expression">
<span id="id1"></span><h2><a class="toc-backref" href="#id8" role="doc-backlink">Why can’t I use an assignment in an expression?</a><a class="headerlink" href="#why-can-t-i-use-an-assignment-in-an-expression" title="Link to this heading">¶</a></h2>
<p>Starting in Python 3.8, you can!</p>
<p>Assignment expressions using the walrus operator <code class="docutils literal notranslate"><span class="pre">:=</span></code> assign a variable in an
expression:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="n">chunk</span> <span class="o">:=</span> <span class="n">fp</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">200</span><span class="p">):</span>
   <span class="nb">print</span><span class="p">(</span><span class="n">chunk</span><span class="p">)</span>
</pre></div>
</div>
<p>See <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0572/"><strong>PEP 572</strong></a> for more information.</p>
</section>
<section id="why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list">
<h2><a class="toc-backref" href="#id9" role="doc-backlink">Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?</a><a class="headerlink" href="#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list" title="Link to this heading">¶</a></h2>
<p>As Guido said:</p>
<blockquote>
<div><p>(a) For some operations, prefix notation just reads better than
postfix – prefix (and infix!) operations have a long tradition in
mathematics which likes notations where the visuals help the
mathematician thinking about a problem. Compare the easy with which we
rewrite a formula like x*(a+b) into x*a + x*b to the clumsiness of
doing the same thing using a raw OO notation.</p>
<p>(b) When I read code that says len(x) I <em>know</em> that it is asking for
the length of something. This tells me two things: the result is an
integer, and the argument is some kind of container. To the contrary,
when I read x.len(), I have to already know that x is some kind of
container implementing an interface or inheriting from a class that
has a standard len(). Witness the confusion we occasionally have when
a class that is not implementing a mapping has a get() or keys()
method, or something that isn’t a file has a write() method.</p>
<p class="attribution">—<a class="reference external" href="https://mail.python.org/pipermail/python-3000/2006-November/004643.html">https://mail.python.org/pipermail/python-3000/2006-November/004643.html</a></p>
</div></blockquote>
</section>
<section id="why-is-join-a-string-method-instead-of-a-list-or-tuple-method">
<h2><a class="toc-backref" href="#id10" role="doc-backlink">Why is join() a string method instead of a list or tuple method?</a><a class="headerlink" href="#why-is-join-a-string-method-instead-of-a-list-or-tuple-method" title="Link to this heading">¶</a></h2>
<p>Strings became much more like other standard types starting in Python 1.6, when
methods were added which give the same functionality that has always been
available using the functions of the string module.  Most of these new methods
have been widely accepted, but the one which appears to make some programmers
feel uncomfortable is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="s2">&quot;, &quot;</span><span class="o">.</span><span class="n">join</span><span class="p">([</span><span class="s1">&#39;1&#39;</span><span class="p">,</span> <span class="s1">&#39;2&#39;</span><span class="p">,</span> <span class="s1">&#39;4&#39;</span><span class="p">,</span> <span class="s1">&#39;8&#39;</span><span class="p">,</span> <span class="s1">&#39;16&#39;</span><span class="p">])</span>
</pre></div>
</div>
<p>which gives the result:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="s2">&quot;1, 2, 4, 8, 16&quot;</span>
</pre></div>
</div>
<p>There are two common arguments against this usage.</p>
<p>The first runs along the lines of: “It looks really ugly using a method of a
string literal (string constant)”, to which the answer is that it might, but a
string literal is just a fixed value. If the methods are to be allowed on names
bound to strings there is no logical reason to make them unavailable on
literals.</p>
<p>The second objection is typically cast as: “I am really telling a sequence to
join its members together with a string constant”.  Sadly, you aren’t.  For some
reason there seems to be much less difficulty with having <a class="reference internal" href="../library/stdtypes.html#str.split" title="str.split"><code class="xref py py-meth docutils literal notranslate"><span class="pre">split()</span></code></a> as
a string method, since in that case it is easy to see that</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="s2">&quot;1, 2, 4, 8, 16&quot;</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s2">&quot;, &quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>is an instruction to a string literal to return the substrings delimited by the
given separator (or, by default, arbitrary runs of white space).</p>
<p><a class="reference internal" href="../library/stdtypes.html#str.join" title="str.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> is a string method because in using it you are telling the
separator string to iterate over a sequence of strings and insert itself between
adjacent elements.  This method can be used with any argument which obeys the
rules for sequence objects, including any new classes you might define yourself.
Similar methods exist for bytes and bytearray objects.</p>
</section>
<section id="how-fast-are-exceptions">
<h2><a class="toc-backref" href="#id11" role="doc-backlink">How fast are exceptions?</a><a class="headerlink" href="#how-fast-are-exceptions" title="Link to this heading">¶</a></h2>
<p>A <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> block is extremely efficient if no exceptions
are raised.  Actually
catching an exception is expensive.  In versions of Python prior to 2.0 it was
common to use this idiom:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
    <span class="n">value</span> <span class="o">=</span> <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span>
<span class="k">except</span> <span class="ne">KeyError</span><span class="p">:</span>
    <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">getvalue</span><span class="p">(</span><span class="n">key</span><span class="p">)</span>
    <span class="n">value</span> <span class="o">=</span> <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span>
</pre></div>
</div>
<p>This only made sense when you expected the dict to have the key almost all the
time.  If that wasn’t the case, you coded it like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">mydict</span><span class="p">:</span>
    <span class="n">value</span> <span class="o">=</span> <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
    <span class="n">value</span> <span class="o">=</span> <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">getvalue</span><span class="p">(</span><span class="n">key</span><span class="p">)</span>
</pre></div>
</div>
<p>For this specific case, you could also use <code class="docutils literal notranslate"><span class="pre">value</span> <span class="pre">=</span> <span class="pre">dict.setdefault(key,</span>
<span class="pre">getvalue(key))</span></code>, but only if the <code class="docutils literal notranslate"><span class="pre">getvalue()</span></code> call is cheap enough because it
is evaluated in all cases.</p>
</section>
<section id="why-isn-t-there-a-switch-or-case-statement-in-python">
<h2><a class="toc-backref" href="#id12" role="doc-backlink">Why isn’t there a switch or case statement in Python?</a><a class="headerlink" href="#why-isn-t-there-a-switch-or-case-statement-in-python" title="Link to this heading">¶</a></h2>
<p>In general, structured switch statements execute one block of code
when an expression has a particular value or set of values.
