Edit File - File Manager

Edit File: simple_stmts.html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 
 <title>6. Simple statements &mdash; Python v2.6.6 documentation</title>
 <link rel="stylesheet" href="../_static/default.css" type="text/css" />
 <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
 <script type="text/javascript">
 var DOCUMENTATION_OPTIONS = {
 URL_ROOT: '../',
 VERSION: '2.6.6',
 COLLAPSE_MODINDEX: false,
 FILE_SUFFIX: '.html',
 HAS_SOURCE: true
 };
 </script>
 <script type="text/javascript" src="../_static/jquery.js"></script>
 <script type="text/javascript" src="../_static/doctools.js"></script>
 <link rel="search" type="application/opensearchdescription+xml"
 title="Search within Python v2.6.6 documentation"
 href="../_static/opensearch.xml"/>
 <link rel="author" title="About these documents" href="../about.html" />
 <link rel="copyright" title="Copyright" href="../copyright.html" />
 <link rel="top" title="Python v2.6.6 documentation" href="../index.html" />
 <link rel="up" title="The Python Language Reference" href="index.html" />
 <link rel="next" title="7. Compound statements" href="compound_stmts.html" />
 <link rel="prev" title="5. Expressions" href="expressions.html" />
 <link rel="shortcut icon" type="image/png" href="../_static/py.png" />

</head>
 <body>
 <div class="related">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 accesskey="I">index</a></li>
 <li class="right" >
 <a href="../modindex.html" title="Global Module Index"
 accesskey="M">modules</a> |</li>
 <li class="right" >
 <a href="compound_stmts.html" title="7. Compound statements"
 accesskey="N">next</a> |</li>
 <li class="right" >
 <a href="expressions.html" title="5. Expressions"
 accesskey="P">previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="../index.html">Python v2.6.6 documentation</a> &raquo;</li>

<li><a href="index.html" accesskey="U">The Python Language Reference</a> &raquo;</li> 
 </ul>
 </div>

<div class="document">
 <div class="documentwrapper">
 <div class="bodywrapper">
 <div class="body">
 
