Python 3.6.5 Documentation >  "__future__" — Future statement definitions

"__future__" — Future statement definitions
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**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.