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<title>6.8 The yield statement </title>
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<H1><A NAME="SECTION008800000000000000000"></A><A NAME="yield"></A>
<BR>
6.8 The <tt class="keyword">yield</tt> statement
</H1>
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<P>
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    <td><a id='tok-yield_stmt' xml:id='tok-yield_stmt'>yield_stmt</a></td>
    <td>::=</td>
    <td>"yield" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a></td></tr>
</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
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<P>
The <tt class="keyword">yield</tt> statement is only used when defining a generator
function, and is only used in the body of the generator function.
Using a <tt class="keyword">yield</tt> statement in a function definition is
sufficient to cause that definition to create a generator function
instead of a normal function.

<P>
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's
<tt class="method">next()</tt> method repeatedly until it raises an exception.

<P>
When a <tt class="keyword">yield</tt> statement is executed, the state of the
generator is frozen and the value of <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> is
returned to <tt class="method">next()</tt>'s caller.  By ``frozen'' 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 <tt class="method">next()</tt> is
invoked, the function can proceed exactly as if the <tt class="keyword">yield</tt>
statement were just another external call.

<P>
The <tt class="keyword">yield</tt> statement is not allowed in the <tt class="keyword">try</tt>
clause of a <tt class="keyword">try</tt> ... <tt class="keyword">finally</tt> construct.  The
difficulty is that there's no guarantee the generator will ever be
resumed, hence no guarantee that the <tt class="keyword">finally</tt> block will ever
get executed.

<P>
<div class="note"><b class="label">Note:</b>
In Python 2.2, the <tt class="keyword">yield</tt> statement is only allowed
when the <code>generators</code> feature has been enabled.  It will always
be enabled in Python 2.3.  This <code>__future__</code> import statement can
be used to enable the feature:

<P>
<div class="verbatim"><pre>
from __future__ import generators
</pre></div>
</div>

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seerfc">
    <dt><a href="http://www.python.org/peps/pep-0255.html"
        title="Simple Generators"
        >PEP 0255, <em>Simple Generators</em></a>
    <dd>The proposal for adding generators and the <tt class="keyword">yield</tt>
          statement to Python.
  </dl>
</div>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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