 <div class="section" id="simple-statements">
<h1>6. Simple statements<a class="headerlink" href="#simple-statements" title="Permalink to this headline">¶</a></h1>
Simple statements are comprised within a single logical line. Several simple
statements may occur on a single line separated by semicolons. The syntax for
simple statements is:
<pre>
simple_stmt ::= <a class="reference internal" href="#grammar-token-expression_stmt"><tt class="xref docutils literal">expression_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-assert_stmt"><tt class="xref docutils literal">assert_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-assignment_stmt"><tt class="xref docutils literal">assignment_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-augmented_assignment_stmt"><tt class="xref docutils literal">augmented_assignment_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-pass_stmt"><tt class="xref docutils literal">pass_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-del_stmt"><tt class="xref docutils literal">del_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-print_stmt"><tt class="xref docutils literal">print_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-return_stmt"><tt class="xref docutils literal">return_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-yield_stmt"><tt class="xref docutils literal">yield_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-raise_stmt"><tt class="xref docutils literal">raise_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-break_stmt"><tt class="xref docutils literal">break_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-continue_stmt"><tt class="xref docutils literal">continue_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-import_stmt"><tt class="xref docutils literal">import_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-global_stmt"><tt class="xref docutils literal">global_stmt</tt></a>
 | <a class="reference internal" href="#grammar-token-exec_stmt"><tt class="xref docutils literal">exec_stmt</tt></a>
</pre>
<div class="section" id="expression-statements">
<h2>6.1. Expression statements<a class="headerlink" href="#expression-statements" title="Permalink to this headline">¶</a></h2>
Expression statements are used (mostly interactively) to compute and write a
value, or (usually) to call a procedure (a function that returns no meaningful
result; in Python, procedures return the value <tt class="xref docutils literal">None</tt>). Other uses of
expression statements are allowed and occasionally useful. The syntax for an
expression statement is:
<pre>
expression_stmt ::= <a class="reference external" href="expressions.html#grammar-token-expression_list"><tt class="xref docutils literal">expression_list</tt></a>
</pre>
An expression statement evaluates the expression list (which may be a single
expression).
In interactive mode, if the value is not <tt class="xref docutils literal">None</tt>, it is converted to a string
using the built-in <a title="repr" class="reference external" href="../library/functions.html#repr"><tt class="xref docutils literal">repr()</tt></a> function and the resulting string is written to
standard output (see section <a class="reference internal" href="#print">The print statement</a>) on a line by itself. (Expression
statements yielding <tt class="xref docutils literal">None</tt> are not written, so that procedure calls do not
cause any output.)
</div>
<div class="section" id="assignment-statements">
<h2>6.2. Assignment statements<a class="headerlink" href="#assignment-statements" title="Permalink to this headline">¶</a></h2>
Assignment statements are used to (re)bind names to values and to modify
attributes or items of mutable objects:
<pre>
assignment_stmt ::= (<a class="reference internal" href="#grammar-token-target_list"><tt class="xref docutils literal">target_list</tt></a> &quot;=&quot;)+ (<a class="reference external" href="expressions.html#grammar-token-expression_list"><tt class="xref docutils literal">expression_list</tt></a> | <a class="reference external" href="expressions.html#grammar-token-yield_expression"><tt class="xref docutils literal">yield_expression</tt></a>)
target_list ::= <a class="reference internal" href="#grammar-token-target"><tt class="xref docutils literal">target</tt></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-target"><tt class="xref docutils literal">target</tt></a>)* [&quot;,&quot;]
target ::= <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a>
 | &quot;(&quot; <a class="reference internal" href="#grammar-token-target_list"><tt class="xref docutils literal">target_list</tt></a> &quot;)&quot;
 | &quot;[&quot; <a class="reference internal" href="#grammar-token-target_list"><tt class="xref docutils literal">target_list</tt></a> &quot;]&quot;
 | <a class="reference external" href="expressions.html#grammar-token-attributeref"><tt class="xref docutils literal">attributeref</tt></a>
 | <a class="reference external" href="expressions.html#grammar-token-subscription"><tt class="xref docutils literal">subscription</tt></a>
 | <a class="reference external" href="expressions.html#grammar-token-slicing"><tt class="xref docutils literal">slicing</tt></a>
</pre>
(See section <a class="reference external" href="expressions.html#primaries">Primaries</a> for the syntax definitions for the last three
symbols.)
An assignment statement evaluates the expression list (remember that this can be
a single expression or a comma-separated list, the latter yielding a tuple) and
assigns the single resulting object to each of the target lists, from left to
right.
Assignment is defined recursively depending on the form of the target (list).
When a target is part of a mutable object (an attribute reference, subscription
or slicing), the mutable object must ultimately perform the assignment and
decide about its validity, and may raise an exception if the assignment is
unacceptable. The rules observed by various types and the exceptions raised are
given with the definition of the object types (see section <a class="reference external" href="datamodel.html#types">The standard type hierarchy</a>).
Assignment of an object to a target list is recursively defined as follows.
<ul class="simple">
<li>If the target list is a single target: The object is assigned to that target.</li>
<li>If the target list is a comma-separated list of targets: The object must be an
iterable with the same number of items as there are targets in the target list,
and the items are assigned, from left to right, to the corresponding targets.
(This rule is relaxed as of Python 1.5; in earlier versions, the object had to
be a tuple. Since strings are sequences, an assignment like <tt class="docutils literal">a, b = &quot;xy&quot;</tt> is
now legal as long as the string has the right length.)</li>
</ul>
Assignment of an object to a single target is recursively defined as follows.
<ul>
<li>If the target is an identifier (name):
<blockquote>
</blockquote>
<ul class="simple" id="index-1019">
<li>If the name does not occur in a <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement in the current
code block: the name is bound to the object in the current local namespace.</li>
<li>Otherwise: the name is bound to the object in the current global namespace.</li>
</ul>
The name is rebound if it was already bound. This may cause the reference count
for the object previously bound to the name to reach zero, causing the object to
be deallocated and its destructor (if it has one) to be called.
</li>
<li>If the target is a target list enclosed in parentheses or in square brackets:
The object must be an iterable with the same number of items as there are
targets in the target list, and its items are assigned, from left to right,
to the corresponding targets.
</li>
<li id="index-1021">If the target is an attribute reference: The primary expression in the
reference is evaluated. It should yield an object with assignable attributes;
if this is not the case, <a title="exceptions.TypeError" class="reference external" href="../library/exceptions.html#exceptions.TypeError"><tt class="xref docutils literal">TypeError</tt></a> is raised. That object is then
asked to assign the assigned object to the given attribute; if it cannot
perform the assignment, it raises an exception (usually but not necessarily
<a title="exceptions.AttributeError" class="reference external" href="../library/exceptions.html#exceptions.AttributeError"><tt class="xref docutils literal">AttributeError</tt></a>).
Note: If the object is a class instance and the attribute reference occurs on
both sides of the assignment operator, the RHS expression, <tt class="docutils literal">a.x</tt> can access
either an instance attribute or (if no instance attribute exists) a class
attribute. The LHS target <tt class="docutils literal">a.x</tt> is always set as an instance attribute,
creating it if necessary. Thus, the two occurrences of <tt class="docutils literal">a.x</tt> do not
necessarily refer to the same attribute: if the RHS expression refers to a
class attribute, the LHS creates a new instance attribute as the target of the
assignment:
<div class="highlight-python"><div class="highlight"><pre>class Cls:
 x = 3 # class variable
inst = Cls()
inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3
</pre></div>
</div>
This description does not necessarily apply to descriptor attributes, such as
properties created with <a title="property" class="reference external" href="../library/functions.html#property"><tt class="xref docutils literal">property()</tt></a>.
</li>
<li id="index-1022">If the target is a subscription: The primary expression in the reference is
evaluated. It should yield either a mutable sequence object (such as a list) or
a mapping object (such as a dictionary). Next, the subscript expression is
evaluated.
If the primary is a mutable sequence object (such as a list), the subscript must
yield a plain integer. If it is negative, the sequence&#8217;s length is added to it.
The resulting value must be a nonnegative integer less than the sequence&#8217;s
length, and the sequence is asked to assign the assigned object to its item with
that index. If the index is out of range, <a title="exceptions.IndexError" class="reference external" href="../library/exceptions.html#exceptions.IndexError"><tt class="xref docutils literal">IndexError</tt></a> is raised
(assignment to a subscripted sequence cannot add new items to a list).
If the primary is a mapping object (such as a dictionary), the subscript must
have a type compatible with the mapping&#8217;s key type, and the mapping is then
asked to create a key/datum pair which maps the subscript to the assigned
object. This can either replace an existing key/value pair with the same key
value, or insert a new key/value pair (if no key with the same value existed).
</li>
<li id="index-1025">If the target is a slicing: The primary expression in the reference is
evaluated. It should yield a mutable sequence object (such as a list). The
assigned object should be a sequence object of the same type. Next, the lower
and upper bound expressions are evaluated, insofar they are present; defaults
are zero and the sequence&#8217;s length. The bounds should evaluate to (small)
integers. If either bound is negative, the sequence&#8217;s length is added to it.
The resulting bounds are clipped to lie between zero and the sequence&#8217;s length,
inclusive. Finally, the sequence object is asked to replace the slice with the
items of the assigned sequence. The length of the slice may be different from
the length of the assigned sequence, thus changing the length of the target
sequence, if the object allows it.
</li>
</ul>
<div class="impl-detail compound">
CPython implementation detail: In the current implementation, the syntax for targets is taken to be the same
as for expressions, and invalid syntax is rejected during the code generation
phase, causing less detailed error messages.
</div>
WARNING: Although the definition of assignment implies that overlaps between the
left-hand side and the right-hand side are &#8216;safe&#8217; (for example <tt class="docutils literal">a, b = b, a</tt>
swaps two variables), overlaps within the collection of assigned-to variables
are not safe! For instance, the following program prints <tt class="docutils literal">[0, 2]</tt>:
<div class="highlight-python"><div class="highlight"><pre>x = [0, 1]
i = 0
i, x[i] = 1, 2
print x
</pre></div>
</div>
<div class="section" id="augmented-assignment-statements">
<h3>6.2.1. Augmented assignment statements<a class="headerlink" href="#augmented-assignment-statements" title="Permalink to this headline">¶</a></h3>
Augmented assignment is the combination, in a single statement, of a binary
operation and an assignment statement:
<pre>
augmented_assignment_stmt ::= <a class="reference internal" href="#grammar-token-augtarget"><tt class="xref docutils literal">augtarget</tt></a> <a class="reference internal" href="#grammar-token-augop"><tt class="xref docutils literal">augop</tt></a> (<a class="reference external" href="expressions.html#grammar-token-expression_list"><tt class="xref docutils literal">expression_list</tt></a> | <a class="reference external" href="expressions.html#grammar-token-yield_expression"><tt class="xref docutils literal">yield_expression</tt></a>)
augtarget ::= <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> | <a class="reference external" href="expressions.html#grammar-token-attributeref"><tt class="xref docutils literal">attributeref</tt></a> | <a class="reference external" href="expressions.html#grammar-token-subscription"><tt class="xref docutils literal">subscription</tt></a> | <a class="reference external" href="expressions.html#grammar-token-slicing"><tt class="xref docutils literal">slicing</tt></a>
augop ::= &quot;+=&quot; | &quot;-=&quot; | &quot;*=&quot; | &quot;/=&quot; | &quot;//=&quot; | &quot;%=&quot; | &quot;**=&quot;
 | &quot;&gt;&gt;=&quot; | &quot;&lt;&lt;=&quot; | &quot;&amp;=&quot; | &quot;^=&quot; | &quot;|=&quot;
</pre>
(See section <a class="reference external" href="expressions.html#primaries">Primaries</a> for the syntax definitions for the last three
symbols.)
An augmented assignment evaluates the target (which, unlike normal assignment
statements, cannot be an unpacking) and the expression list, performs the binary
operation specific to the type of assignment on the two operands, and assigns
the result to the original target. The target is only evaluated once.
An augmented assignment expression like <tt class="docutils literal">x += 1</tt> can be rewritten as <tt class="docutils literal">x = x +
1</tt> to achieve a similar, but not exactly equal effect. In the augmented
version, <tt class="docutils literal">x</tt> is only evaluated once. Also, when possible, the actual operation
is performed in-place, meaning that rather than creating a new object and
assigning that to the target, the old object is modified instead.
With the exception of assigning to tuples and multiple targets in a single
statement, the assignment done by augmented assignment statements is handled the
same way as normal assignments. Similarly, with the exception of the possible
in-place behavior, the binary operation performed by augmented assignment is
the same as the normal binary operations.
For targets which are attribute references, the same <a class="reference internal" href="#attr-target-note">caveat about class
and instance attributes</a> applies as for regular assignments.
</div>
</div>
<div class="section" id="the-assert-statement">
<h2>6.3. The <a class="reference internal" href="#assert"><tt class="xref docutils literal">assert</tt></a> statement<a class="headerlink" href="#the-assert-statement" title="Permalink to this headline">¶</a></h2>
Assert statements are a convenient way to insert debugging assertions into a
program:
<pre>
assert_stmt ::= &quot;assert&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a> [&quot;,&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a>]
</pre>
The simple form, <tt class="docutils literal">assert expression</tt>, is equivalent to
<div class="highlight-python"><div class="highlight"><pre>if __debug__:
 if not expression: raise AssertionError
</pre></div>
</div>
The extended form, <tt class="docutils literal">assert expression1, expression2</tt>, is equivalent to
<div class="highlight-python"><div class="highlight"><pre>if __debug__:
 if not expression1: raise AssertionError(expression2)
</pre></div>
</div>
These equivalences assume that <a title="__debug__" class="reference external" href="../library/constants.html#__debug__"><tt class="xref docutils literal">__debug__</tt></a> and <a title="exceptions.AssertionError" class="reference external" href="../library/exceptions.html#exceptions.AssertionError"><tt class="xref docutils literal">AssertionError</tt></a> refer to
the built-in variables with those names. In the current implementation, the
built-in variable <a title="__debug__" class="reference external" href="../library/constants.html#__debug__"><tt class="xref docutils literal">__debug__</tt></a> is <tt class="xref docutils literal">True</tt> under normal circumstances,
<tt class="xref docutils literal">False</tt> when optimization is requested (command line option -O). The current
code generator emits no code for an assert statement when optimization is
requested at compile time. Note that it is unnecessary to include the source
code for the expression that failed in the error message; it will be displayed
as part of the stack trace.
Assignments to <a title="__debug__" class="reference external" href="../library/constants.html#__debug__"><tt class="xref docutils literal">__debug__</tt></a> are illegal. The value for the built-in variable
is determined when the interpreter starts.
</div>
<div class="section" id="the-pass-statement">
<h2>6.4. The <a class="reference internal" href="#pass"><tt class="xref docutils literal">pass</tt></a> statement<a class="headerlink" href="#the-pass-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1029">
pass_stmt ::= &quot;pass&quot;
</pre>
<a class="reference internal" href="#pass"><tt class="xref docutils literal">pass</tt></a> is a null operation &#8212; when it is executed, nothing happens.
It is useful as a placeholder when a statement is required syntactically, but no
code needs to be executed, for example:
<div class="highlight-python"><div class="highlight"><pre>def f(arg): pass # a function that does nothing (yet)

