cpython/Doc/library/__future__.rst

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:mod:`__future__` --- Future statement definitions
==================================================
.. module:: __future__
:synopsis: Future statement definitions
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**Source code:** :source:`Lib/__future__.py`
--------------
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: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).
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* To document when incompatible changes were introduced, and when they will be
--- or were --- made mandatory. This is a form of executable documentation, and
Merged revisions 64722,64729,64753,64845-64846,64849,64871,64880-64882,64885,64888,64897,64900-64901,64915,64926-64929,64938-64941,64944,64961,64966,64973 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r64722 | georg.brandl | 2008-07-05 12:13:36 +0200 (Sat, 05 Jul 2008) | 4 lines #2663: support an *ignore* argument to shutil.copytree(). Patch by Tarek Ziade. This is a new feature, but Barry authorized adding it in the beta period. ........ r64729 | mark.dickinson | 2008-07-05 13:33:52 +0200 (Sat, 05 Jul 2008) | 5 lines Issue 3188: accept float('infinity') as well as float('inf'). This makes the float constructor behave in the same way as specified by various other language standards, including C99, IEEE 754r, and the IBM Decimal standard. ........ r64753 | gregory.p.smith | 2008-07-06 05:35:58 +0200 (Sun, 06 Jul 2008) | 4 lines - Issue #2862: Make int and float freelist management consistent with other freelists. Changes their CompactFreeList apis into ClearFreeList apis and calls them via gc.collect(). ........ r64845 | raymond.hettinger | 2008-07-10 16:03:19 +0200 (Thu, 10 Jul 2008) | 1 line Issue 3301: Bisect functions behaved badly when lo was negative. ........ r64846 | raymond.hettinger | 2008-07-10 16:34:57 +0200 (Thu, 10 Jul 2008) | 1 line Issue 3285: Fractions from_float() and from_decimal() accept Integral arguments. ........ r64849 | andrew.kuchling | 2008-07-10 16:43:31 +0200 (Thu, 10 Jul 2008) | 1 line Wording changes ........ r64871 | raymond.hettinger | 2008-07-11 14:00:21 +0200 (Fri, 11 Jul 2008) | 1 line Add cautionary note on the use of PySequence_Fast_ITEMS. ........ r64880 | amaury.forgeotdarc | 2008-07-11 23:28:25 +0200 (Fri, 11 Jul 2008) | 5 lines #3317 in zipfile module, restore the previous names of global variables: some applications relied on them. Also remove duplicated lines. ........ r64881 | amaury.forgeotdarc | 2008-07-11 23:45:06 +0200 (Fri, 11 Jul 2008) | 3 lines #3342: In tracebacks, printed source lines were not indented since r62555. #3343: Py_DisplaySourceLine should be a private function. Rename it to _Py_DisplaySourceLine. ........ r64882 | josiah.carlson | 2008-07-12 00:17:14 +0200 (Sat, 12 Jul 2008) | 2 lines Fix for the AttributeError in test_asynchat. ........ r64885 | josiah.carlson | 2008-07-12 01:26:59 +0200 (Sat, 12 Jul 2008) | 2 lines Fixed test for asyncore. ........ r64888 | matthias.klose | 2008-07-12 09:51:48 +0200 (Sat, 12 Jul 2008) | 2 lines - Fix bashisms in Tools/faqwiz/move-faqwiz.sh ........ r64897 | benjamin.peterson | 2008-07-12 22:16:19 +0200 (Sat, 12 Jul 2008) | 1 line fix various doc typos #3320 ........ r64900 | alexandre.vassalotti | 2008-07-13 00:06:53 +0200 (Sun, 13 Jul 2008) | 2 lines Fixed typo. ........ r64901 | benjamin.peterson | 2008-07-13 01:41:19 +0200 (Sun, 13 Jul 2008) | 1 line #1778443 robotparser fixes from Aristotelis Mikropoulos ........ r64915 | nick.coghlan | 2008-07-13 16:52:36 +0200 (Sun, 13 Jul 2008) | 1 line Fix issue 3221 by emitting a RuntimeWarning instead of raising SystemError when the parent module can't be found during an absolute import (likely due to non-PEP 361 aware code which sets a module level __package__ attribute) ........ r64926 | martin.v.loewis | 2008-07-13 22:31:49 +0200 (Sun, 13 Jul 2008) | 2 lines Add turtle into the module index. ........ r64927 | alexandre.vassalotti | 2008-07-13 22:42:44 +0200 (Sun, 13 Jul 2008) | 3 lines Issue #3274: Use a less common identifier for the temporary variable in Py_CLEAR(). ........ r64928 | andrew.kuchling | 2008-07-13 23:43:25 +0200 (Sun, 13 Jul 2008) | 1 line Re-word ........ r64929 | andrew.kuchling | 2008-07-13 23:43:52 +0200 (Sun, 13 Jul 2008) | 1 line Add various items; move ctypes items into a subsection of their own ........ r64938 | andrew.kuchling | 2008-07-14 02:35:32 +0200 (Mon, 14 Jul 2008) | 1 line Typo fixes ........ r64939 | andrew.kuchling | 2008-07-14 02:40:55 +0200 (Mon, 14 Jul 2008) | 1 line Typo fix ........ r64940 | andrew.kuchling | 2008-07-14 03:18:16 +0200 (Mon, 14 Jul 2008) | 1 line Typo fix ........ r64941 | andrew.kuchling | 2008-07-14 03:18:31 +0200 (Mon, 14 Jul 2008) | 1 line Expand the multiprocessing section ........ r64944 | gregory.p.smith | 2008-07-14 08:06:48 +0200 (Mon, 14 Jul 2008) | 7 lines Fix posix.fork1() / os.fork1() to only call PyOS_AfterFork() in the child process rather than both parent and child. Does anyone actually use fork1()? It appears to be a Solaris thing but if Python is built with pthreads on Solaris, fork1() and fork() should be the same. ........ r64961 | jesse.noller | 2008-07-15 15:47:33 +0200 (Tue, 15 Jul 2008) | 1 line multiprocessing/connection.py patch to remove fqdn oddness for issue 3270 ........ r64966 | nick.coghlan | 2008-07-15 17:40:22 +0200 (Tue, 15 Jul 2008) | 1 line Add missing NEWS entry for r64962 ........ r64973 | jesse.noller | 2008-07-15 20:29:18 +0200 (Tue, 15 Jul 2008) | 1 line Revert 3270 patch: self._address is in pretty widespread use, need to revisit ........
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can be inspected programmatically via importing :mod:`__future__` and examining
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its contents.
Each statement in :file:`__future__.py` is of the form::
Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r61687 | jeffrey.yasskin | 2008-03-21 06:02:44 +0100 (Fri, 21 Mar 2008) | 2 lines Speed up test_signal from ~24s to 4s by avoiding nearly all of the sleep calls. ........ r61688 | jeffrey.yasskin | 2008-03-21 06:51:37 +0100 (Fri, 21 Mar 2008) | 5 lines Try to fix test_signal breakages on Linux due to r61687. It appears that at least two of the linux build bots aren't leaving zombie processes around for os.waitpid to wait for, causing ECHILD errors. This would be a symptom of a bug somewhere, but probably not in signal itself. ........ r61696 | georg.brandl | 2008-03-21 15:32:33 +0100 (Fri, 21 Mar 2008) | 2 lines Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. ........ r61700 | georg.brandl | 2008-03-21 18:19:29 +0100 (Fri, 21 Mar 2008) | 2 lines Fix markup. ........ r61704 | jeffrey.yasskin | 2008-03-21 19:25:06 +0100 (Fri, 21 Mar 2008) | 3 lines Try to fix test_signal on FreeBSD. I'm assuming that os.kill is failing to raise a signal, but switching to subprocess makes the code cleaner anyway. ........ r61705 | jeffrey.yasskin | 2008-03-21 19:48:04 +0100 (Fri, 21 Mar 2008) | 7 lines Speed test_threading up from 14s to .5s, and avoid a deadlock on certain failures. The test for enumerate-after-join is now a little less rigorous, but the bug it references says the error happened in the first couple iterations, so 100 iterations should still be enough. cProfile was useful for identifying the slow tests here. ........ r61707 | georg.brandl | 2008-03-21 20:14:38 +0100 (Fri, 21 Mar 2008) | 2 lines Fix a code block in __future__ docs. ........ r61708 | georg.brandl | 2008-03-21 20:20:21 +0100 (Fri, 21 Mar 2008) | 2 lines Add docs for __func__ and __self__ on methods. ........ r61709 | georg.brandl | 2008-03-21 20:37:57 +0100 (Fri, 21 Mar 2008) | 2 lines Add docs for print_function and future_builtins. Fixes #2442. ........ r61711 | georg.brandl | 2008-03-21 20:54:00 +0100 (Fri, 21 Mar 2008) | 2 lines #2136: allow single quotes in realm spec. ........ r61712 | georg.brandl | 2008-03-21 21:01:51 +0100 (Fri, 21 Mar 2008) | 3 lines Issue #2432: give DictReader the dialect and line_num attributes advertised in the docs. ........ r61714 | georg.brandl | 2008-03-21 21:11:46 +0100 (Fri, 21 Mar 2008) | 2 lines #2358: add py3k warning to sys.exc_clear(). ........ r61715 | georg.brandl | 2008-03-21 21:21:46 +0100 (Fri, 21 Mar 2008) | 2 lines #2346/#2347: add py3k warning for __methods__ and __members__. Patch by Jack Diederich. ........ r61716 | georg.brandl | 2008-03-21 21:38:24 +0100 (Fri, 21 Mar 2008) | 2 lines #2348: add py3k warning for file.softspace. ........ r61718 | georg.brandl | 2008-03-21 21:55:20 +0100 (Fri, 21 Mar 2008) | 2 lines #2160: document PyImport_GetImporter. ........ r61719 | georg.brandl | 2008-03-21 21:55:51 +0100 (Fri, 21 Mar 2008) | 2 lines Update doc ACKS. ........ r61720 | steven.bethard | 2008-03-21 22:01:18 +0100 (Fri, 21 Mar 2008) | 1 line Replace hack in regrtest.py with use of sys.py3kwarning. ........ r61721 | georg.brandl | 2008-03-21 22:05:03 +0100 (Fri, 21 Mar 2008) | 2 lines Add missing versionadded tag. ........ r61722 | christian.heimes | 2008-03-22 00:49:44 +0100 (Sat, 22 Mar 2008) | 3 lines Applied patch #1657 epoll and kqueue wrappers for the select module The patch adds wrappers for the Linux epoll syscalls and the BSD kqueue syscalls. Thanks to Thomas Herve and the Twisted people for their support and help. TODO: Finish documentation documentation ........
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FeatureName = _Feature(OptionalRelease, MandatoryRelease,
CompilerFlag)
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where, normally, *OptionalRelease* is less than *MandatoryRelease*, and both are
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5-tuples of the same form as :data:`sys.version_info`::
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(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
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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* |
+------------------+-------------+--------------+---------------------------------------------+
| generator_stop | 3.5.0b1 | 3.7 | :pep:`479`: |
| | | | *StopIteration handling inside generators* |
+------------------+-------------+--------------+---------------------------------------------+
| annotations | 3.7.0b1 | TBD [1]_ | :pep:`563`: |
| | | | *Postponed evaluation of annotations* |
+------------------+-------------+--------------+---------------------------------------------+
.. XXX Adding a new entry? Remember to update simple_stmts.rst, too.
.. [1]
``from __future__ import annotations`` was previously scheduled to
become mandatory in Python 3.10, but the Python Steering Council
twice decided to delay the change
(`announcement for Python 3.10 <https://mail.python.org/archives/list/python-dev@python.org/message/CLVXXPQ2T2LQ5MP2Y53VVQFCXYWQJHKZ/>`__;
`announcement for Python 3.11 <https://mail.python.org/archives/list/python-dev@python.org/message/VIZEBX5EYMSYIJNDBF6DMUMZOCWHARSO/>`__).
No final decision has been made yet. See also :pep:`563` and :pep:`649`.
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.. seealso::
:ref:`future`
How the compiler treats future imports.