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 <section id="compound-statements">
<h1>7. Compound statements<a class="headerlink" href="#compound-statements" title="Permalink to this headline">¶</a></h1>
Compound statements contain (groups of) other statements; they affect or control
the execution of those other statements in some way. In general, compound
statements span multiple lines, although in simple incarnations a whole compound
statement may be contained in one line.
The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a>, <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> and <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> statements implement
traditional control flow constructs. <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> specifies exception
handlers and/or cleanup code for a group of statements. Function and class
definitions are also syntactically compound statements.
Compound statements consist of one or more ‘clauses.’ A clause consists of a
header and a ‘suite.’ The clause headers of a particular compound statement are
all at the same indentation level. Each clause header begins with a uniquely
identifying keyword and ends with a colon. A suite is a group of statements
controlled by a clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header’s colon, or it
can be one or more indented statements on subsequent lines. Only the latter
form of suite can contain nested compound statements; the following is illegal,
mostly because it wouldn’t be clear to which <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> clause a following
<a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause would belong:
<div class="highlight-default notranslate"><div class="highlight"><pre>if test1: if test2: print x
</pre></div>
</div>
Also note that the semicolon binds tighter than the colon in this context, so
that in the following example, either all or none of the <a class="reference internal" href="simple_stmts.html#print"><code class="xref std std-keyword docutils literal notranslate">print</code></a>
statements are executed:
<div class="highlight-default notranslate"><div class="highlight"><pre>if x &lt; y &lt; z: print x; print y; print z
</pre></div>
</div>
Summarizing:
<pre>
compound_stmt ::= <a class="reference internal" href="#grammar-token-if_stmt"><code class="xref docutils literal notranslate">if_stmt</code></a>
 | <a class="reference internal" href="#grammar-token-while_stmt"><code class="xref docutils literal notranslate">while_stmt</code></a>
 | <a class="reference internal" href="#grammar-token-for_stmt"><code class="xref docutils literal notranslate">for_stmt</code></a>
 | <a class="reference internal" href="#grammar-token-try_stmt"><code class="xref docutils literal notranslate">try_stmt</code></a>
 | <a class="reference internal" href="#grammar-token-with_stmt"><code class="xref docutils literal notranslate">with_stmt</code></a>
 | <a class="reference internal" href="#grammar-token-funcdef"><code class="xref docutils literal notranslate">funcdef</code></a>
 | <a class="reference internal" href="#grammar-token-classdef"><code class="xref docutils literal notranslate">classdef</code></a>
 | <a class="reference internal" href="#grammar-token-decorated"><code class="xref docutils literal notranslate">decorated</code></a>
suite ::= <a class="reference internal" href="#grammar-token-stmt_list"><code class="xref docutils literal notranslate">stmt_list</code></a> NEWLINE | NEWLINE INDENT <a class="reference internal" href="#grammar-token-statement"><code class="xref docutils literal notranslate">statement</code></a>+ DEDENT
statement ::= <a class="reference internal" href="#grammar-token-stmt_list"><code class="xref docutils literal notranslate">stmt_list</code></a> NEWLINE | <a class="reference internal" href="#grammar-token-compound_stmt"><code class="xref docutils literal notranslate">compound_stmt</code></a>
stmt_list ::= <a class="reference internal" href="simple_stmts.html#grammar-token-simple_stmt"><code class="xref docutils literal notranslate">simple_stmt</code></a> (&quot;;&quot; <a class="reference internal" href="simple_stmts.html#grammar-token-simple_stmt"><code class="xref docutils literal notranslate">simple_stmt</code></a>)* [&quot;;&quot;]
</pre>
Note that statements always end in a <code class="docutils literal notranslate">NEWLINE</code> possibly followed by a
<code class="docutils literal notranslate">DEDENT</code>. Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities (the
‘dangling <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a>’ problem is solved in Python by requiring nested
<a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> statements to be indented).
The formatting of the grammar rules in the following sections places each clause
on a separate line for clarity.