class C: pass # a class with no methods (yet)
</pre></div>
</div>
</div>
<div class="section" id="the-del-statement">
<h2>6.5. The <a class="reference internal" href="#del"><tt class="xref docutils literal">del</tt></a> statement<a class="headerlink" href="#the-del-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1030">
del_stmt ::= &quot;del&quot; <a class="reference internal" href="#grammar-token-target_list"><tt class="xref docutils literal">target_list</tt></a>
</pre>
Deletion is recursively defined very similar to the way assignment is defined.
Rather that spelling it out in full details, here are some hints.
Deletion of a target list recursively deletes each target, from left to right.
Deletion of a name removes the binding of that name from the local or global
namespace, depending on whether the name occurs in a <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement
in the same code block. If the name is unbound, a <a title="exceptions.NameError" class="reference external" href="../library/exceptions.html#exceptions.NameError"><tt class="xref docutils literal">NameError</tt></a> exception
will be raised.
It is illegal to delete a name from the local namespace if it occurs as a free
variable in a nested block.
Deletion of attribute references, subscriptions and slicings is passed to the
primary object involved; deletion of a slicing is in general equivalent to
assignment of an empty slice of the right type (but even this is determined by
the sliced object).
</div>
<div class="section" id="the-print-statement">
<h2>6.6. The <a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a> statement<a class="headerlink" href="#the-print-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1034">
print_stmt ::= &quot;print&quot; ([<a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a> (&quot;,&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a>)* [&quot;,&quot;]]
 | &quot;&gt;&gt;&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a> [(&quot;,&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a>)+ [&quot;,&quot;]])
</pre>
<a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a> evaluates each expression in turn and writes the resulting
object to standard output (see below). If an object is not a string, it is
first converted to a string using the rules for string conversions. The
(resulting or original) string is then written. A space is written before each
object is (converted and) written, unless the output system believes it is
positioned at the beginning of a line. This is the case (1) when no characters
have yet been written to standard output, (2) when the last character written to
standard output is a whitespace character except <tt class="docutils literal">' '</tt>, or (3) when the last
write operation on standard output was not a <a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a> statement.
(In some cases it may be functional to write an empty string to standard output
for this reason.)
<div class="admonition note">
Note
Objects which act like file objects but which are not the built-in file objects
often do not properly emulate this aspect of the file object&#8217;s behavior, so it
is best not to rely on this.
</div>
A <tt class="docutils literal">'\n'</tt> character is written at the end, unless the <a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a>
statement ends with a comma. This is the only action if the statement contains
just the keyword <a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a>.
Standard output is defined as the file object named <tt class="docutils literal">stdout</tt> in the built-in
module <a title="Access system-specific parameters and functions." class="reference external" href="../library/sys.html#module-sys"><tt class="xref docutils literal">sys</tt></a>. If no such object exists, or if it does not have a
<tt class="xref docutils literal">write()</tt> method, a <a title="exceptions.RuntimeError" class="reference external" href="../library/exceptions.html#exceptions.RuntimeError"><tt class="xref docutils literal">RuntimeError</tt></a> exception is raised.
<a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a> also has an extended form, defined by the second portion of the
syntax described above. This form is sometimes referred to as &#8220;<a class="reference internal" href="#print"><tt class="xref docutils literal">print</tt></a>
chevron.&#8221; In this form, the first expression after the <tt class="docutils literal">&gt;&gt;</tt> must evaluate to a
&#8220;file-like&#8221; object, specifically an object that has a <tt class="xref docutils literal">write()</tt> method as
described above. With this extended form, the subsequent expressions are
printed to this file object. If the first expression evaluates to <tt class="xref docutils literal">None</tt>,
then <tt class="docutils literal">sys.stdout</tt> is used as the file for output.
</div>
<div class="section" id="the-return-statement">
<h2>6.7. The <a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a> statement<a class="headerlink" href="#the-return-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1038">
return_stmt ::= &quot;return&quot; [<a class="reference external" href="expressions.html#grammar-token-expression_list"><tt class="xref docutils literal">expression_list</tt></a>]
</pre>
<a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a> may only occur syntactically nested in a function definition,
not within a nested class definition.
If an expression list is present, it is evaluated, else <tt class="xref docutils literal">None</tt> is substituted.
<a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a> leaves the current function call with the expression list (or
<tt class="xref docutils literal">None</tt>) as return value.
When <a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a> passes control out of a <a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a> statement with a
<a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause, that <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause is executed before
really leaving the function.
In a generator function, the <a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a> statement is not allowed to
include an <a class="reference external" href="expressions.html#grammar-token-expression_list">expression_list</a>. In that context, a bare <a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a>
indicates that the generator is done and will cause <a title="exceptions.StopIteration" class="reference external" href="../library/exceptions.html#exceptions.StopIteration"><tt class="xref docutils literal">StopIteration</tt></a> to be
raised.
</div>
<div class="section" id="the-yield-statement">
<h2>6.8. The <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statement<a class="headerlink" href="#the-yield-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1040">
yield_stmt ::= <a class="reference external" href="expressions.html#grammar-token-yield_expression"><tt class="xref docutils literal">yield_expression</tt></a>
</pre>
The <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statement is only used when defining a generator function,
and is only used in the body of the generator function. Using a <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a>
statement in a function definition is sufficient to cause that definition to
create a generator function instead of a normal function.
When a generator function is called, it returns an iterator known as a generator
iterator, or more commonly, a generator. The body of the generator function is
executed by calling the generator&#8217;s <a title="next" class="reference external" href="../library/functions.html#next"><tt class="xref docutils literal">next()</tt></a> method repeatedly until it
raises an exception.
When a <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statement is executed, the state of the generator is
frozen and the value of <a class="reference external" href="expressions.html#grammar-token-expression_list">expression_list</a> is returned to <a title="next" class="reference external" href="../library/functions.html#next"><tt class="xref docutils literal">next()</tt></a>&#8216;s
caller. By &#8220;frozen&#8221; we mean that all local state is retained, including the
current bindings of local variables, the instruction pointer, and the internal
evaluation stack: enough information is saved so that the next time <a title="next" class="reference external" href="../library/functions.html#next"><tt class="xref docutils literal">next()</tt></a>
is invoked, the function can proceed exactly as if the <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a>
statement were just another external call.
As of Python version 2.5, the <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statement is now allowed in the
<a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a> clause of a <a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a> ... <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> construct. If
the generator is not resumed before it is finalized (by reaching a zero
reference count or by being garbage collected), the generator-iterator&#8217;s
<tt class="xref docutils literal">close()</tt> method will be called, allowing any pending <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a>
clauses to execute.
<div class="admonition note">
Note
In Python 2.2, the <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statement was only allowed when the
<tt class="docutils literal">generators</tt> feature has been enabled. This <tt class="docutils literal">__future__</tt>
import statement was used to enable the feature:
<div class="last highlight-python"><div class="highlight"><pre>from __future__ import generators
</pre></div>
</div>
</div>
<div class="admonition-see-also admonition seealso">
See also
<dl class="last docutils">
<dt><a class="reference external" href="http://www.python.org/dev/peps/pep-0255">PEP 0255</a> - Simple Generators</dt>
<dd>The proposal for adding generators and the <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statement to Python.</dd>
<dt><a class="reference external" href="http://www.python.org/dev/peps/pep-0342">PEP 0342</a> - Coroutines via Enhanced Generators</dt>
<dd>The proposal that, among other generator enhancements, proposed allowing
<a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> to appear inside a <a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a> ... <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> block.</dd>
</dl>
</div>
</div>
<div class="section" id="the-raise-statement">
<h2>6.9. The <a class="reference internal" href="#raise"><tt class="xref docutils literal">raise</tt></a> statement<a class="headerlink" href="#the-raise-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1043">