Since Python 3.10 one can easily match literal values, or constants
within a namespace, with a <code class="docutils literal notranslate"><span class="pre">match</span> <span class="pre">...</span> <span class="pre">case</span></code> statement.
An older alternative is a sequence of <code class="docutils literal notranslate"><span class="pre">if...</span> <span class="pre">elif...</span> <span class="pre">elif...</span> <span class="pre">else</span></code>.</p>
<p>For cases where you need to choose from a very large number of possibilities,
you can create a dictionary mapping case values to functions to call.  For
example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">functions</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;a&#39;</span><span class="p">:</span> <span class="n">function_1</span><span class="p">,</span>
             <span class="s1">&#39;b&#39;</span><span class="p">:</span> <span class="n">function_2</span><span class="p">,</span>
             <span class="s1">&#39;c&#39;</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">method_1</span><span class="p">}</span>

<span class="n">func</span> <span class="o">=</span> <span class="n">functions</span><span class="p">[</span><span class="n">value</span><span class="p">]</span>
<span class="n">func</span><span class="p">()</span>
</pre></div>
</div>
<p>For calling methods on objects, you can simplify yet further by using the
<a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">getattr()</span></code></a> built-in to retrieve methods with a particular name:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyVisitor</span><span class="p">:</span>
    <span class="k">def</span> <span class="nf">visit_a</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="o">...</span>

    <span class="k">def</span> <span class="nf">dispatch</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
        <span class="n">method_name</span> <span class="o">=</span> <span class="s1">&#39;visit_&#39;</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
        <span class="n">method</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">method_name</span><span class="p">)</span>
        <span class="n">method</span><span class="p">()</span>
</pre></div>
</div>
<p>It’s suggested that you use a prefix for the method names, such as <code class="docutils literal notranslate"><span class="pre">visit_</span></code> in
this example.  Without such a prefix, if values are coming from an untrusted
source, an attacker would be able to call any method on your object.</p>
<p>Imitating switch with fallthrough, as with C’s switch-case-default,
is possible, much harder, and less needed.</p>
</section>
<section id="can-t-you-emulate-threads-in-the-interpreter-instead-of-relying-on-an-os-specific-thread-implementation">
<h2><a class="toc-backref" href="#id13" role="doc-backlink">Can’t you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?</a><a class="headerlink" href="#can-t-you-emulate-threads-in-the-interpreter-instead-of-relying-on-an-os-specific-thread-implementation" title="Link to this heading">¶</a></h2>
<p>Answer 1: Unfortunately, the interpreter pushes at least one C stack frame for
each Python stack frame.  Also, extensions can call back into Python at almost
random moments.  Therefore, a complete threads implementation requires thread
support for C.</p>
<p>Answer 2: Fortunately, there is <a class="reference external" href="https://github.com/stackless-dev/stackless/wiki">Stackless Python</a>,
which has a completely redesigned interpreter loop that avoids the C stack.</p>
</section>
<section id="why-can-t-lambda-expressions-contain-statements">
<h2><a class="toc-backref" href="#id14" role="doc-backlink">Why can’t lambda expressions contain statements?</a><a class="headerlink" href="#why-can-t-lambda-expressions-contain-statements" title="Link to this heading">¶</a></h2>
<p>Python lambda expressions cannot contain statements because Python’s syntactic
framework can’t handle statements nested inside expressions.  However, in
Python, this is not a serious problem.  Unlike lambda forms in other languages,
where they add functionality, Python lambdas are only a shorthand notation if
you’re too lazy to define a function.</p>
<p>Functions are already first class objects in Python, and can be declared in a
local scope.  Therefore the only advantage of using a lambda instead of a
locally defined function is that you don’t need to invent a name for the
function – but that’s just a local variable to which the function object (which
is exactly the same type of object that a lambda expression yields) is assigned!</p>
</section>
<section id="can-python-be-compiled-to-machine-code-c-or-some-other-language">
<h2><a class="toc-backref" href="#id15" role="doc-backlink">Can Python be compiled to machine code, C or some other language?</a><a class="headerlink" href="#can-python-be-compiled-to-machine-code-c-or-some-other-language" title="Link to this heading">¶</a></h2>
<p><a class="reference external" href="https://cython.org/">Cython</a> compiles a modified version of Python with
optional annotations into C extensions.  <a class="reference external" href="https://www.nuitka.net/">Nuitka</a> is
an up-and-coming compiler of Python into C++ code, aiming to support the full
Python language.</p>
</section>
<section id="how-does-python-manage-memory">
<h2><a class="toc-backref" href="#id16" role="doc-backlink">How does Python manage memory?</a><a class="headerlink" href="#how-does-python-manage-memory" title="Link to this heading">¶</a></h2>
<p>The details of Python memory management depend on the implementation.  The
standard implementation of Python, <a class="reference internal" href="../glossary.html#term-CPython"><span class="xref std std-term">CPython</span></a>, uses reference counting to
detect inaccessible objects, and another mechanism to collect reference cycles,
periodically executing a cycle detection algorithm which looks for inaccessible
cycles and deletes the objects involved. The <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module provides functions
to perform a garbage collection, obtain debugging statistics, and tune the
collector’s parameters.</p>
<p>Other implementations (such as <a class="reference external" href="https://www.jython.org">Jython</a> or
