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