raise_stmt ::= &quot;raise&quot; [<a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a> [&quot;,&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a> [&quot;,&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a>]]]
</pre>
If no expressions are present, <a class="reference internal" href="#raise"><tt class="xref docutils literal">raise</tt></a> re-raises the last exception
that was active in the current scope. If no exception is active in the current
scope, a <a title="exceptions.TypeError" class="reference external" href="../library/exceptions.html#exceptions.TypeError"><tt class="xref docutils literal">TypeError</tt></a> exception is raised indicating that this is an error
(if running under IDLE, a <a title="Queue.Empty" class="reference external" href="../library/queue.html#Queue.Empty"><tt class="xref docutils literal">Queue.Empty</tt></a> exception is raised instead).
Otherwise, <a class="reference internal" href="#raise"><tt class="xref docutils literal">raise</tt></a> evaluates the expressions to get three objects,
using <tt class="xref docutils literal">None</tt> as the value of omitted expressions. The first two objects are
used to determine the type and value of the exception.
If the first object is an instance, the type of the exception is the class of
the instance, the instance itself is the value, and the second object must be
<tt class="xref docutils literal">None</tt>.
If the first object is a class, it becomes the type of the exception. The second
object is used to determine the exception value: If it is an instance of the
class, the instance becomes the exception value. If the second object is a
tuple, it is used as the argument list for the class constructor; if it is
<tt class="xref docutils literal">None</tt>, an empty argument list is used, and any other object is treated as a
single argument to the constructor. The instance so created by calling the
constructor is used as the exception value.
If a third object is present and not <tt class="xref docutils literal">None</tt>, it must be a traceback object
(see section <a class="reference external" href="datamodel.html#types">The standard type hierarchy</a>), and it is substituted instead of the current
location as the place where the exception occurred. If the third object is
present and not a traceback object or <tt class="xref docutils literal">None</tt>, a <a title="exceptions.TypeError" class="reference external" href="../library/exceptions.html#exceptions.TypeError"><tt class="xref docutils literal">TypeError</tt></a> exception is
raised. The three-expression form of <a class="reference internal" href="#raise"><tt class="xref docutils literal">raise</tt></a> is useful to re-raise an
exception transparently in an except clause, but <a class="reference internal" href="#raise"><tt class="xref docutils literal">raise</tt></a> with no
expressions should be preferred if the exception to be re-raised was the most
recently active exception in the current scope.
Additional information on exceptions can be found in section <a class="reference external" href="executionmodel.html#exceptions">Exceptions</a>,
and information about handling exceptions is in section <a class="reference external" href="compound_stmts.html#try">The try statement</a>.
</div>
<div class="section" id="the-break-statement">
<h2>6.10. The <a class="reference internal" href="#break"><tt class="xref docutils literal">break</tt></a> statement<a class="headerlink" href="#the-break-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1045">
break_stmt ::= &quot;break&quot;
</pre>
<a class="reference internal" href="#break"><tt class="xref docutils literal">break</tt></a> may only occur syntactically nested in a <a class="reference external" href="compound_stmts.html#for"><tt class="xref docutils literal">for</tt></a> or
<a class="reference external" href="compound_stmts.html#while"><tt class="xref docutils literal">while</tt></a> loop, but not nested in a function or class definition within
that loop.
It terminates the nearest enclosing loop, skipping the optional <a class="reference external" href="compound_stmts.html#else"><tt class="xref docutils literal">else</tt></a>
clause if the loop has one.
If a <a class="reference external" href="compound_stmts.html#for"><tt class="xref docutils literal">for</tt></a> loop is terminated by <a class="reference internal" href="#break"><tt class="xref docutils literal">break</tt></a>, the loop control
target keeps its current value.
When <a class="reference internal" href="#break"><tt class="xref docutils literal">break</tt></a> passes control out of a <a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a> statement with a
<a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause, that <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause is executed before
really leaving the loop.
</div>
<div class="section" id="the-continue-statement">
<h2>6.11. The <a class="reference internal" href="#continue"><tt class="xref docutils literal">continue</tt></a> statement<a class="headerlink" href="#the-continue-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1049">
continue_stmt ::= &quot;continue&quot;
</pre>
<a class="reference internal" href="#continue"><tt class="xref docutils literal">continue</tt></a> may only occur syntactically nested in a <a class="reference external" href="compound_stmts.html#for"><tt class="xref docutils literal">for</tt></a> or
<a class="reference external" href="compound_stmts.html#while"><tt class="xref docutils literal">while</tt></a> loop, but not nested in a function or class definition or
<a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause within that loop. It continues with the next
cycle of the nearest enclosing loop.
When <a class="reference internal" href="#continue"><tt class="xref docutils literal">continue</tt></a> passes control out of a <a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal">try</tt></a> statement with a
<a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause, that <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal">finally</tt></a> clause is executed before
really starting the next loop cycle.
</div>
<div class="section" id="the-import-statement">
<h2>6.12. The <a class="reference internal" href="#import"><tt class="xref docutils literal">import</tt></a> statement<a class="headerlink" href="#the-import-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1050">
import_stmt ::= &quot;import&quot; <a class="reference internal" href="#grammar-token-module"><tt class="xref docutils literal">module</tt></a> [&quot;as&quot; <a class="reference internal" href="#grammar-token-name"><tt class="xref docutils literal">name</tt></a>] ( &quot;,&quot; <a class="reference internal" href="#grammar-token-module"><tt class="xref docutils literal">module</tt></a> [&quot;as&quot; <a class="reference internal" href="#grammar-token-name"><tt class="xref docutils literal">name</tt></a>] )*
 | &quot;from&quot; <a class="reference internal" href="#grammar-token-relative_module"><tt class="xref docutils literal">relative_module</tt></a> &quot;import&quot; <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> [&quot;as&quot; <a class="reference internal" href="#grammar-token-name"><tt class="xref docutils literal">name</tt></a>]
 ( &quot;,&quot; <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> [&quot;as&quot; <a class="reference internal" href="#grammar-token-name"><tt class="xref docutils literal">name</tt></a>] )*
 | &quot;from&quot; <a class="reference internal" href="#grammar-token-relative_module"><tt class="xref docutils literal">relative_module</tt></a> &quot;import&quot; &quot;(&quot; <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> [&quot;as&quot; <a class="reference internal" href="#grammar-token-name"><tt class="xref docutils literal">name</tt></a>]
 ( &quot;,&quot; <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> [&quot;as&quot; <a class="reference internal" href="#grammar-token-name"><tt class="xref docutils literal">name</tt></a>] )* [&quot;,&quot;] &quot;)&quot;
 | &quot;from&quot; <a class="reference internal" href="#grammar-token-module"><tt class="xref docutils literal">module</tt></a> &quot;import&quot; &quot;*&quot;
module ::= (<a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> &quot;.&quot;)* <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a>
relative_module ::= &quot;.&quot;* <a class="reference internal" href="#grammar-token-module"><tt class="xref docutils literal">module</tt></a> | &quot;.&quot;+
name ::= <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a>
</pre>
Import statements are executed in two steps: (1) find a module, and initialize
it if necessary; (2) define a name or names in the local namespace (of the scope
where the <a class="reference internal" href="#import"><tt class="xref docutils literal">import</tt></a> statement occurs). The statement comes in two
forms differing on whether it uses the <a class="reference internal" href="#from"><tt class="xref docutils literal">from</tt></a> keyword. The first form
(without <a class="reference internal" href="#from"><tt class="xref docutils literal">from</tt></a>) repeats these steps for each identifier in the list.
The form with <a class="reference internal" href="#from"><tt class="xref docutils literal">from</tt></a> performs step (1) once, and then performs step
(2) repeatedly.
To understand how step (1) occurs, one must first understand how Python handles
hierarchical naming of modules. To help organize modules and provide a
hierarchy in naming, Python has a concept of packages. A package can contain
other packages and modules while modules cannot contain other modules or
packages. From a file system perspective, packages are directories and modules
are files. The original <a class="reference external" href="http://www.python.org/doc/essays/packages.html">specification for packages</a> is still available to read,
although minor details have changed since the writing of that document.
Once the name of the module is known (unless otherwise specified, the term
&#8220;module&#8221; will refer to both packages and modules), searching
for the module or package can begin. The first place checked is
<a title="sys.modules" class="reference external" href="../library/sys.html#sys.modules"><tt class="xref docutils literal">sys.modules</tt></a>, the cache of all modules that have been imported
previously. If the module is found there then it is used in step (2) of import.
If the module is not found in the cache, then <a title="sys.meta_path" class="reference external" href="../library/sys.html#sys.meta_path"><tt class="xref docutils literal">sys.meta_path</tt></a> is searched
(the specification for <a title="sys.meta_path" class="reference external" href="../library/sys.html#sys.meta_path"><tt class="xref docutils literal">sys.meta_path</tt></a> can be found in <a class="reference external" href="http://www.python.org/dev/peps/pep-0302">PEP 302</a>).