<a class="reference external" href="https://www.pypy.org">PyPy</a>), however, can rely on a different mechanism
such as a full-blown garbage collector.  This difference can cause some
subtle porting problems if your Python code depends on the behavior of the
reference counting implementation.</p>
<p>In some Python implementations, the following code (which is fine in CPython)
will probably run out of file descriptors:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">file</span> <span class="ow">in</span> <span class="n">very_long_list_of_files</span><span class="p">:</span>
    <span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">file</span><span class="p">)</span>
    <span class="n">c</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
<p>Indeed, using CPython’s reference counting and destructor scheme, each new
assignment to <code class="docutils literal notranslate"><span class="pre">f</span></code> closes the previous file.  With a traditional GC, however,
those file objects will only get collected (and closed) at varying and possibly
long intervals.</p>
<p>If you want to write code that will work with any Python implementation,
you should explicitly close the file or use the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement;
this will work regardless of memory management scheme:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">file</span> <span class="ow">in</span> <span class="n">very_long_list_of_files</span><span class="p">:</span>
    <span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">file</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
        <span class="n">c</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="why-doesn-t-cpython-use-a-more-traditional-garbage-collection-scheme">
<h2><a class="toc-backref" href="#id17" role="doc-backlink">Why doesn’t CPython use a more traditional garbage collection scheme?</a><a class="headerlink" href="#why-doesn-t-cpython-use-a-more-traditional-garbage-collection-scheme" title="Link to this heading">¶</a></h2>
<p>For one thing, this is not a C standard feature and hence it’s not portable.
(Yes, we know about the Boehm GC library.  It has bits of assembler code for
<em>most</em> common platforms, not for all of them, and although it is mostly
transparent, it isn’t completely transparent; patches are required to get
Python to work with it.)</p>
<p>Traditional GC also becomes a problem when Python is embedded into other
applications.  While in a standalone Python it’s fine to replace the standard
<code class="docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="docutils literal notranslate"><span class="pre">free()</span></code> with versions provided by the GC library, an application
embedding Python may want to have its <em>own</em> substitute for <code class="docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="docutils literal notranslate"><span class="pre">free()</span></code>,
and may not want Python’s.  Right now, CPython works with anything that
implements <code class="docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="docutils literal notranslate"><span class="pre">free()</span></code> properly.</p>
</section>
<section id="why-isn-t-all-memory-freed-when-cpython-exits">
<h2><a class="toc-backref" href="#id18" role="doc-backlink">Why isn’t all memory freed when CPython exits?</a><a class="headerlink" href="#why-isn-t-all-memory-freed-when-cpython-exits" title="Link to this heading">¶</a></h2>
<p>Objects referenced from the global namespaces of Python modules are not always
deallocated when Python exits.  This may happen if there are circular
references.  There are also certain bits of memory that are allocated by the C
library that are impossible to free (e.g. a tool like Purify will complain about
these).  Python is, however, aggressive about cleaning up memory on exit and
does try to destroy every single object.</p>
<p>If you want to force Python to delete certain things on deallocation use the
<a class="reference internal" href="../library/atexit.html#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">atexit</span></code></a> module to run a function that will force those deletions.</p>
</section>
<section id="why-are-there-separate-tuple-and-list-data-types">
<h2><a class="toc-backref" href="#id19" role="doc-backlink">Why are there separate tuple and list data types?</a><a class="headerlink" href="#why-are-there-separate-tuple-and-list-data-types" title="Link to this heading">¶</a></h2>
<p>Lists and tuples, while similar in many respects, are generally used in
fundamentally different ways.  Tuples can be thought of as being similar to
Pascal <code class="docutils literal notranslate"><span class="pre">records</span></code> or C <code class="docutils literal notranslate"><span class="pre">structs</span></code>; they’re small collections of related data which may
be of different types which are operated on as a group.  For example, a
Cartesian coordinate is appropriately represented as a tuple of two or three
numbers.</p>
<p>Lists, on the other hand, are more like arrays in other languages.  They tend to
hold a varying number of objects all of which have the same type and which are
operated on one-by-one.  For example, <a class="reference internal" href="../library/os.html#os.listdir" title="os.listdir"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.listdir('.')</span></code></a>
returns a list of
strings representing the files in the current directory.  Functions which
operate on this output would generally not break if you added another file or
two to the directory.</p>
<p>Tuples are immutable, meaning that once a tuple has been created, you can’t
replace any of its elements with a new value.  Lists are mutable, meaning that
you can always change a list’s elements.  Only immutable elements can be used as
dictionary keys, and hence only tuples and not lists can be used as keys.</p>
</section>
<section id="how-are-lists-implemented-in-cpython">
<h2><a class="toc-backref" href="#id20" role="doc-backlink">How are lists implemented in CPython?</a><a class="headerlink" href="#how-are-lists-implemented-in-cpython" title="Link to this heading">¶</a></h2>
<p>CPython’s lists are really variable-length arrays, not Lisp-style linked lists.
