29.10. `future` — 未来的语句定义 ¶

__future__ 是真实模块，且服务 3 种目的：

To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.
要确保未来语句 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.

每条语句在 __future__.py 的形式：

FeatureName = _Feature(OptionalRelease, MandatoryRelease,
                       CompilerFlag)

通常，其中 OptionalRelease 小于 MandatoryRelease ，且两者是相同形式的 5 元组如 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 记录接受特征的第一个发行。

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 还可以为 None ，意味着计划特征被丢弃。

实例化的类 _Feature 拥有 2 相应方法， 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 实例。

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

另请参阅

未来语句: 编译器如何对待未来导入。

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29.10. __future__ — 未来的语句定义 ¶

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29.10. `future` — 未来的语句定义 ¶