The object is a list of <a class="reference external" href="../glossary.html#term-finder">finder</a> objects which are queried in order as to
whether they know how to load the module by calling their <tt class="xref docutils literal">find_module()</tt>
method with the name of the module. If the module happens to be contained
within a package (as denoted by the existence of a dot in the name), then a
second argument to <tt class="xref docutils literal">find_module()</tt> is given as the value of the
<tt class="xref docutils literal">__path__</tt> attribute from the parent package (everything up to the last
dot in the name of the module being imported). If a finder can find the module
it returns a <a class="reference external" href="../glossary.html#term-loader">loader</a> (discussed later) or returns <tt class="xref xref docutils literal">None</tt>.
If none of the finders on <a title="sys.meta_path" class="reference external" href="../library/sys.html#sys.meta_path"><tt class="xref docutils literal">sys.meta_path</tt></a> are able to find the module
then some implicitly defined finders are queried. Implementations of Python
vary in what implicit meta path finders are defined. The one they all do
define, though, is one that handles <a title="sys.path_hooks" class="reference external" href="../library/sys.html#sys.path_hooks"><tt class="xref docutils literal">sys.path_hooks</tt></a>,
<a title="sys.path_importer_cache" class="reference external" href="../library/sys.html#sys.path_importer_cache"><tt class="xref docutils literal">sys.path_importer_cache</tt></a>, and <a title="sys.path" class="reference external" href="../library/sys.html#sys.path"><tt class="xref docutils literal">sys.path</tt></a>.
The implicit finder searches for the requested module in the &#8220;paths&#8221; specified
in one of two places (&#8220;paths&#8221; do not have to be file system paths). If the
module being imported is supposed to be contained within a package then the
second argument passed to <tt class="xref docutils literal">find_module()</tt>, <tt class="xref docutils literal">__path__</tt> on the parent
package, is used as the source of paths. If the module is not contained in a
package then <a title="sys.path" class="reference external" href="../library/sys.html#sys.path"><tt class="xref docutils literal">sys.path</tt></a> is used as the source of paths.
Once the source of paths is chosen it is iterated over to find a finder that
can handle that path. The dict at <a title="sys.path_importer_cache" class="reference external" href="../library/sys.html#sys.path_importer_cache"><tt class="xref docutils literal">sys.path_importer_cache</tt></a> caches
finders for paths and is checked for a finder. If the path does not have a
finder cached then <a title="sys.path_hooks" class="reference external" href="../library/sys.html#sys.path_hooks"><tt class="xref docutils literal">sys.path_hooks</tt></a> is searched by calling each object in
the list with a single argument of the path, returning a finder or raises
<a title="exceptions.ImportError" class="reference external" href="../library/exceptions.html#exceptions.ImportError"><tt class="xref docutils literal">ImportError</tt></a>. If a finder is returned then it is cached in
<a title="sys.path_importer_cache" class="reference external" href="../library/sys.html#sys.path_importer_cache"><tt class="xref docutils literal">sys.path_importer_cache</tt></a> and then used for that path entry. If no finder
can be found but the path exists then a value of <tt class="xref xref docutils literal">None</tt> is
stored in <a title="sys.path_importer_cache" class="reference external" href="../library/sys.html#sys.path_importer_cache"><tt class="xref docutils literal">sys.path_importer_cache</tt></a> to signify that an implicit,
file-based finder that handles modules stored as individual files should be
used for that path. If the path does not exist then a finder which always
returns <tt class="xref xref docutils literal">None</tt> is placed in the cache for the path.
If no finder can find the module then <a title="exceptions.ImportError" class="reference external" href="../library/exceptions.html#exceptions.ImportError"><tt class="xref docutils literal">ImportError</tt></a> is raised. Otherwise
some finder returned a loader whose <tt class="xref docutils literal">load_module()</tt> method is called with
the name of the module to load (see <a class="reference external" href="http://www.python.org/dev/peps/pep-0302">PEP 302</a> for the original definition of
loaders). A loader has several responsibilities to perform on a module it
loads. First, if the module already exists in <a title="sys.modules" class="reference external" href="../library/sys.html#sys.modules"><tt class="xref docutils literal">sys.modules</tt></a> (a
possibility if the loader is called outside of the import machinery) then it
is to use that module for initialization and not a new module. But if the
module does not exist in <a title="sys.modules" class="reference external" href="../library/sys.html#sys.modules"><tt class="xref docutils literal">sys.modules</tt></a> then it is to be added to that
dict before initialization begins. If an error occurs during loading of the
module and it was added to <a title="sys.modules" class="reference external" href="../library/sys.html#sys.modules"><tt class="xref docutils literal">sys.modules</tt></a> it is to be removed from the
dict. If an error occurs but the module was already in <a title="sys.modules" class="reference external" href="../library/sys.html#sys.modules"><tt class="xref docutils literal">sys.modules</tt></a> it
is left in the dict.
The loader must set several attributes on the module. <tt class="xref docutils literal">__name__</tt> is to be
set to the name of the module. <tt class="xref docutils literal">__file__</tt> is to be the &#8220;path&#8221; to the file
unless the module is built-in (and thus listed in
<a title="sys.builtin_module_names" class="reference external" href="../library/sys.html#sys.builtin_module_names"><tt class="xref docutils literal">sys.builtin_module_names</tt></a>) in which case the attribute is not set.
If what is being imported is a package then <tt class="xref docutils literal">__path__</tt> is to be set to a
list of paths to be searched when looking for modules and packages contained
within the package being imported. <tt class="xref docutils literal">__package__</tt> is optional but should
be set to the name of package that contains the module or package (the empty
string is used for module not contained in a package). <tt class="xref docutils literal">__loader__</tt> is
also optional but should be set to the loader object that is loading the
module.
If an error occurs during loading then the loader raises <a title="exceptions.ImportError" class="reference external" href="../library/exceptions.html#exceptions.ImportError"><tt class="xref docutils literal">ImportError</tt></a> if
some other exception is not already being propagated. Otherwise the loader
returns the module that was loaded and initialized.
When step (1) finishes without raising an exception, step (2) can begin.
The first form of <a class="reference internal" href="#import"><tt class="xref docutils literal">import</tt></a> statement binds the module name in the local
namespace to the module object, and then goes on to import the next identifier,
if any. If the module name is followed by <a class="reference external" href="compound_stmts.html#as"><tt class="xref docutils literal">as</tt></a>, the name following
<a class="reference external" href="compound_stmts.html#as"><tt class="xref docutils literal">as</tt></a> is used as the local name for the module.
The <a class="reference internal" href="#from"><tt class="xref docutils literal">from</tt></a> form does not bind the module name: it goes through the list
of identifiers, looks each one of them up in the module found in step (1), and
binds the name in the local namespace to the object thus found. As with the
first form of <a class="reference internal" href="#import"><tt class="xref docutils literal">import</tt></a>, an alternate local name can be supplied by
specifying &#8220;<a class="reference external" href="compound_stmts.html#as"><tt class="xref docutils literal">as</tt></a> localname&#8221;. If a name is not found,
<a title="exceptions.ImportError" class="reference external" href="../library/exceptions.html#exceptions.ImportError"><tt class="xref docutils literal">ImportError</tt></a> is raised. If the list of identifiers is replaced by a star
(<tt class="docutils literal">'*'</tt>), all public names defined in the module are bound in the local
namespace of the <a class="reference internal" href="#import"><tt class="xref docutils literal">import</tt></a> statement..
The public names defined by a module are determined by checking the module&#8217;s
namespace for a variable named <tt class="docutils literal">__all__</tt>; if defined, it must be a sequence of
strings which are names defined or imported by that module. The names given in
<tt class="docutils literal">__all__</tt> are all considered public and are required to exist. If <tt class="docutils literal">__all__</tt>
is not defined, the set of public names includes all names found in the module&#8217;s
namespace which do not begin with an underscore character (<tt class="docutils literal">'_'</tt>).
<tt class="docutils literal">__all__</tt> should contain the entire public API. It is intended to avoid
accidentally exporting items that are not part of the API (such as library
modules which were imported and used within the module).
The <a class="reference internal" href="#from"><tt class="xref docutils literal">from</tt></a> form with <tt class="docutils literal">*</tt> may only occur in a module scope. If the
wild card form of import &#8212; <tt class="docutils literal">import *</tt> &#8212; is used in a function and the
function contains or is a nested block with free variables, the compiler will
raise a <a title="exceptions.SyntaxError" class="reference external" href="../library/exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal">SyntaxError</tt></a>.
When specifying what module to import you do not have to specify the absolute
name of the module. When a module or package is contained within another
package it is possible to make a relative import within the same top package
without having to mention the package name. By using leading dots in the
specified module or package after <a class="reference internal" href="#from"><tt class="xref docutils literal">from</tt></a> you can specify how high to
traverse up the current package hierarchy without specifying exact names. One
leading dot means the current package where the module making the import
exists. Two dots means up one package level. Three dots is up two levels, etc.
