mirror of
https://github.com/python/cpython.git
synced 2024-12-12 19:33:52 +08:00
96 lines
4.8 KiB
ReStructuredText
96 lines
4.8 KiB
ReStructuredText
:mod:`__future__` --- Future statement definitions
|
|
==================================================
|
|
|
|
.. module:: __future__
|
|
:synopsis: Future statement definitions
|
|
|
|
**Source code:** :source:`Lib/__future__.py`
|
|
|
|
--------------
|
|
|
|
: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 :data:`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 | 3.0 | :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.
|