Commit | Line | Data |
---|---|---|
920dae64 AT |
1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> |
2 | <html> | |
3 | <head> | |
4 | <link rel="STYLESHEET" href="ref.css" type='text/css' /> | |
5 | <link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" /> | |
6 | <link rel='start' href='../index.html' title='Python Documentation Index' /> | |
7 | <link rel="first" href="ref.html" title='Python Reference Manual' /> | |
8 | <link rel='contents' href='contents.html' title="Contents" /> | |
9 | <link rel='index' href='genindex.html' title='Index' /> | |
10 | <link rel='last' href='about.html' title='About this document...' /> | |
11 | <link rel='help' href='about.html' title='About this document...' /> | |
12 | <link rel="next" href="raise.html" /> | |
13 | <link rel="prev" href="return.html" /> | |
14 | <link rel="parent" href="simple.html" /> | |
15 | <link rel="next" href="raise.html" /> | |
16 | <meta name='aesop' content='information' /> | |
17 | <title>6.8 The yield statement </title> | |
18 | </head> | |
19 | <body> | |
20 | <DIV CLASS="navigation"> | |
21 | <div id='top-navigation-panel' xml:id='top-navigation-panel'> | |
22 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
23 | <tr> | |
24 | <td class='online-navigation'><a rel="prev" title="6.7 The return statement" | |
25 | href="return.html"><img src='../icons/previous.png' | |
26 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
27 | <td class='online-navigation'><a rel="parent" title="6. Simple statements" | |
28 | href="simple.html"><img src='../icons/up.png' | |
29 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
30 | <td class='online-navigation'><a rel="next" title="6.9 The raise statement" | |
31 | href="raise.html"><img src='../icons/next.png' | |
32 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
33 | <td align="center" width="100%">Python Reference Manual</td> | |
34 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
35 | href="contents.html"><img src='../icons/contents.png' | |
36 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
37 | <td class='online-navigation'><img src='../icons/blank.png' | |
38 | border='0' height='32' alt='' width='32' /></td> | |
39 | <td class='online-navigation'><a rel="index" title="Index" | |
40 | href="genindex.html"><img src='../icons/index.png' | |
41 | border='0' height='32' alt='Index' width='32' /></A></td> | |
42 | </tr></table> | |
43 | <div class='online-navigation'> | |
44 | <b class="navlabel">Previous:</b> | |
45 | <a class="sectref" rel="prev" href="return.html">6.7 The return statement</A> | |
46 | <b class="navlabel">Up:</b> | |
47 | <a class="sectref" rel="parent" href="simple.html">6. Simple statements</A> | |
48 | <b class="navlabel">Next:</b> | |
49 | <a class="sectref" rel="next" href="raise.html">6.9 The raise statement</A> | |
50 | </div> | |
51 | <hr /></div> | |
52 | </DIV> | |
53 | <!--End of Navigation Panel--> | |
54 | ||
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. | |
131 | </dl> | |
132 | </div> | |
133 | ||
134 | <P> | |
135 | ||
136 | <DIV CLASS="navigation"> | |
137 | <div class='online-navigation'> | |
138 | <p></p><hr /> | |
139 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
140 | <tr> | |
141 | <td class='online-navigation'><a rel="prev" title="6.7 The return statement" | |
142 | href="return.html"><img src='../icons/previous.png' | |
143 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
144 | <td class='online-navigation'><a rel="parent" title="6. Simple statements" | |
145 | href="simple.html"><img src='../icons/up.png' | |
146 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
147 | <td class='online-navigation'><a rel="next" title="6.9 The raise statement" | |
148 | href="raise.html"><img src='../icons/next.png' | |
149 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
150 | <td align="center" width="100%">Python Reference Manual</td> | |
151 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
152 | href="contents.html"><img src='../icons/contents.png' | |
153 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
154 | <td class='online-navigation'><img src='../icons/blank.png' | |
155 | border='0' height='32' alt='' width='32' /></td> | |
156 | <td class='online-navigation'><a rel="index" title="Index" | |
157 | href="genindex.html"><img src='../icons/index.png' | |
158 | border='0' height='32' alt='Index' width='32' /></A></td> | |
159 | </tr></table> | |
160 | <div class='online-navigation'> | |
161 | <b class="navlabel">Previous:</b> | |
162 | <a class="sectref" rel="prev" href="return.html">6.7 The return statement</A> | |
163 | <b class="navlabel">Up:</b> | |
164 | <a class="sectref" rel="parent" href="simple.html">6. Simple statements</A> | |
165 | <b class="navlabel">Next:</b> | |
166 | <a class="sectref" rel="next" href="raise.html">6.9 The raise statement</A> | |
167 | </div> | |
168 | </div> | |
169 | <hr /> | |
170 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
171 | </DIV> | |
172 | <!--End of Navigation Panel--> | |
173 | <ADDRESS> | |
174 | See <i><a href="about.html">About this document...</a></i> for information on suggesting changes. | |
175 | </ADDRESS> | |
176 | </BODY> | |
177 | </HTML> |