So if you execute <tt class="docutils literal">from . import mod</tt> from a module in the <tt class="docutils literal">pkg</tt> package
then you will end up importing <tt class="docutils literal">pkg.mod</tt>. If you execute <tt class="docutils literal">from ..subpkg2
imprt mod</tt> from within <tt class="docutils literal">pkg.subpkg1</tt> you will import <tt class="docutils literal">pkg.subpkg2.mod</tt>.
The specification for relative imports is contained within <a class="reference external" href="http://www.python.org/dev/peps/pep-0328">PEP 328</a>.
The built-in function <a title="__import__" class="reference external" href="../library/functions.html#__import__"><tt class="xref docutils literal">__import__()</tt></a> is provided to support applications
that determine which modules need to be loaded dynamically; refer to
<a class="reference external" href="../library/functions.html#built-in-funcs">Built-in Functions</a> for additional information.
<div class="section" id="future-statements">
<h3>6.12.1. Future statements<a class="headerlink" href="#future-statements" title="Permalink to this headline">¶</a></h3>
A future statement is a directive to the compiler that a particular
module should be compiled using syntax or semantics that will be available in a
specified future release of Python. The future statement is intended to ease
migration to future versions of Python that introduce incompatible changes to
the language. It allows use of the new features on a per-module basis before
the release in which the feature becomes standard.
<pre>
future_statement ::= &quot;from&quot; &quot;__future__&quot; &quot;import&quot; feature [&quot;as&quot; name]
 (&quot;,&quot; feature [&quot;as&quot; name])*
 | &quot;from&quot; &quot;__future__&quot; &quot;import&quot; &quot;(&quot; feature [&quot;as&quot; name]
 (&quot;,&quot; feature [&quot;as&quot; name])* [&quot;,&quot;] &quot;)&quot;
feature ::= identifier
name ::= identifier
</pre>
A future statement must appear near the top of the module. The only lines that
can appear before a future statement are:
<ul class="simple">
<li>the module docstring (if any),</li>
<li>comments,</li>
<li>blank lines, and</li>
<li>other future statements.</li>
</ul>
The features recognized by Python 2.6 are <tt class="docutils literal">unicode_literals</tt>,
<tt class="docutils literal">print_function</tt>, <tt class="docutils literal">absolute_import</tt>, <tt class="docutils literal">division</tt>, <tt class="docutils literal">generators</tt>,
<tt class="docutils literal">nested_scopes</tt> and <tt class="docutils literal">with_statement</tt>. <tt class="docutils literal">generators</tt>, <tt class="docutils literal">with_statement</tt>,
<tt class="docutils literal">nested_scopes</tt> are redundant in Python version 2.6 and above because they are
always enabled.
A future statement is recognized and treated specially at compile time: Changes
to the semantics of core constructs are often implemented by generating
different code. It may even be the case that a new feature introduces new
incompatible syntax (such as a new reserved word), in which case the compiler
may need to parse the module differently. Such decisions cannot be pushed off
until runtime.
For any given release, the compiler knows which feature names have been defined,
and raises a compile-time error if a future statement contains a feature not
known to it.
The direct runtime semantics are the same as for any import statement: there is
a standard module <a title="Future statement definitions" class="reference external" href="../library/__future__.html#module-__future__"><tt class="xref docutils literal">__future__</tt></a>, described later, and it will be imported in
the usual way at the time the future statement is executed.
The interesting runtime semantics depend on the specific feature enabled by the
future statement.
Note that there is nothing special about the statement:
<div class="highlight-python"><pre>import __future__ [as name]</pre>
</div>
That is not a future statement; it&#8217;s an ordinary import statement with no
special semantics or syntax restrictions.
Code compiled by an <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a> statement or calls to the built-in functions
<a title="compile" class="reference external" href="../library/functions.html#compile"><tt class="xref docutils literal">compile()</tt></a> and <a title="execfile" class="reference external" href="../library/functions.html#execfile"><tt class="xref docutils literal">execfile()</tt></a> that occur in a module <tt class="xref docutils literal">M</tt> containing
a future statement will, by default, use the new syntax or semantics associated
with the future statement. This can, starting with Python 2.2 be controlled by
optional arguments to <a title="compile" class="reference external" href="../library/functions.html#compile"><tt class="xref docutils literal">compile()</tt></a> &#8212; see the documentation of that function
for details.
A future statement typed at an interactive interpreter prompt will take effect
for the rest of the interpreter session. If an interpreter is started with the
<a class="reference external" href="../using/cmdline.html#cmdoption-i">-i</a> option, is passed a script name to execute, and the script includes
a future statement, it will be in effect in the interactive session started
after the script is executed.
<div class="admonition-see-also admonition seealso">
See also
<dl class="last docutils">
<dt><a class="reference external" href="http://www.python.org/dev/peps/pep-0236">PEP 236</a> - Back to the __future__</dt>
<dd>The original proposal for the __future__ mechanism.</dd>
</dl>
</div>
</div>
</div>
<div class="section" id="the-global-statement">
<h2>6.13. The <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement<a class="headerlink" href="#the-global-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1067">
global_stmt ::= &quot;global&quot; <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a> (&quot;,&quot; <a class="reference external" href="lexical_analysis.html#grammar-token-identifier"><tt class="xref docutils literal">identifier</tt></a>)*
</pre>
The <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement is a declaration which holds for the entire
current code block. It means that the listed identifiers are to be interpreted
as globals. It would be impossible to assign to a global variable without
<a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a>, although free variables may refer to globals without being
declared global.
Names listed in a <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement must not be used in the same code
block textually preceding that <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement.
Names listed in a <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement must not be defined as formal
parameters or in a <a class="reference external" href="compound_stmts.html#for"><tt class="xref docutils literal">for</tt></a> loop control target, <a class="reference external" href="compound_stmts.html#class"><tt class="xref docutils literal">class</tt></a>
definition, function definition, or <a class="reference internal" href="#import"><tt class="xref docutils literal">import</tt></a> statement.
<div class="impl-detail compound">
CPython implementation detail: The current implementation does not enforce the latter two restrictions, but
programs should not abuse this freedom, as future implementations may enforce
them or silently change the meaning of the program.
</div>
Programmer&#8217;s note: the <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> is a directive to the parser. It
applies only to code parsed at the same time as the <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement.
In particular, a <a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statement contained in an <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a>
statement does not affect the code block containing the <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a>
statement, and code contained in an <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a> statement is unaffected by
<a class="reference internal" href="#global"><tt class="xref docutils literal">global</tt></a> statements in the code containing the <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a>
statement. The same applies to the <a title="eval" class="reference external" href="../library/functions.html#eval"><tt class="xref docutils literal">eval()</tt></a>, <a title="execfile" class="reference external" href="../library/functions.html#execfile"><tt class="xref docutils literal">execfile()</tt></a> and
<a title="compile" class="reference external" href="../library/functions.html#compile"><tt class="xref docutils literal">compile()</tt></a> functions.
</div>
<div class="section" id="the-exec-statement">
<h2>6.14. The <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a> statement<a class="headerlink" href="#the-exec-statement" title="Permalink to this headline">¶</a></h2>
<pre id="index-1069">
exec_stmt ::= &quot;exec&quot; <a class="reference external" href="expressions.html#grammar-token-or_expr"><tt class="xref docutils literal">or_expr</tt></a> [&quot;in&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a> [&quot;,&quot; <a class="reference external" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal">expression</tt></a>]]
</pre>
This statement supports dynamic execution of Python code. The first expression
should evaluate to either a string, an open file object, or a code object. If
it is a string, the string is parsed as a suite of Python statements which is
then executed (unless a syntax error occurs). <a class="footnote-reference" href="#id3" id="id2">[1]</a> If it is an open file, the file
is parsed until EOF and executed. If it is a code object, it is simply
executed. In all cases, the code that&#8217;s executed is expected to be valid as
file input (see section <a class="reference external" href="toplevel_components.html#file-input">File input</a>). Be aware that the
<a class="reference internal" href="#return"><tt class="xref docutils literal">return</tt></a> and <a class="reference internal" href="#yield"><tt class="xref docutils literal">yield</tt></a> statements may not be used outside of
function definitions even within the context of code passed to the
<a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a> statement.
In all cases, if the optional parts are omitted, the code is executed in the
current scope. If only the first expression after <a class="reference external" href="expressions.html#in"><tt class="xref docutils literal">in</tt></a> is specified,
it should be a dictionary, which will be used for both the global and the local
variables. If two expressions are given, they are used for the global and local
variables, respectively. If provided, locals can be any mapping object.

