Python 3.6.5 Documentation > "__future__" — Future statement definitions
"__future__" — Future statement definitions *******************************************
**Source code:** Lib/__future__.py
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"__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 future statements run under releases prior to 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).
* 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 "__future__" and examining its contents.
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* | +--------------------+---------------+----------------+-----------------------------------------------+ | generator_stop | 3.5.0b1 | 3.7 | **PEP 479**: *StopIteration handling inside | | | | | generators* | +--------------------+---------------+----------------+-----------------------------------------------+
See also:
Future statements How the compiler treats future imports.
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