空列表的正确类型提示是什么? [英] What is the correct type hint for an empty list?

问题描述

x = [] 的正确类型提示是什么?

What is the correct type hint for x = []?

我的 PyCharm 编辑器中的类型检查器将此标记为错误:

The type checker in my PyCharm editor flagged this as an error:

labelframes: List[ttk.LabelFrame] = []

'Optional' 不是一个选项:

'Optional' is not an option as in:

labelframes: List[Optional[ttk.LabelFrame]] = []

因为 typing.Optional 的文档说明这等效于:

because the documentation for typing.Optional states that this is equivalent to:

labelframes: List[Union[ttk.LabelFrame, None]] = []

和 [None] 不是 [].

我应该提到 PyCharm 也不喜欢这样:

I ought to mention that PyCharm doesn't like this either:

labelframes: List[Union[ttk.LabelFrame, None]] = [None]

无论我尝试什么类型的提示.PyCharm 将其标记为错误，希望返回我的类型提示，但没有返回，"所以我尝试了:

Whatever type hint I try. PyCharm flags it as an error with, "Expected to return my type hint here, got no return," so I tried:

labelframes: Optional[List[ttk.LabelFrame, None]] = []

那没有用.

我知道 PEP 526 有许多遵循这种模式的示例:

I am aware that PEP 526 has numerous examples which follow the pattern:

x: List[str] = []

推荐答案

查看 PEP 484，特别是 关于类型注释的部分(变量注释的前身)这确实看起来像是 PyCharm 的检查器的一个错误.

Looking at PEP 484, specifically the section on type comments (the precursor to variable annotations) this does indeed seem like a bug with PyCharm's checker.

引自 PEP:

在非存根代码中，也有类似的特例:

In non-stub code, there is a similar special case:

from typing import IO

stream = None  # type: IO[str]

类型检查器不应对此抱怨(尽管值 None 与给定类型不匹配)，也不应将推断类型更改为 Optional[...](尽管规则对默认值为 None 的带注释的参数执行此操作).这里的假设是其他代码将确保变量被赋予了正确类型的值，并且所有使用都可以假设变量具有给定类型.

Type checkers should not complain about this (despite the value None not matching the given type), nor should they change the inferred type to Optional[...] (despite the rule that does this for annotated arguments with a default value of None ). The assumption here is that other code will ensure that the variable is given a value of the proper type, and all uses can assume that the variable has the given type.

因此，以类似的方式，类型检查器(如 mypy 当前所做的那样)不应该抱怨您提供的初始化值与注释不严格匹配的事实.

So, in a similar fashion, type-checkers (as mypy currently does), should not complain about the fact that the initializing value you provide doesn't strictly match the annotation.

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