Source code: Lib/__future__.py
__future__ is a real module, and serves three purposes:
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.
Instances of class _Feature have two corresponding methods, getOptionalRelease() and getMandatoryRelease().
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 compiler_flag attribute on _Feature instances.
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 “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|