Changed in version 2.4: Formerly, locals was required to be a dictionary.
As a side effect, an implementation may insert additional keys into the
dictionaries given besides those corresponding to variable names set by the
executed code. For example, the current implementation may add a reference to
the dictionary of the built-in module <a title="The module that provides the built-in namespace." class="reference external" href="../library/__builtin__.html#module-__builtin__"><tt class="xref docutils literal">__builtin__</tt></a> under the key
<tt class="docutils literal">__builtins__</tt> (!).
Programmer&#8217;s hints: dynamic evaluation of expressions is supported by the
built-in function <a title="eval" class="reference external" href="../library/functions.html#eval"><tt class="xref docutils literal">eval()</tt></a>. The built-in functions <a title="globals" class="reference external" href="../library/functions.html#globals"><tt class="xref docutils literal">globals()</tt></a> and
<a title="locals" class="reference external" href="../library/functions.html#locals"><tt class="xref docutils literal">locals()</tt></a> return the current global and local dictionary, respectively,
which may be useful to pass around for use by <a class="reference internal" href="#exec"><tt class="xref docutils literal">exec</tt></a>.
Footnotes
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[1]</a></td><td>Note that the parser only accepts the Unix-style end of line convention.
If you are reading the code from a file, make sure to use universal
newline mode to convert Windows or Mac-style newlines.</td></tr>
</tbody>
</table>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference external" href="#">6. Simple statements</a><ul>
<li><a class="reference external" href="#expression-statements">6.1. Expression statements</a></li>
<li><a class="reference external" href="#assignment-statements">6.2. Assignment statements</a><ul>
<li><a class="reference external" href="#augmented-assignment-statements">6.2.1. Augmented assignment statements</a></li>
</ul>
</li>
<li><a class="reference external" href="#the-assert-statement">6.3. The <tt class="docutils literal">assert</tt> statement</a></li>
<li><a class="reference external" href="#the-pass-statement">6.4. The <tt class="docutils literal">pass</tt> statement</a></li>
<li><a class="reference external" href="#the-del-statement">6.5. The <tt class="docutils literal">del</tt> statement</a></li>
<li><a class="reference external" href="#the-print-statement">6.6. The <tt class="docutils literal">print</tt> statement</a></li>
<li><a class="reference external" href="#the-return-statement">6.7. The <tt class="docutils literal">return</tt> statement</a></li>
<li><a class="reference external" href="#the-yield-statement">6.8. The <tt class="docutils literal">yield</tt> statement</a></li>
<li><a class="reference external" href="#the-raise-statement">6.9. The <tt class="docutils literal">raise</tt> statement</a></li>
<li><a class="reference external" href="#the-break-statement">6.10. The <tt class="docutils literal">break</tt> statement</a></li>
<li><a class="reference external" href="#the-continue-statement">6.11. The <tt class="docutils literal">continue</tt> statement</a></li>
<li><a class="reference external" href="#the-import-statement">6.12. The <tt class="docutils literal">import</tt> statement</a><ul>
<li><a class="reference external" href="#future-statements">6.12.1. Future statements</a></li>
</ul>
</li>
<li><a class="reference external" href="#the-global-statement">6.13. The <tt class="docutils literal">global</tt> statement</a></li>
<li><a class="reference external" href="#the-exec-statement">6.14. The <tt class="docutils literal">exec</tt> statement</a></li>
</ul>
</li>
</ul>