The implementation uses a contiguous array of references to other objects, and
keeps a pointer to this array and the array’s length in a list head structure.</p>
<p>This makes indexing a list <code class="docutils literal notranslate"><span class="pre">a[i]</span></code> an operation whose cost is independent of
the size of the list or the value of the index.</p>
<p>When items are appended or inserted, the array of references is resized.  Some
cleverness is applied to improve the performance of appending items repeatedly;
when the array must be grown, some extra space is allocated so the next few
times don’t require an actual resize.</p>
</section>
<section id="how-are-dictionaries-implemented-in-cpython">
<h2><a class="toc-backref" href="#id21" role="doc-backlink">How are dictionaries implemented in CPython?</a><a class="headerlink" href="#how-are-dictionaries-implemented-in-cpython" title="Link to this heading">¶</a></h2>
<p>CPython’s dictionaries are implemented as resizable hash tables.  Compared to
B-trees, this gives better performance for lookup (the most common operation by
far) under most circumstances, and the implementation is simpler.</p>
<p>Dictionaries work by computing a hash code for each key stored in the dictionary
using the <a class="reference internal" href="../library/functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate"><span class="pre">hash()</span></code></a> built-in function.  The hash code varies widely depending
on the key and a per-process seed; for example, <code class="docutils literal notranslate"><span class="pre">'Python'</span></code> could hash to
<code class="docutils literal notranslate"><span class="pre">-539294296</span></code> while <code class="docutils literal notranslate"><span class="pre">'python'</span></code>, a string that differs by a single bit, could hash
to <code class="docutils literal notranslate"><span class="pre">1142331976</span></code>.  The hash code is then used to calculate a location in an
internal array where the value will be stored.  Assuming that you’re storing
keys that all have different hash values, this means that dictionaries take
constant time – <em>O</em>(1), in Big-O notation – to retrieve a key.</p>
</section>
<section id="why-must-dictionary-keys-be-immutable">
<h2><a class="toc-backref" href="#id22" role="doc-backlink">Why must dictionary keys be immutable?</a><a class="headerlink" href="#why-must-dictionary-keys-be-immutable" title="Link to this heading">¶</a></h2>
<p>The hash table implementation of dictionaries uses a hash value calculated from
the key value to find the key.  If the key were a mutable object, its value
could change, and thus its hash could also change.  But since whoever changes
the key object can’t tell that it was being used as a dictionary key, it can’t
move the entry around in the dictionary.  Then, when you try to look up the same
object in the dictionary it won’t be found because its hash value is different.
If you tried to look up the old value it wouldn’t be found either, because the
value of the object found in that hash bin would be different.</p>
<p>If you want a dictionary indexed with a list, simply convert the list to a tuple
first; the function <code class="docutils literal notranslate"><span class="pre">tuple(L)</span></code> creates a tuple with the same entries as the
list <code class="docutils literal notranslate"><span class="pre">L</span></code>.  Tuples are immutable and can therefore be used as dictionary keys.</p>
<p>Some unacceptable solutions that have been proposed:</p>
<ul>
<li><p>Hash lists by their address (object ID).  This doesn’t work because if you
construct a new list with the same value it won’t be found; e.g.:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">mydict</span> <span class="o">=</span> <span class="p">{[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">]:</span> <span class="s1">&#39;12&#39;</span><span class="p">}</span>
<span class="nb">print</span><span class="p">(</span><span class="n">mydict</span><span class="p">[[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">]])</span>
</pre></div>
</div>
<p>would raise a <a class="reference internal" href="../library/exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> exception because the id of the <code class="docutils literal notranslate"><span class="pre">[1,</span> <span class="pre">2]</span></code> used in the
second line differs from that in the first line.  In other words, dictionary
keys should be compared using <code class="docutils literal notranslate"><span class="pre">==</span></code>, not using <a class="reference internal" href="../reference/expressions.html#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a>.</p>
</li>
<li><p>Make a copy when using a list as a key.  This doesn’t work because the list,
being a mutable object, could contain a reference to itself, and then the
copying code would run into an infinite loop.</p></li>
<li><p>Allow lists as keys but tell the user not to modify them.  This would allow a
class of hard-to-track bugs in programs when you forgot or modified a list by
accident. It also invalidates an important invariant of dictionaries: every
value in <code class="docutils literal notranslate"><span class="pre">d.keys()</span></code> is usable as a key of the dictionary.</p></li>
<li><p>Mark lists as read-only once they are used as a dictionary key.  The problem
is that it’s not just the top-level object that could change its value; you
could use a tuple containing a list as a key.  Entering anything as a key into
a dictionary would require marking all objects reachable from there as
read-only – and again, self-referential objects could cause an infinite loop.</p></li>
</ul>
<p>There is a trick to get around this if you need to, but use it at your own risk:
You can wrap a mutable structure inside a class instance which has both a
<a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> and a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method.
