__future__ — Future statement definitions¶
Source code:Lib/__future__.py
Imports of the formfrom__future__importfeature are calledfuture statements. These are special-cased by the Python compilerto allow the use of new Python features in modules containing the future statementbefore the release in which the feature becomes standard.
While these future statements are given additional special meaning by thePython compiler, they are still executed like any other import statement andthe__future__ exists and is handled by the import system the same wayany other Python module would be. This design serves three purposes:
To avoid confusing existing tools that analyze import statements and expect tofind the modules they’re importing.
To document when incompatible changes were introduced, and when they will be— or were — made mandatory. This is a form of executable documentation, andcan be inspected programmatically via importing
__future__and examiningits contents.To ensure thatfuture statements run under releases prior toPython 2.1 at least yield runtime exceptions (the import of
__future__will fail, because there was no module of that name prior to 2.1).
Module Contents¶
No feature description will ever be deleted from__future__. Since itsintroduction in Python 2.1 the following features have found their way into thelanguage 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 |
- class__future__._Feature¶
Each statement in
__future__.pyis of the form:FeatureName=_Feature(OptionalRelease,MandatoryRelease,CompilerFlag)
where, normally,OptionalRelease is less thanMandatoryRelease, and both are5-tuples of the same form as
sys.version_info:(PY_MAJOR_VERSION,# the 2 in 2.1.0a3; an intPY_MINOR_VERSION,# the 1; an intPY_MICRO_VERSION,# the 0; an intPY_RELEASE_LEVEL,# "alpha", "beta", "candidate" or "final"; stringPY_RELEASE_SERIAL# the 3; an int)
- _Feature.getOptionalRelease()¶
OptionalRelease records the first release in which the feature was accepted.
- _Feature.getMandatoryRelease()¶
In the case of aMandatoryRelease that has not yet occurred,MandatoryRelease predicts the release in which the feature will become part ofthe language.
ElseMandatoryRelease records when the feature became part of the language; inreleases at or after that, modules no longer need a future statement to use thefeature in question, but may continue to use such imports.
MandatoryRelease may also be
None, meaning that a planned feature gotdropped or that it is not yet decided.
- _Feature.compiler_flag¶
CompilerFlag is the (bitfield) flag that should be passed in the fourthargument to the built-in function
compile()to enable the feature indynamically compiled code. This flag is stored in the_Feature.compiler_flagattribute on_Featureinstances.
from__future__importannotations was previously scheduled tobecome mandatory in Python 3.10, but the Python Steering Counciltwice decided to delay the change(announcement for Python 3.10;announcement for Python 3.11).No final decision has been made yet. See alsoPEP 563 andPEP 649.
See also
- Future statements
How the compiler treats future imports.
- PEP 236 - Back to the __future__
The original proposal for the __future__ mechanism.