<h4>Previous topic</h4>
 <a href="expressions.html"
 title="previous chapter">5. Expressions</a>
 <h4>Next topic</h4>
 <a href="compound_stmts.html"
 title="next chapter">7. Compound statements</a>
<h3>This Page</h3>
<ul class="this-page-menu">
 <li><a href="../bugs.html">Report a Bug</a></li>
 <li><a href="../_sources/reference/simple_stmts.txt"
 rel="nofollow">Show Source</a></li>
</ul>

<div id="searchbox" style="display: none">
 <h3>Quick search</h3>
 <form class="search" action="../search.html" method="get">
 <input type="text" name="q" size="18" />
 <input type="submit" value="Go" />
 <input type="hidden" name="check_keywords" value="yes" />
 <input type="hidden" name="area" value="default" />
 </form>
 
 Enter search terms or a module, class or function name.
 
 </div>
 <script type="text/javascript">$('#searchbox').show(0);</script>
 </div>
 </div>
 <div class="clearer"></div>
 </div>
 <div class="related">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 >index</a></li>
 <li class="right" >
 <a href="../modindex.html" title="Global Module Index"
 >modules</a> |</li>
 <li class="right" >
 <a href="compound_stmts.html" title="7. Compound statements"
 >next</a> |</li>
 <li class="right" >
 <a href="expressions.html" title="5. Expressions"
 >previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="../index.html">Python v2.6.6 documentation</a> &raquo;</li>

<li><a href="index.html" >The Python Language Reference</a> &raquo;</li> 
 </ul>
 </div>
 <div class="footer">
 &copy; <a href="../copyright.html">Copyright</a> 1990-2011, Python Software Foundation.
 
 The Python Software Foundation is a non-profit corporation. 
 <a href="http://www.python.org/psf/donations/">Please donate.</a>
 
 Last updated on Jul 20, 2011.
 <a href="../bugs.html">Found a bug</a>?
 
 Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 0.6.6.
 </div>

</body>
</html>

Back to File Manager