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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 | ||
145 | <P> | |
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