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| 55 | <H1><A NAME="SECTION008800000000000000000"></A><A NAME="yield"></A> | |
| 56 | <BR> | |
| 57 | 6.8 The <tt class="keyword">yield</tt> statement | |
| 58 | </H1> | |
| 59 | <a id='l2h-510' xml:id='l2h-510'></a> | |
| 60 | <P> | |
| 61 | <dl><dd class="grammar"> | |
| 62 | <div class="productions"> | |
| 63 | <table> | |
| 64 | <tr> | |
| 65 | <td><a id='tok-yield_stmt' xml:id='tok-yield_stmt'>yield_stmt</a></td> | |
| 66 | <td>::=</td> | |
| 67 | <td>"yield" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a></td></tr> | |
| 68 | </table> | |
| 69 | </div> | |
| 70 | <a class="grammar-footer" | |
| 71 | href="grammar.txt" type="text/plain" | |
| 72 | >Download entire grammar as text.</a> | |
| 73 | </dd></dl> | |
| 74 | ||
| 75 | <P> | |
| 76 | <a id='l2h-512' xml:id='l2h-512'></a> | |
| 77 | <a id='l2h-511' xml:id='l2h-511'></a> | |
| 78 | <P> | |
| 79 | The <tt class="keyword">yield</tt> statement is only used when defining a generator | |
| 80 | function, and is only used in the body of the generator function. | |
| 81 | Using a <tt class="keyword">yield</tt> statement in a function definition is | |
| 82 | sufficient to cause that definition to create a generator function | |
| 83 | instead of a normal function. | |
| 84 | ||
| 85 | <P> | |
| 86 | When a generator function is called, it returns an iterator known as a | |
| 87 | generator iterator, or more commonly, a generator. The body of the | |
| 88 | generator function is executed by calling the generator's | |
| 89 | <tt class="method">next()</tt> method repeatedly until it raises an exception. | |
| 90 | ||
| 91 | <P> | |
| 92 | When a <tt class="keyword">yield</tt> statement is executed, the state of the | |
| 93 | generator is frozen and the value of <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> is | |
| 94 | returned to <tt class="method">next()</tt>'s caller. By ``frozen'' we mean that all | |
| 95 | local state is retained, including the current bindings of local | |
| 96 | variables, the instruction pointer, and the internal evaluation stack: | |
| 97 | enough information is saved so that the next time <tt class="method">next()</tt> is | |
| 98 | invoked, the function can proceed exactly as if the <tt class="keyword">yield</tt> | |
| 99 | statement were just another external call. | |
| 100 | ||
| 101 | <P> | |
| 102 | The <tt class="keyword">yield</tt> statement is not allowed in the <tt class="keyword">try</tt> | |
| 103 | clause of a <tt class="keyword">try</tt> ... <tt class="keyword">finally</tt> construct. The | |
| 104 | difficulty is that there's no guarantee the generator will ever be | |
| 105 | resumed, hence no guarantee that the <tt class="keyword">finally</tt> block will ever | |
| 106 | get executed. | |
| 107 | ||
| 108 | <P> | |
| 109 | <div class="note"><b class="label">Note:</b> | |
| 110 | In Python 2.2, the <tt class="keyword">yield</tt> statement is only allowed | |
| 111 | when the <code>generators</code> feature has been enabled. It will always | |
| 112 | be enabled in Python 2.3. This <code>__future__</code> import statement can | |
| 113 | be used to enable the feature: | |
| 114 | ||
| 115 | <P> | |
| 116 | <div class="verbatim"><pre> | |
| 117 | from __future__ import generators | |
| 118 | </pre></div> | |
| 119 | </div> | |
| 120 | ||
| 121 | <P> | |
| 122 | <div class="seealso"> | |
| 123 | <p class="heading">See Also:</p> | |
| 124 | ||
| 125 | <dl compact="compact" class="seerfc"> | |
| 126 | <dt><a href="http://www.python.org/peps/pep-0255.html" | |
| 127 | title="Simple Generators" | |
| 128 | >PEP 0255, <em>Simple Generators</em></a> | |
| 129 | <dd>The proposal for adding generators and the <tt class="keyword">yield</tt> | |
| 130 | statement to Python. | |
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| 170 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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