You must then make sure that the
hash value for all such wrapper objects that reside in a dictionary (or other
hash based structure), remain fixed while the object is in the dictionary (or
other structure).</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ListWrapper</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">the_list</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">the_list</span> <span class="o">=</span> <span class="n">the_list</span>

    <span class="k">def</span> <span class="fm">__eq__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">the_list</span> <span class="o">==</span> <span class="n">other</span><span class="o">.</span><span class="n">the_list</span>

    <span class="k">def</span> <span class="fm">__hash__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="n">l</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">the_list</span>
        <span class="n">result</span> <span class="o">=</span> <span class="mi">98767</span> <span class="o">-</span> <span class="nb">len</span><span class="p">(</span><span class="n">l</span><span class="p">)</span><span class="o">*</span><span class="mi">555</span>
        <span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">el</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">l</span><span class="p">):</span>
            <span class="k">try</span><span class="p">:</span>
                <span class="n">result</span> <span class="o">=</span> <span class="n">result</span> <span class="o">+</span> <span class="p">(</span><span class="nb">hash</span><span class="p">(</span><span class="n">el</span><span class="p">)</span> <span class="o">%</span> <span class="mi">9999999</span><span class="p">)</span> <span class="o">*</span> <span class="mi">1001</span> <span class="o">+</span> <span class="n">i</span>
            <span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
                <span class="n">result</span> <span class="o">=</span> <span class="p">(</span><span class="n">result</span> <span class="o">%</span> <span class="mi">7777777</span><span class="p">)</span> <span class="o">+</span> <span class="n">i</span> <span class="o">*</span> <span class="mi">333</span>
        <span class="k">return</span> <span class="n">result</span>
</pre></div>
</div>
<p>Note that the hash computation is complicated by the possibility that some
members of the list may be unhashable and also by the possibility of arithmetic
overflow.</p>
<p>Furthermore it must always be the case that if <code class="docutils literal notranslate"><span class="pre">o1</span> <span class="pre">==</span> <span class="pre">o2</span></code> (ie <code class="docutils literal notranslate"><span class="pre">o1.__eq__(o2)</span>
<span class="pre">is</span> <span class="pre">True</span></code>) then <code class="docutils literal notranslate"><span class="pre">hash(o1)</span> <span class="pre">==</span> <span class="pre">hash(o2)</span></code> (ie, <code class="docutils literal notranslate"><span class="pre">o1.__hash__()</span> <span class="pre">==</span> <span class="pre">o2.__hash__()</span></code>),
regardless of whether the object is in a dictionary or not.  If you fail to meet
these restrictions dictionaries and other hash based structures will misbehave.</p>
<p>In the case of <code class="xref py py-class docutils literal notranslate"><span class="pre">ListWrapper</span></code>, whenever the wrapper object is in a dictionary the
wrapped list must not change to avoid anomalies.  Don’t do this unless you are
prepared to think hard about the requirements and the consequences of not
meeting them correctly.  Consider yourself warned.</p>
</section>
<section id="why-doesn-t-list-sort-return-the-sorted-list">
<h2><a class="toc-backref" href="#id23" role="doc-backlink">Why doesn’t list.sort() return the sorted list?</a><a class="headerlink" href="#why-doesn-t-list-sort-return-the-sorted-list" title="Link to this heading">¶</a></h2>
<p>In situations where performance matters, making a copy of the list just to sort
it would be wasteful. Therefore, <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code></a> sorts the list in place. In
order to remind you of that fact, it does not return the sorted list.  This way,
you won’t be fooled into accidentally overwriting a list when you need a sorted
copy but also need to keep the unsorted version around.</p>
<p>If you want to return a new list, use the built-in <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> function
instead.  This function creates a new list from a provided iterable, sorts
it and returns it.  For example, here’s how to iterate over the keys of a
dictionary in sorted order:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">mydict</span><span class="p">):</span>
    <span class="o">...</span>  <span class="c1"># do whatever with mydict[key]...</span>
</pre></div>
</div>
</section>
<section id="how-do-you-specify-and-enforce-an-interface-spec-in-python">
<h2><a class="toc-backref" href="#id24" role="doc-backlink">How do you specify and enforce an interface spec in Python?</a><a class="headerlink" href="#how-do-you-specify-and-enforce-an-interface-spec-in-python" title="Link to this heading">¶</a></h2>
<p>An interface specification for a module as provided by languages such as C++ and
Java describes the prototypes for the methods and functions of the module.  Many
feel that compile-time enforcement of interface specifications helps in the
construction of large programs.</p>
<p>Python 2.6 adds an <a class="reference internal" href="../library/abc.html#module-abc" title="abc: Abstract base classes according to :pep:`3119`."><code class="xref py py-mod docutils literal notranslate"><span class="pre">abc</span></code></a> module that lets you define Abstract Base Classes
(ABCs).  You can then use <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> and <a class="reference internal" href="../library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal notranslate"><span class="pre">issubclass()</span></code></a> to check
whether an instance or a class implements a particular ABC.  The
<a class="reference internal" href="../library/collections.abc.html#module-collections.abc" title="collections.abc: Abstract base classes for containers"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections.abc</span></code></a> module defines a set of useful ABCs such as
<a class="reference internal" href="../library/collections.abc.html#collections.abc.Iterable" title="collections.abc.Iterable"><code class="xref py py-class docutils literal notranslate"><span class="pre">Iterable</span></code></a>, <a class="reference internal" href="../library/collections.abc.html#collections.abc.Container" title="collections.abc.Container"><code class="xref py py-class docutils literal notranslate"><span class="pre">Container</span></code></a>, and
<a class="reference internal" href="../library/collections.abc.html#collections.abc.MutableMapping" title="collections.abc.MutableMapping"><code class="xref py py-class docutils literal notranslate"><span class="pre">MutableMapping</span></code></a>.</p>
<p>For Python, many of the advantages of interface specifications can be obtained
by an appropriate test discipline for components.</p>
<p>A good test suite for a module can both provide a regression test and serve as a
module interface specification and a set of examples.  Many Python modules can
be run as a script to provide a simple “self test.”  Even modules which use
complex external interfaces can often be tested in isolation using trivial
“stub” emulations of the external interface.  The <a class="reference internal" href="../library/doctest.html#module-doctest" title="doctest: Test pieces of code within docstrings."><code class="xref py py-mod docutils literal notranslate"><span class="pre">doctest</span></code></a> and
<a class="reference internal" href="../library/unittest.html#module-unittest" title="unittest: Unit testing framework for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">unittest</span></code></a> modules or third-party test frameworks can be used to construct
exhaustive test suites that exercise every line of code in a module.</p>
<p>An appropriate testing discipline can help build large complex applications in
Python as well as having interface specifications would.  In fact, it can be
better because an interface specification cannot test certain properties of a
program.  For example, the <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.append()</span></code> method is expected to add new elements
to the end of some internal list; an interface specification cannot test that
your <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.append()</span></code> implementation will actually do this correctly, but it’s
trivial to check this property in a test suite.</p>
<p>Writing test suites is very helpful, and you might want to design your code to
make it easily tested. One increasingly popular technique, test-driven
development, calls for writing parts of the test suite first, before you write
any of the actual code.  Of course Python allows you to be sloppy and not write
test cases at all.</p>
</section>
<section id="why-is-there-no-goto">
<h2><a class="toc-backref" href="#id25" role="doc-backlink">Why is there no goto?</a><a class="headerlink" href="#why-is-there-no-goto" title="Link to this heading">¶</a></h2>
<p>In the 1970s people realized that unrestricted goto could lead
to messy “spaghetti” code that was hard to understand and revise.
