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54 | <H2><A NAME="SECTION0081210000000000000000"></A><A NAME="future"></A> | |
55 | <BR> | |
56 | 6.12.1 Future statements | |
57 | </H2> | |
58 | ||
59 | <P> | |
60 | A <i class="dfn">future statement</i><a id='l2h-556' xml:id='l2h-556'></a> is a directive to | |
61 | the compiler that a particular module should be compiled using syntax | |
62 | or semantics that will be available in a specified future release of | |
63 | Python. The future statement is intended to ease migration to future | |
64 | versions of Python that introduce incompatible changes to the | |
65 | language. It allows use of the new features on a per-module basis | |
66 | before the release in which the feature becomes standard. | |
67 | ||
68 | <P> | |
69 | <dl><dd class="grammar"> | |
70 | <div class="productions"> | |
71 | <table> | |
72 | ||
73 | <tr> | |
74 | <td>future_statement</td> | |
75 | <td>::=</td> | |
76 | <td>"from" "__future__" "import" feature ["as" name] ("," feature ["as" name])*</td></tr> | |
77 | <tr> | |
78 | <td></td> | |
79 | <td></td> | |
80 | <td><code>| "from" "__future__" "import" "(" feature ["as" name] ("," feature ["as" name])* [","] ")"</code></td></tr> | |
81 | <tr> | |
82 | <td>feature</td> | |
83 | <td>::=</td> | |
84 | <td>identifier</td></tr> | |
85 | <tr> | |
86 | <td>name</td> | |
87 | <td>::=</td> | |
88 | <td>identifier</td></tr> | |
89 | </table> | |
90 | </div> | |
91 | </dd></dl> | |
92 | ||
93 | <P> | |
94 | A future statement must appear near the top of the module. The only | |
95 | lines that can appear before a future statement are: | |
96 | ||
97 | <P> | |
98 | ||
99 | <UL> | |
100 | <LI>the module docstring (if any), | |
101 | </LI> | |
102 | <LI>comments, | |
103 | </LI> | |
104 | <LI>blank lines, and | |
105 | </LI> | |
106 | <LI>other future statements. | |
107 | ||
108 | <P> | |
109 | </LI> | |
110 | </UL> | |
111 | ||
112 | <P> | |
113 | The features recognized by Python 2.3 are "<tt class="samp">generators</tt>", | |
114 | "<tt class="samp">division</tt>" and "<tt class="samp">nested_scopes</tt>". "<tt class="samp">generators</tt>" and | |
115 | "<tt class="samp">nested_scopes</tt>" are redundant in 2.3 because they are always | |
116 | enabled. | |
117 | ||
118 | <P> | |
119 | A future statement is recognized and treated specially at compile | |
120 | time: Changes to the semantics of core constructs are often | |
121 | implemented by generating different code. It may even be the case | |
122 | that a new feature introduces new incompatible syntax (such as a new | |
123 | reserved word), in which case the compiler may need to parse the | |
124 | module differently. Such decisions cannot be pushed off until | |
125 | runtime. | |
126 | ||
127 | <P> | |
128 | For any given release, the compiler knows which feature names have been | |
129 | defined, and raises a compile-time error if a future statement contains | |
130 | a feature not known to it. | |
131 | ||
132 | <P> | |
133 | The direct runtime semantics are the same as for any import statement: | |
134 | there is a standard module <tt class="module">__future__</tt>, described later, and | |
135 | it will be imported in the usual way at the time the future statement | |
136 | is executed. | |
137 | ||
138 | <P> | |
139 | The interesting runtime semantics depend on the specific feature | |
140 | enabled by the future statement. | |
141 | ||
142 | <P> | |
143 | Note that there is nothing special about the statement: | |
144 | ||
145 | <P> | |
146 | <div class="verbatim"><pre> | |
147 | import __future__ [as name] | |
148 | </pre></div> | |
149 | ||
150 | <P> | |
151 | That is not a future statement; it's an ordinary import statement with | |
152 | no special semantics or syntax restrictions. | |
153 | ||
154 | <P> | |
155 | Code compiled by an exec statement or calls to the builtin functions | |
156 | <tt class="function">compile()</tt> and <tt class="function">execfile()</tt> that occur in a module | |
157 | <tt class="module">M</tt> containing a future statement will, by default, use the new | |
158 | syntax or semantics associated with the future statement. This can, | |
159 | starting with Python 2.2 be controlled by optional arguments to | |
160 | <tt class="function">compile()</tt> -- see the documentation of that function in the | |
161 | library reference for details. | |
162 | ||
163 | <P> | |
164 | A future statement typed at an interactive interpreter prompt will | |
165 | take effect for the rest of the interpreter session. If an | |
166 | interpreter is started with the <b class="programopt">-i</b> option, is passed a | |
167 | script name to execute, and the script includes a future statement, it | |
168 | will be in effect in the interactive session started after the script | |
169 | is executed. | |
170 | ||
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