__future__
— 未来的语句定义
¶
源代码： Lib/__future__.py
__future__
是真实模块，且服务 3 种目的：
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。
要确保
未来语句
运行在 2.1 之前的发行下至少产生运行时异常 (导入
__future__
将失败，因为 2.1 之前没有该名称的模块)。
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。
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 |
| annotations | 3.7.0b1 | 3.10 | PEP 563 : Postponed evaluation of annotations |
另请参阅
编译器如何对待未来导入。