In a high-level language, it is also unneeded as long as there
are ways to branch (in Python, with <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statements and <a class="reference internal" href="../reference/expressions.html#or"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">or</span></code></a>,
<a class="reference internal" href="../reference/expressions.html#and"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">and</span></code></a>, and <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> expressions) and loop (with <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a>
and <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statements, possibly containing <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> and <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a>).</p>
<p>One can also use exceptions to provide a “structured goto”
that works even across
function calls.  Many feel that exceptions can conveniently emulate all
reasonable uses of the <code class="docutils literal notranslate"><span class="pre">go</span></code> or <code class="docutils literal notranslate"><span class="pre">goto</span></code> constructs of C, Fortran, and other
languages.  For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">label</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span> <span class="k">pass</span>  <span class="c1"># declare a label</span>

<span class="k">try</span><span class="p">:</span>
    <span class="o">...</span>
    <span class="k">if</span> <span class="n">condition</span><span class="p">:</span> <span class="k">raise</span> <span class="n">label</span><span class="p">()</span>  <span class="c1"># goto label</span>
    <span class="o">...</span>
<span class="k">except</span> <span class="n">label</span><span class="p">:</span>  <span class="c1"># where to goto</span>
    <span class="k">pass</span>
<span class="o">...</span>
</pre></div>
</div>
<p>This doesn’t allow you to jump into the middle of a loop, but that’s usually
considered an abuse of <code class="docutils literal notranslate"><span class="pre">goto</span></code> anyway.  Use sparingly.</p>
</section>
<section id="why-can-t-raw-strings-r-strings-end-with-a-backslash">
<h2><a class="toc-backref" href="#id26" role="doc-backlink">Why can’t raw strings (r-strings) end with a backslash?</a><a class="headerlink" href="#why-can-t-raw-strings-r-strings-end-with-a-backslash" title="Link to this heading">¶</a></h2>
<p>More precisely, they can’t end with an odd number of backslashes: the unpaired
backslash at the end escapes the closing quote character, leaving an
unterminated string.</p>
<p>Raw strings were designed to ease creating input for processors (chiefly regular
expression engines) that want to do their own backslash escape processing. Such
processors consider an unmatched trailing backslash to be an error anyway, so
raw strings disallow that.  In return, they allow you to pass on the string
quote character by escaping it with a backslash.  These rules work well when
r-strings are used for their intended purpose.</p>
<p>If you’re trying to build Windows pathnames, note that all Windows system calls
accept forward slashes too:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s2">&quot;/mydir/file.txt&quot;</span><span class="p">)</span>  <span class="c1"># works fine!</span>
</pre></div>
</div>
<p>If you’re trying to build a pathname for a DOS command, try e.g. one of</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">dir</span> <span class="o">=</span> <span class="sa">r</span><span class="s2">&quot;\this\is\my\dos\dir&quot;</span> <span class="s2">&quot;</span><span class="se">\\</span><span class="s2">&quot;</span>
<span class="nb">dir</span> <span class="o">=</span> <span class="sa">r</span><span class="s2">&quot;\this\is\my\dos\dir\ &quot;</span><span class="p">[:</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="nb">dir</span> <span class="o">=</span> <span class="s2">&quot;</span><span class="se">\\</span><span class="s2">this</span><span class="se">\\</span><span class="s2">is</span><span class="se">\\</span><span class="s2">my</span><span class="se">\\</span><span class="s2">dos</span><span class="se">\\</span><span class="s2">dir</span><span class="se">\\</span><span class="s2">&quot;</span>
</pre></div>
</div>
</section>
<section id="why-doesn-t-python-have-a-with-statement-for-attribute-assignments">
<h2><a class="toc-backref" href="#id27" role="doc-backlink">Why doesn’t Python have a “with” statement for attribute assignments?</a><a class="headerlink" href="#why-doesn-t-python-have-a-with-statement-for-attribute-assignments" title="Link to this heading">¶</a></h2>
<p>Python has a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement that wraps the execution of a block, calling code
on the entrance and exit from the block.  Some languages have a construct that
looks like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">obj</span><span class="p">:</span>
    <span class="n">a</span> <span class="o">=</span> <span class="mi">1</span>               <span class="c1"># equivalent to obj.a = 1</span>
    <span class="n">total</span> <span class="o">=</span> <span class="n">total</span> <span class="o">+</span> <span class="mi">1</span>   <span class="c1"># obj.total = obj.total + 1</span>
</pre></div>
</div>
<p>In Python, such a construct would be ambiguous.</p>
<p>Other languages, such as Object Pascal, Delphi, and C++, use static types, so
it’s possible to know, in an unambiguous way, what member is being assigned
to. This is the main point of static typing – the compiler <em>always</em> knows the
scope of every variable at compile time.</p>
<p>Python uses dynamic types. It is impossible to know in advance which attribute
will be referenced at runtime. Member attributes may be added or removed from
objects on the fly. This makes it impossible to know, from a simple reading,
what attribute is being referenced: a local one, a global one, or a member
attribute?</p>
<p>For instance, take the following incomplete snippet:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">):</span>
    <span class="k">with</span> <span class="n">a</span><span class="p">:</span>
        <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
<p>The snippet assumes that <code class="docutils literal notranslate"><span class="pre">a</span></code> must have a member attribute called <code class="docutils literal notranslate"><span class="pre">x</span></code>. However,
there is nothing in Python that tells the interpreter this. What should happen
