__future__ â Future statement definitions¶
Source code: Lib/__future__.py
Imports of the form from __future__ import feature are called future statements. These are special-cased by the Python compiler to allow the use of new Python features in modules containing the future statement before the release in which the feature becomes standard.
While these future statements are given additional special meaning by the Python compiler, they are still executed like any other import statement and the __future__ exists and is handled by the import system the same way any other Python module would be. This design serves three purposes:
To avoid confusing existing tools that analyze import statements and expect to find the modules theyâre importing.
To document when incompatible changes were introduced, and when they will be â or were â made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing __future__ and examining its contents.
To ensure that future statements run under releases prior to Python 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).
No feature description will ever be deleted from __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 âwithâ 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
Each statement in __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 or that it is not yet decided.
CompilerFlag is the (bitfield) flag that should be passed in the fourth argument to the built-in function compile() to enable the feature in dynamically compiled code. This flag is stored in the _Feature.compiler_flag attribute on _Feature instances.
See also
How the compiler treats future imports.
The original proposal for the __future__ mechanism.
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