<section id="the-if-statement">
<h2>7.1. The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> statement<a class="headerlink" href="#the-if-statement" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> statement is used for conditional execution:
<pre>
if_stmt ::= &quot;if&quot; <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
 ( &quot;elif&quot; <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a> )*
 [&quot;else&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>]
</pre>
It selects exactly one of the suites by evaluating the expressions one by one
until one is found to be true (see section <a class="reference internal" href="expressions.html#booleans">Boolean operations</a> for the definition of
true and false); then that suite is executed (and no other part of the
<a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> statement is executed or evaluated). If all expressions are
false, the suite of the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause, if present, is executed.
</section>
<section id="the-while-statement">
<h2>7.2. The <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> statement<a class="headerlink" href="#the-while-statement" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> statement is used for repeated execution as long as an
expression is true:
<pre>
while_stmt ::= &quot;while&quot; <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
 [&quot;else&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>]
</pre>
This repeatedly tests the expression and, if it is true, executes the first
suite; if the expression is false (which may be the first time it is tested) the
suite of the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause, if present, is executed and the loop
terminates.
A <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement executed in the first suite terminates the loop
without executing the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause’s suite. A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a>
statement executed in the first suite skips the rest of the suite and goes back
to testing the expression.
</section>
<section id="the-for-statement">
<h2>7.3. The <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> statement<a class="headerlink" href="#the-for-statement" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> statement is used to iterate over the elements of a sequence
(such as a string, tuple or list) or other iterable object:
<pre>
for_stmt ::= &quot;for&quot; <a class="reference internal" href="simple_stmts.html#grammar-token-target_list"><code class="xref docutils literal notranslate">target_list</code></a> &quot;in&quot; <a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate">expression_list</code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
 [&quot;else&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>]
</pre>
The expression list is evaluated once; it should yield an iterable object. An
iterator is created for the result of the <code class="docutils literal notranslate">expression_list</code>. The suite is
then executed once for each item provided by the iterator, in the order of
ascending indices. Each item in turn is assigned to the target list using the
standard rules for assignments, and then the suite is executed. When the items
are exhausted (which is immediately when the sequence is empty), the suite in
the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause, if present, is executed, and the loop terminates.
A <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement executed in the first suite terminates the loop
without executing the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause’s suite. A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a>
statement executed in the first suite skips the rest of the suite and continues
with the next item, or with the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause if there was no next
item.
The suite may assign to the variable(s) in the target list; this does not affect
the next item assigned to it.
The target list is not deleted when the loop is finished, but if the sequence is
empty, it will not have been assigned to at all by the loop. Hint: the built-in
function <a class="reference internal" href="../library/functions.html#range" title="range"><code class="xref py py-func docutils literal notranslate">range()</code></a> returns a sequence of integers suitable to emulate the
effect of Pascal’s <code class="docutils literal notranslate">for i := a to b do</code>; e.g., <code class="docutils literal notranslate">range(3)</code> returns the list
<code class="docutils literal notranslate">[0, 1, 2]</code>.
<div class="admonition note">
Note
There is a subtlety when the sequence is being modified by the loop (this can
only occur for mutable sequences, e.g. lists). An internal counter is used to
keep track of which item is used next, and this is incremented on each
iteration. When this counter has reached the length of the sequence the loop
terminates. This means that if the suite deletes the current (or a previous)
item from the sequence, the next item will be skipped (since it gets the index
of the current item which has already been treated). Likewise, if the suite
inserts an item in the sequence before the current item, the current item will
be treated again the next time through the loop. This can lead to nasty bugs
that can be avoided by making a temporary copy using a slice of the whole
sequence, e.g.,
<div class="highlight-default notranslate"><div class="highlight"><pre>for x in a[:]:
 if x &lt; 0: a.remove(x)
</pre></div>
</div>
</div>
</section>
<section id="the-try-statement">
<h2>7.4. The <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement<a class="headerlink" href="#the-try-statement" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement specifies exception handlers and/or cleanup code
for a group of statements:
<pre>
try_stmt ::= try1_stmt | try2_stmt
try1_stmt ::= &quot;try&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
 (&quot;except&quot; [<a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> [(&quot;as&quot; | &quot;,&quot;) <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>]] &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>)+
 [&quot;else&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>]
 [&quot;finally&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>]
try2_stmt ::= &quot;try&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
 &quot;finally&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
</pre>
<div class="versionchanged">
Changed in version 2.5: In previous versions of Python, <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> did not work. <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> had to be
nested in <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a>.