if <code class="docutils literal notranslate"><span class="pre">a</span></code> is, let us say, an integer?  If there is a global variable named <code class="docutils literal notranslate"><span class="pre">x</span></code>,
will it be used inside the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> block?  As you see, the dynamic nature of Python
makes such choices much harder.</p>
<p>The primary benefit of <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> and similar language features (reduction of code
volume) can, however, easily be achieved in Python by assignment.  Instead of:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">function</span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="o">.</span><span class="n">mydict</span><span class="p">[</span><span class="n">index</span><span class="p">][</span><span class="n">index</span><span class="p">]</span><span class="o">.</span><span class="n">a</span> <span class="o">=</span> <span class="mi">21</span>
<span class="n">function</span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="o">.</span><span class="n">mydict</span><span class="p">[</span><span class="n">index</span><span class="p">][</span><span class="n">index</span><span class="p">]</span><span class="o">.</span><span class="n">b</span> <span class="o">=</span> <span class="mi">42</span>
<span class="n">function</span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="o">.</span><span class="n">mydict</span><span class="p">[</span><span class="n">index</span><span class="p">][</span><span class="n">index</span><span class="p">]</span><span class="o">.</span><span class="n">c</span> <span class="o">=</span> <span class="mi">63</span>
</pre></div>
</div>
<p>write this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">ref</span> <span class="o">=</span> <span class="n">function</span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="o">.</span><span class="n">mydict</span><span class="p">[</span><span class="n">index</span><span class="p">][</span><span class="n">index</span><span class="p">]</span>
<span class="n">ref</span><span class="o">.</span><span class="n">a</span> <span class="o">=</span> <span class="mi">21</span>
<span class="n">ref</span><span class="o">.</span><span class="n">b</span> <span class="o">=</span> <span class="mi">42</span>
<span class="n">ref</span><span class="o">.</span><span class="n">c</span> <span class="o">=</span> <span class="mi">63</span>
</pre></div>
</div>
<p>This also has the side-effect of increasing execution speed because name
bindings are resolved at run-time in Python, and the second version only needs
to perform the resolution once.</p>
<p>Similar proposals that would introduce syntax to further reduce code volume,
such as using a ‘leading dot’, have been rejected in favour of explicitness (see
<a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2016-May/040070.html">https://mail.python.org/pipermail/python-ideas/2016-May/040070.html</a>).</p>
</section>
<section id="why-don-t-generators-support-the-with-statement">
<h2><a class="toc-backref" href="#id28" role="doc-backlink">Why don’t generators support the with statement?</a><a class="headerlink" href="#why-don-t-generators-support-the-with-statement" title="Link to this heading">¶</a></h2>
<p>For technical reasons, a generator used directly as a context manager
would not work correctly.  When, as is most common, a generator is used as
an iterator run to completion, no closing is needed.  When it is, wrap
it as <a class="reference internal" href="../library/contextlib.html#contextlib.closing" title="contextlib.closing"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextlib.closing(generator)</span></code></a>
in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p>
</section>
<section id="why-are-colons-required-for-the-if-while-def-class-statements">
<h2><a class="toc-backref" href="#id29" role="doc-backlink">Why are colons required for the if/while/def/class statements?</a><a class="headerlink" href="#why-are-colons-required-for-the-if-while-def-class-statements" title="Link to this heading">¶</a></h2>
<p>The colon is required primarily to enhance readability (one of the results of
the experimental ABC language).  Consider this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">a</span> <span class="o">==</span> <span class="n">b</span>
    <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
</pre></div>
</div>
<p>versus</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">a</span> <span class="o">==</span> <span class="n">b</span><span class="p">:</span>
    <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
</pre></div>
</div>
<p>Notice how the second one is slightly easier to read.  Notice further how a
colon sets off the example in this FAQ answer; it’s a standard usage in English.</p>
<p>Another minor reason is that the colon makes it easier for editors with syntax
highlighting; they can look for colons to decide when indentation needs to be
increased instead of having to do a more elaborate parsing of the program text.</p>
</section>
<section id="why-does-python-allow-commas-at-the-end-of-lists-and-tuples">
<h2><a class="toc-backref" href="#id30" role="doc-backlink">Why does Python allow commas at the end of lists and tuples?</a><a class="headerlink" href="#why-does-python-allow-commas-at-the-end-of-lists-and-tuples" title="Link to this heading">¶</a></h2>
<p>Python lets you add a trailing comma at the end of lists, tuples, and
dictionaries:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,]</span>
<span class="p">(</span><span class="s1">&#39;a&#39;</span><span class="p">,</span> <span class="s1">&#39;b&#39;</span><span class="p">,</span> <span class="s1">&#39;c&#39;</span><span class="p">,)</span>
<span class="n">d</span> <span class="o">=</span> <span class="p">{</span>
    <span class="s2">&quot;A&quot;</span><span class="p">:</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">5</span><span class="p">],</span>
    <span class="s2">&quot;B&quot;</span><span class="p">:</span> <span class="p">[</span><span class="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">],</span>  <span class="c1"># last trailing comma is optional but good style</span>
<span class="p">}</span>
</pre></div>
</div>
<p>There are several reasons to allow this.</p>
<p>When you have a literal value for a list, tuple, or dictionary spread across
multiple lines, it’s easier to add more elements because you don’t have to
remember to add a comma to the previous line.  The lines can also be reordered
without creating a syntax error.</p>
<p>Accidentally omitting the comma can lead to errors that are hard to diagnose.
