Staging
v0.5.1
v0.5.1
https://github.com/python/cpython
Tip revision: 74f4bd53e03ded8408bcc2de67cf0f5a4ac5b1a1 authored by Barry Warsaw on 23 February 2012, 15:59:38 UTC
Bump some more copyright years (as per PEP 101), since this is the first
Bump some more copyright years (as per PEP 101), since this is the first
Tip revision: 74f4bd5
__future__.rst
: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 :ref:`future statements <future>` 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 built-in 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__`. Since its
introduction in Python 2.1 the following features have found their way into the
language using this mechanism:
+------------------+-------------+--------------+---------------------------------------------+
| feature | optional in | mandatory in | effect |
+==================+=============+==============+=============================================+
| nested_scopes | 2.1.0b1 | 2.2 | :pep:`227`: |
| | | | *Statically Nested Scopes* |
+------------------+-------------+--------------+---------------------------------------------+
| generators | 2.2.0a1 | 2.3 | :pep:`255`: |
| | | | *Simple Generators* |
+------------------+-------------+--------------+---------------------------------------------+
| division | 2.2.0a2 | 3.0 | :pep:`238`: |
| | | | *Changing the Division Operator* |
+------------------+-------------+--------------+---------------------------------------------+
| absolute_import | 2.5.0a1 | 2.7 | :pep:`328`: |
| | | | *Imports: Multi-Line and Absolute/Relative* |
+------------------+-------------+--------------+---------------------------------------------+
| with_statement | 2.5.0a1 | 2.6 | :pep:`343`: |
| | | | *The "with" Statement* |
+------------------+-------------+--------------+---------------------------------------------+
| print_function | 2.6.0a2 | 3.0 | :pep:`3105`: |
| | | | *Make print a function* |
+------------------+-------------+--------------+---------------------------------------------+
| unicode_literals | 2.6.0a2 | 3.0 | :pep:`3112`: |
| | | | *Bytes literals in Python 3000* |
+------------------+-------------+--------------+---------------------------------------------+
.. seealso::
:ref:`future`
How the compiler treats future imports.