</div>
The <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause(s) specify one or more exception handlers. When no
exception occurs in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause, no exception handler is executed.
When an exception occurs in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> suite, a search for an exception
handler is started. This search inspects the except clauses in turn until one
is found that matches the exception. An expression-less except clause, if
present, must be last; it matches any exception. For an except clause with an
expression, that expression is evaluated, and the clause matches the exception
if the resulting object is “compatible” with the exception. An object is
compatible with an exception if it is the class or a base class of the exception
object, or a tuple containing an item compatible with the exception.
If no except clause matches the exception, the search for an exception handler
continues in the surrounding code and on the invocation stack. <a class="footnote-reference brackets" href="#id4" id="id1">1</a>
If the evaluation of an expression in the header of an except clause raises an
exception, the original search for a handler is canceled and a search starts for
the new exception in the surrounding code and on the call stack (it is treated
as if the entire <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement raised the exception).
When a matching except clause is found, the exception is assigned to the target
specified in that except clause, if present, and the except clause’s suite is
executed. All except clauses must have an executable block. When the end of
this block is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner handler, the
outer handler will not handle the exception.)
Before an except clause’s suite is executed, details about the exception are
assigned to three variables in the <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate">sys</code></a> module: <code class="docutils literal notranslate">sys.exc_type</code> receives
the object identifying the exception; <code class="docutils literal notranslate">sys.exc_value</code> receives the exception’s
parameter; <code class="docutils literal notranslate">sys.exc_traceback</code> receives a traceback object (see section
<a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>) identifying the point in the program where the exception
occurred. These details are also available through the <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate">sys.exc_info()</code></a>
function, which returns a tuple <code class="docutils literal notranslate">(exc_type, exc_value, exc_traceback)</code>. Use
of the corresponding variables is deprecated in favor of this function, since
their use is unsafe in a threaded program. As of Python 1.5, the variables are
restored to their previous values (before the call) when returning from a
function that handled an exception.
The optional <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause is executed if the control flow leaves the
<a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> suite, no exception was raised, and no <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>,
<a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a>, or <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement was executed. Exceptions in
the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause are not handled by the preceding <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>
clauses.
If <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> is present, it specifies a ‘cleanup’ handler. The
<a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause is executed, including any <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> and
<a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clauses. If an exception occurs in any of the clauses and is
not handled, the exception is temporarily saved. The <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause
is executed. If there is a saved exception, it is re-raised at the end of the
<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause. If the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause raises another
exception or executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> or <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement, the
saved exception is discarded:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def f():
... try:
... 1/0
... finally:
... return 42
...
&gt;&gt;&gt; f()
42
</pre></div>
</div>
The exception information is not available to the program during execution of
the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause.
When a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>, <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> or <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement is
executed in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> suite of a <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a>
statement, the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause is also executed ‘on the way out.’ A
<a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement is illegal in the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause. (The
reason is a problem with the current implementation — this restriction may be
lifted in the future).
The return value of a function is determined by the last <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>
statement executed. Since the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause always executes, a
<a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement executed in the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause will
always be the last one executed:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def foo():
... try:
... return &#39;try&#39;
... finally:
... return &#39;finally&#39;
...
&gt;&gt;&gt; foo()
&#39;finally&#39;
</pre></div>
</div>
Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions">Exceptions</a>,
and information on using the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement to generate exceptions
may be found in section <a class="reference internal" href="simple_stmts.html#raise">The raise statement</a>.
</section>
<section id="the-with-statement">
<h2>7.5. The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement<a class="headerlink" href="#the-with-statement" title="Permalink to this headline">¶</a></h2>
<div class="versionadded" id="index-15">
New in version 2.5.
</div>
The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement is used to wrap the execution of a block with
methods defined by a context manager (see section <a class="reference internal" href="datamodel.html#context-managers">With Statement Context Managers</a>). This
allows common <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> usage
patterns to be encapsulated for convenient reuse.
