2 :mod:`__future__` --- Future statement definitions
3 ==================================================
6 :synopsis: Future statement definitions
9 :mod:`__future__` is a real module, and serves three purposes:
11 * To avoid confusing existing tools that analyze import statements and expect to
12 find the modules they're importing.
14 * To ensure that future_statements run under releases prior to 2.1 at least
15 yield runtime exceptions (the import of :mod:`__future__` will fail, because
16 there was no module of that name prior to 2.1).
18 * To document when incompatible changes were introduced, and when they will be
19 --- or were --- made mandatory. This is a form of executable documentation, and
20 can be inspected programatically via importing :mod:`__future__` and examining
23 Each statement in :file:`__future__.py` is of the form::
25 FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ","
29 where, normally, *OptionalRelease* is less than *MandatoryRelease*, and both are
30 5-tuples of the same form as ``sys.version_info``::
32 (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
33 PY_MINOR_VERSION, # the 1; an int
34 PY_MICRO_VERSION, # the 0; an int
35 PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
36 PY_RELEASE_SERIAL # the 3; an int
39 *OptionalRelease* records the first release in which the feature was accepted.
41 In the case of a *MandatoryRelease* that has not yet occurred,
42 *MandatoryRelease* predicts the release in which the feature will become part of
45 Else *MandatoryRelease* records when the feature became part of the language; in
46 releases at or after that, modules no longer need a future statement to use the
47 feature in question, but may continue to use such imports.
49 *MandatoryRelease* may also be ``None``, meaning that a planned feature got
52 Instances of class :class:`_Feature` have two corresponding methods,
53 :meth:`getOptionalRelease` and :meth:`getMandatoryRelease`.
55 *CompilerFlag* is the (bitfield) flag that should be passed in the fourth
56 argument to the builtin function :func:`compile` to enable the feature in
57 dynamically compiled code. This flag is stored in the :attr:`compiler_flag`
58 attribute on :class:`_Feature` instances.
60 No feature description will ever be deleted from :mod:`__future__`.