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| 54 | <H1><A NAME="SECTION0053400000000000000000"> | |
| 55 | 3.34 <tt class="module">__future__</tt> -- | |
| 56 | Future statement definitions</A> | |
| 57 | </H1> | |
| 58 | ||
| 59 | <P> | |
| 60 | <A NAME="module-future"></A> | |
| 61 | ||
| 62 | <P> | |
| 63 | <tt class="module">__future__</tt> is a real module, and serves three purposes: | |
| 64 | ||
| 65 | <P> | |
| 66 | ||
| 67 | <UL> | |
| 68 | <LI>To avoid confusing existing tools that analyze import statements | |
| 69 | and expect to find the modules they're importing. | |
| 70 | ||
| 71 | <P> | |
| 72 | </LI> | |
| 73 | <LI>To ensure that future_statements run under releases prior to 2.1 | |
| 74 | at least yield runtime exceptions (the import of | |
| 75 | <tt class="module">__future__</tt> will fail, because there was no module of | |
| 76 | that name prior to 2.1). | |
| 77 | ||
| 78 | <P> | |
| 79 | </LI> | |
| 80 | <LI>To document when incompatible changes were introduced, and when they | |
| 81 | will be -- or were -- made mandatory. This is a form of executable | |
| 82 | documentation, and can be inspected programatically via importing | |
| 83 | <tt class="module">__future__</tt> and examining its contents. | |
| 84 | ||
| 85 | <P> | |
| 86 | </LI> | |
| 87 | </UL> | |
| 88 | ||
| 89 | <P> | |
| 90 | Each statement in <span class="file">__future__.py</span> is of the form: | |
| 91 | ||
| 92 | <P> | |
| 93 | <div class="verbatim"><pre><TT> | |
| 94 | FeatureName = "_Feature(" <var>OptionalRelease</var> "," <var>MandatoryRelease</var> "," | |
| 95 | <var>CompilerFlag</var> ")" | |
| 96 | </TT></pre></div> | |
| 97 | ||
| 98 | <P> | |
| 99 | where, normally, <var>OptionalRelease</var> is less than | |
| 100 | <var>MandatoryRelease</var>, and both are 5-tuples of the same form as | |
| 101 | <code>sys.version_info</code>: | |
| 102 | ||
| 103 | <P> | |
| 104 | <div class="verbatim"><pre> | |
| 105 | (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int | |
| 106 | PY_MINOR_VERSION, # the 1; an int | |
| 107 | PY_MICRO_VERSION, # the 0; an int | |
| 108 | PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string | |
| 109 | PY_RELEASE_SERIAL # the 3; an int | |
| 110 | ) | |
| 111 | </pre></div> | |
| 112 | ||
| 113 | <P> | |
| 114 | <var>OptionalRelease</var> records the first release in which the feature | |
| 115 | was accepted. | |
| 116 | ||
| 117 | <P> | |
| 118 | In the case of a <var>MandatoryRelease</var> that has not yet occurred, | |
| 119 | <var>MandatoryRelease</var> predicts the release in which the feature will | |
| 120 | become part of the language. | |
| 121 | ||
| 122 | <P> | |
| 123 | Else <var>MandatoryRelease</var> records when the feature became part of | |
| 124 | the language; in releases at or after that, modules no longer need a | |
| 125 | future statement to use the feature in question, but may continue to | |
| 126 | use such imports. | |
| 127 | ||
| 128 | <P> | |
| 129 | <var>MandatoryRelease</var> may also be <code>None</code>, meaning that a planned | |
| 130 | feature got dropped. | |
| 131 | ||
| 132 | <P> | |
| 133 | Instances of class <tt class="class">_Feature</tt> have two corresponding methods, | |
| 134 | <tt class="method">getOptionalRelease()</tt> and <tt class="method">getMandatoryRelease()</tt>. | |
| 135 | ||
| 136 | <P> | |
| 137 | <var>CompilerFlag</var> is the (bitfield) flag that should be passed in the | |
| 138 | fourth argument to the builtin function <tt class="function">compile()</tt> to enable | |
| 139 | the feature in dynamically compiled code. This flag is stored in the | |
| 140 | <tt class="member">compiler_flag</tt> attribute on <tt class="class">_Future</tt> instances. | |
| 141 | ||
| 142 | <P> | |
| 143 | No feature description will ever be deleted from <tt class="module">__future__</tt>. | |
| 144 | ||
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