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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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