<pre>
with_stmt ::= &quot;with&quot; with_item (&quot;,&quot; with_item)* &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
with_item ::= <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> [&quot;as&quot; <a class="reference internal" href="simple_stmts.html#grammar-token-target"><code class="xref docutils literal notranslate">target</code></a>]
</pre>
The execution of the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement with one “item” proceeds as follows:
<ol class="arabic">
<li>The context expression (the expression given in the <a class="reference internal" href="#grammar-token-with_item"><code class="xref std std-token docutils literal notranslate">with_item</code></a>) is
evaluated to obtain a context manager.</li>
<li>The context manager’s <code class="xref py py-meth docutils literal notranslate">__exit__()</code> is loaded for later use.</li>
<li>The context manager’s <code class="xref py py-meth docutils literal notranslate">__enter__()</code> method is invoked.</li>
<li>If a target was included in the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement, the return value
from <code class="xref py py-meth docutils literal notranslate">__enter__()</code> is assigned to it.
<div class="admonition note">
Note
The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement guarantees that if the <code class="xref py py-meth docutils literal notranslate">__enter__()</code> method
returns without an error, then <code class="xref py py-meth docutils literal notranslate">__exit__()</code> will always be called. Thus, if
an error occurs during the assignment to the target list, it will be treated the
same as an error occurring within the suite would be. See step 6 below.
</div>
</li>
<li>The suite is executed.</li>
<li>The context manager’s <code class="xref py py-meth docutils literal notranslate">__exit__()</code> method is invoked. If an exception
caused the suite to be exited, its type, value, and traceback are passed as
arguments to <code class="xref py py-meth docutils literal notranslate">__exit__()</code>. Otherwise, three <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> arguments are
supplied.
If the suite was exited due to an exception, and the return value from the
<code class="xref py py-meth docutils literal notranslate">__exit__()</code> method was false, the exception is reraised. If the return
value was true, the exception is suppressed, and execution continues with the
statement following the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement.
If the suite was exited for any reason other than an exception, the return value
from <code class="xref py py-meth docutils literal notranslate">__exit__()</code> is ignored, and execution proceeds at the normal location
for the kind of exit that was taken.
</li>
</ol>
With more than one item, the context managers are processed as if multiple
<a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statements were nested:
<div class="highlight-default notranslate"><div class="highlight"><pre>with A() as a, B() as b:
 suite
</pre></div>
</div>
is equivalent to
<div class="highlight-default notranslate"><div class="highlight"><pre>with A() as a:
 with B() as b:
 suite
</pre></div>
</div>
<div class="admonition note">
Note
In Python 2.5, the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement is only allowed when the
<code class="docutils literal notranslate">with_statement</code> feature has been enabled. It is always enabled in
Python 2.6.
</div>
<div class="versionchanged">
Changed in version 2.7: Support for multiple context expressions.
</div>
<div class="admonition seealso">
See also
<dl class="simple">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343">PEP 343</a> - The “with” statement</dt><dd>The specification, background, and examples for the Python <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statement.
</dd>
</dl>
</div>
</section>
<section id="function-definitions">
<h2>7.6. Function definitions<a class="headerlink" href="#function-definitions" title="Permalink to this headline">¶</a></h2>
A function definition defines a user-defined function object (see section
<a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>):
<pre>
decorated ::= decorators (classdef | funcdef)
decorators ::= <a class="reference internal" href="#grammar-token-decorator"><code class="xref docutils literal notranslate">decorator</code></a>+
decorator ::= &quot;&#64;&quot; <a class="reference internal" href="#grammar-token-dotted_name"><code class="xref docutils literal notranslate">dotted_name</code></a> [&quot;(&quot; [<a class="reference internal" href="expressions.html#grammar-token-argument_list"><code class="xref docutils literal notranslate">argument_list</code></a> [&quot;,&quot;]] &quot;)&quot;] NEWLINE
funcdef ::= &quot;def&quot; <a class="reference internal" href="#grammar-token-funcname"><code class="xref docutils literal notranslate">funcname</code></a> &quot;(&quot; [<a class="reference internal" href="#grammar-token-parameter_list"><code class="xref docutils literal notranslate">parameter_list</code></a>] &quot;)&quot; &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
dotted_name ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a> (&quot;.&quot; <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>)*
parameter_list ::= (<a class="reference internal" href="#grammar-token-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a> &quot;,&quot;)*
 ( &quot;*&quot; <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a> [&quot;,&quot; &quot;**&quot; <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>]
 | &quot;**&quot; <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>
 | <a class="reference internal" href="#grammar-token-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a> [&quot;,&quot;] )
defparameter ::= <a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate">parameter</code></a> [&quot;=&quot; <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>]
sublist ::= <a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate">parameter</code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate">parameter</code></a>)* [&quot;,&quot;]
parameter ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a> | &quot;(&quot; <a class="reference internal" href="#grammar-token-sublist"><code class="xref docutils literal notranslate">sublist</code></a> &quot;)&quot;
funcname ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>
</pre>
A function definition is an executable statement. Its execution binds the
function name in the current local namespace to a function object (a wrapper
around the executable code for the function). This function object contains a
reference to the current global namespace as the global namespace to be used
when the function is called.