For example:</p>
<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="s2">&quot;fee&quot;</span><span class="p">,</span>
  <span class="s2">&quot;fie&quot;</span>
  <span class="s2">&quot;foo&quot;</span><span class="p">,</span>
  <span class="s2">&quot;fum&quot;</span>
<span class="p">]</span>
</pre></div>
</div>
<p>This list looks like it has four elements, but it actually contains three:
“fee”, “fiefoo” and “fum”.  Always adding the comma avoids this source of error.</p>
<p>Allowing the trailing comma may also make programmatic code generation easier.</p>
</section>
</section>


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    <ul>
<li><a class="reference internal" href="#">Design and History FAQ</a><ul>
<li><a class="reference internal" href="#why-does-python-use-indentation-for-grouping-of-statements">Why does Python use indentation for grouping of statements?</a></li>
<li><a class="reference internal" href="#why-am-i-getting-strange-results-with-simple-arithmetic-operations">Why am I getting strange results with simple arithmetic operations?</a></li>
<li><a class="reference internal" href="#why-are-floating-point-calculations-so-inaccurate">Why are floating-point calculations so inaccurate?</a></li>
<li><a class="reference internal" href="#why-are-python-strings-immutable">Why are Python strings immutable?</a></li>
<li><a class="reference internal" href="#why-must-self-be-used-explicitly-in-method-definitions-and-calls">Why must ‘self’ be used explicitly in method definitions and calls?</a></li>
<li><a class="reference internal" href="#why-can-t-i-use-an-assignment-in-an-expression">Why can’t I use an assignment in an expression?</a></li>
<li><a class="reference internal" href="#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list">Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?</a></li>
<li><a class="reference internal" href="#why-is-join-a-string-method-instead-of-a-list-or-tuple-method">Why is join() a string method instead of a list or tuple method?</a></li>
<li><a class="reference internal" href="#how-fast-are-exceptions">How fast are exceptions?</a></li>
<li><a class="reference internal" href="#why-isn-t-there-a-switch-or-case-statement-in-python">Why isn’t there a switch or case statement in Python?</a></li>
<li><a class="reference internal" href="#can-t-you-emulate-threads-in-the-interpreter-instead-of-relying-on-an-os-specific-thread-implementation">Can’t you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?</a></li>
<li><a class="reference internal" href="#why-can-t-lambda-expressions-contain-statements">Why can’t lambda expressions contain statements?</a></li>
<li><a class="reference internal" href="#can-python-be-compiled-to-machine-code-c-or-some-other-language">Can Python be compiled to machine code, C or some other language?</a></li>
<li><a class="reference internal" href="#how-does-python-manage-memory">How does Python manage memory?</a></li>
<li><a class="reference internal" href="#why-doesn-t-cpython-use-a-more-traditional-garbage-collection-scheme">Why doesn’t CPython use a more traditional garbage collection scheme?</a></li>
<li><a class="reference internal" href="#why-isn-t-all-memory-freed-when-cpython-exits">Why isn’t all memory freed when CPython exits?</a></li>
<li><a class="reference internal" href="#why-are-there-separate-tuple-and-list-data-types">Why are there separate tuple and list data types?</a></li>
<li><a class="reference internal" href="#how-are-lists-implemented-in-cpython">How are lists implemented in CPython?</a></li>
<li><a class="reference internal" href="#how-are-dictionaries-implemented-in-cpython">How are dictionaries implemented in CPython?</a></li>
<li><a class="reference internal" href="#why-must-dictionary-keys-be-immutable">Why must dictionary keys be immutable?</a></li>
<li><a class="reference internal" href="#why-doesn-t-list-sort-return-the-sorted-list">Why doesn’t list.sort() return the sorted list?</a></li>
<li><a class="reference internal" href="#how-do-you-specify-and-enforce-an-interface-spec-in-python">How do you specify and enforce an interface spec in Python?</a></li>
<li><a class="reference internal" href="#why-is-there-no-goto">Why is there no goto?</a></li>
<li><a class="reference internal" href="#why-can-t-raw-strings-r-strings-end-with-a-backslash">Why can’t raw strings (r-strings) end with a backslash?</a></li>
<li><a class="reference internal" href="#why-doesn-t-python-have-a-with-statement-for-attribute-assignments">Why doesn’t Python have a “with” statement for attribute assignments?</a></li>
<li><a class="reference internal" href="#why-don-t-generators-support-the-with-statement">Why don’t generators support the with statement?</a></li>
<li><a class="reference internal" href="#why-are-colons-required-for-the-if-while-def-class-statements">Why are colons required for the if/while/def/class statements?</a></li>
<li><a class="reference internal" href="#why-does-python-allow-commas-at-the-end-of-lists-and-tuples">Why does Python allow commas at the end of lists and tuples?</a></li>
</ul>
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