NoReturn vs. None in “void"函数 - Python 3.6 中的类型注释 [英] NoReturn vs. None in "void" functions - type annotations in Python 3.6

问题描述

Python 3.6 支持类型注解，比如:

Python 3.6 supports type annotation, like:

def foo() -> int:
    return 42

但是当一个函数没有返回任何东西时，应该使用什么?PEP484 示例大多使用 None 作为返回类型，但也有来自 typing 包的 NoReturn 类型.

But what is expected to use when a function hasn't return anything? PEP484 examples mostly use None as a return type, but there is also NoReturn type from typing package.

因此，问题是什么更可取，什么被认为是最佳实践:

So, the question is what is preferable to use and what is considered a best practice:

def foo() -> None:
    #do smth

或

from typing import NoReturn

def foo() -> NoReturn:
    #do smth

推荐答案

NoReturn 表示函数从不返回值.

函数要么不终止，要么总是抛出异常:"typing 模块提供了一个特殊的类型 NoReturn 来注释永远不会正常返回的函数.例如，一个无条件引发异常的函数.......

from typing import NoReturn

def stop() -> NoReturn:
    raise RuntimeError('no way')

也就是说，x = foo_None() 是类型有效但可疑的，而 x = foo_NoReturn() 是无效的.

That is, x = foo_None() is type-valid but suspect while x = foo_NoReturn() is invalid.

除了从来没有可赋值的结果外，NoReturn 在分支分析中还有其他含义:foo_NoReturn();无法访问...在 'A NoReturn 类型是需要的#165'中有进一步的讨论门票.

Besides never having an assignable result, NoReturn also has other implications in branch analysis: foo_NoReturn(); unreachable... There is further discussion in the 'A NoReturn type is needed #165' ticket.

为了进行分支分析，需要知道哪些调用永远不会正常返回.例如 sys.exit(总是通过异常返回)和 os.exit(从不返回).

In order to perform branch analysis, it is necessary to know which calls will never return normally. Examples are sys.exit (which always returns via exception) and os.exit (which never returns)..

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