The function definition does not execute the function body; this gets executed
only when the function is called. <a class="footnote-reference brackets" href="#id5" id="id2">2</a>
A function definition may be wrapped by one or more <a class="reference internal" href="../glossary.html#term-decorator">decorator</a> expressions.
Decorator expressions are evaluated when the function is defined, in the scope
that contains the function definition. The result must be a callable, which is
invoked with the function object as the only argument. The returned value is
bound to the function name instead of the function object. Multiple decorators
are applied in nested fashion. For example, the following code:
<div class="highlight-default notranslate"><div class="highlight"><pre>@f1(arg)
@f2
def func(): pass
</pre></div>
</div>
is equivalent to:
<div class="highlight-default notranslate"><div class="highlight"><pre>def func(): pass
func = f1(arg)(f2(func))
</pre></div>
</div>
When one or more top-level <a class="reference internal" href="../glossary.html#term-parameter">parameters</a> have the form
parameter <code class="docutils literal notranslate">=</code> expression, the function is said to have “default parameter
values.” For a parameter with a default value, the corresponding
<a class="reference internal" href="../glossary.html#term-argument">argument</a> may be omitted from a call, in which
case the parameter’s default value is substituted. If a
parameter has a default value, all following parameters must also have a default
value — this is a syntactic restriction that is not expressed by the grammar.
Default parameter values are evaluated when the function definition is
executed. This means that the expression is evaluated once, when the function
is defined, and that the same “pre-computed” value is used for each call. This
is especially important to understand when a default parameter is a mutable
object, such as a list or a dictionary: if the function modifies the object
(e.g. by appending an item to a list), the default value is in effect modified.
This is generally not what was intended. A way around this is to use <code class="docutils literal notranslate">None</code>
as the default, and explicitly test for it in the body of the function, e.g.:
<div class="highlight-default notranslate"><div class="highlight"><pre>def whats_on_the_telly(penguin=None):
 if penguin is None:
 penguin = []
 penguin.append(&quot;property of the zoo&quot;)
 return penguin
</pre></div>
</div>
Function call semantics are described in more detail in section <a class="reference internal" href="expressions.html#calls">Calls</a>. A
function call always assigns values to all parameters mentioned in the parameter
list, either from position arguments, from keyword arguments, or from default
values. If the form “<code class="docutils literal notranslate">*identifier</code>” is present, it is initialized to a tuple
receiving any excess positional parameters, defaulting to the empty tuple. If
the form “<code class="docutils literal notranslate">**identifier</code>” is present, it is initialized to a new dictionary
receiving any excess keyword arguments, defaulting to a new empty dictionary.
It is also possible to create anonymous functions (functions not bound to a
name), for immediate use in expressions. This uses lambda expressions, described in
section <a class="reference internal" href="expressions.html#lambda">Lambdas</a>. Note that the lambda expression is merely a shorthand for a
simplified function definition; a function defined in a “<a class="reference internal" href="#def"><code class="xref std std-keyword docutils literal notranslate">def</code></a>”
statement can be passed around or assigned to another name just like a function
defined by a lambda expression. The “<a class="reference internal" href="#def"><code class="xref std std-keyword docutils literal notranslate">def</code></a>” form is actually more powerful
since it allows the execution of multiple statements.
Programmer’s note: Functions are first-class objects. A “<code class="docutils literal notranslate">def</code>” form
executed inside a function definition defines a local function that can be
returned or passed around. Free variables used in the nested function can
access the local variables of the function containing the def. See section
<a class="reference internal" href="executionmodel.html#naming">Naming and binding</a> for details.
