/Doc/library/__future__.rst
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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 programmatically 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__`.
- .. seealso::
- :ref:`future`
- How the compiler treats future imports.