__future__ — 未来的语句定义 ¶

源代码： Lib/__future__.py

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Imports of the form from __future__ import feature are called 未来语句 . 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:

• 为避免混淆现有工具，分析 import 语句并期望找到正导入它们的模块。

• 要文档当引入不兼容改变时，和当它们将是 — 或是 — 强制性的。这是可执行文档编制形式，和能以编程方式审查凭借导入 __future__ 并审查其内容。

• 要确保 未来语句 run under releases prior to Python 2.1 at least yield runtime exceptions (the import of __future__ 将失败，因为 2.1 之前没有该名称的模块)。

模块内容 ¶

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
annotations	3.7.0b1	TBD [ 1 ]	PEP 563 : Postponed evaluation of annotations

class __future__. _Feature ¶

每条语句在 __future__.py 的形式：

FeatureName = _Feature(OptionalRelease, MandatoryRelease,
                       CompilerFlag)