</section>
<section id="class-definitions">
<h2>7.7. Class definitions<a class="headerlink" href="#class-definitions" title="Permalink to this headline">¶</a></h2>
A class definition defines a class object (see section <a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>):
<pre>
classdef ::= &quot;class&quot; <a class="reference internal" href="#grammar-token-classname"><code class="xref docutils literal notranslate">classname</code></a> [<a class="reference internal" href="#grammar-token-inheritance"><code class="xref docutils literal notranslate">inheritance</code></a>] &quot;:&quot; <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate">suite</code></a>
inheritance ::= &quot;(&quot; [<a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate">expression_list</code></a>] &quot;)&quot;
classname ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>
</pre>
A class definition is an executable statement. It first evaluates the
inheritance list, if present. Each item in the inheritance list should evaluate
to a class object or class type which allows subclassing. The class’s suite is
then executed in a new execution frame (see section <a class="reference internal" href="executionmodel.html#naming">Naming and binding</a>), using a
newly created local namespace and the original global namespace. (Usually, the
suite contains only function definitions.) When the class’s suite finishes
execution, its execution frame is discarded but its local namespace is
saved. <a class="footnote-reference brackets" href="#id6" id="id3">3</a> A class object is then created using the inheritance list for the
base classes and the saved local namespace for the attribute dictionary. The
class name is bound to this class object in the original local namespace.
Programmer’s note: Variables defined in the class definition are class
variables; they are shared by all instances. To create instance variables, they
can be set in a method with <code class="docutils literal notranslate">self.name = value</code>. Both class and instance
variables are accessible through the notation “<code class="docutils literal notranslate">self.name</code>”, and an instance
variable hides a class variable with the same name when accessed in this way.
Class variables can be used as defaults for instance variables, but using
mutable values there can lead to unexpected results. For <a class="reference internal" href="../glossary.html#term-new-style-class">new-style
class</a>es, descriptors can be used to create instance variables with different
implementation details.
Class definitions, like function definitions, may be wrapped by one or more
<a class="reference internal" href="../glossary.html#term-decorator">decorator</a> expressions. The evaluation rules for the decorator
expressions are the same as for functions. The result must be a class object,
which is then bound to the class name.
Footnotes
<dl class="footnote brackets">
<dt class="label" id="id4"><a class="fn-backref" href="#id1">1</a></dt>
<dd>The exception is propagated to the invocation stack unless
there is a <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause which happens to raise another
exception. That new exception causes the old one to be lost.
</dd>
<dt class="label" id="id5"><a class="fn-backref" href="#id2">2</a></dt>
<dd>A string literal appearing as the first statement in the function body is
transformed into the function’s <code class="docutils literal notranslate">__doc__</code> attribute and therefore the
function’s <a class="reference internal" href="../glossary.html#term-docstring">docstring</a>.
</dd>
<dt class="label" id="id6"><a class="fn-backref" href="#id3">3</a></dt>
<dd>A string literal appearing as the first statement in the class body is
transformed into the namespace’s <code class="docutils literal notranslate">__doc__</code> item and therefore the class’s
<a class="reference internal" href="../glossary.html#term-docstring">docstring</a>.
</dd>
</dl>
</section>
</section>

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 <ul>
<li><a class="reference internal" href="#">7. Compound statements</a><ul>
<li><a class="reference internal" href="#the-if-statement">7.1. The <code class="xref std std-keyword docutils literal notranslate">if</code> statement</a></li>
<li><a class="reference internal" href="#the-while-statement">7.2. The <code class="xref std std-keyword docutils literal notranslate">while</code> statement</a></li>
<li><a class="reference internal" href="#the-for-statement">7.3. The <code class="xref std std-keyword docutils literal notranslate">for</code> statement</a></li>
<li><a class="reference internal" href="#the-try-statement">7.4. The <code class="xref std std-keyword docutils literal notranslate">try</code> statement</a></li>
<li><a class="reference internal" href="#the-with-statement">7.5. The <code class="xref std std-keyword docutils literal notranslate">with</code> statement</a></li>
<li><a class="reference internal" href="#function-definitions">7.6. Function definitions</a></li>
<li><a class="reference internal" href="#class-definitions">7.7. Class definitions</a></li>
</ul>
</li>
</ul>

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