从带mypy注释的python函数返回None,多种返回类型 [英] Return None from python function annotated with mypy, multiple return types
问题描述
我来自打字稿背景.我正在将静态类型检查引入到我正在研究的python项目中(使用mypy).
I come from a Typescript background. I'm bringing static type checking into a python project I'm working on (using mypy).
在Typescript中,从带有注释以返回其他内容(即字符串)的函数中返回null是有效的:
In Typescript, it is valid to return null from a function that is annotated to return something else, i.e. a string:
function test(flag: boolean): string {
if(flag) {
return 'success';
} else {
return null;
}
}
注释函数具有多种可能的返回类型(即字符串或布尔值)也是有效的:
It is also valid to annotate your function to have multiple potential return types, i.e. string or boolean:
function test(flag: boolean): string | boolean {
if(flag) {
return 'success';
} else {
return false;
}
}
但是,在使用mypy的python中,我不允许从带有注释以返回 str
的函数中返回None.
But, in python using mypy, I'm disallowed from returning None from a function that is annotated to return str
.
def test(flag: bool) -> str:
if flag:
return 'success'
else:
return None
# [mypy] error:Incompatible return value type (got "None", expected "str")
此外,我看不到注释多种返回类型的方法,即 str |没有
.
Furthermore, I don't see a way to annotate multiple return types, i.e. str | None
.
我应该如何使用mypy处理类似的事情?从错误状态返回None的函数遍布我的代码库.
How should I approach something like this using mypy? Functions that return None from the error state are all over my codebase.
推荐答案
好吧,感谢mypy gitter上的@zsol,我发现文档中缺少的内容!
Okay, I found what I was missing in the documentation thanks to @zsol on the mypy gitter!
mypy的两个有用功能是Optional和Union类型,可以从python的输入模块中导入它们.此处的文档.
Two helpful mypy features are the Optional and Union types that can be imported from python's typing module. Documentation here.
如果要注释的是,该函数除主要类型外还可能返回None,例如 str
,使用 Optional
:
If you want to annotate that the function can potentially return None in addition to the primary type, e.g. str
, use Optional
:
from typing import Optional
def test(flag: bool) -> Optional[str]:
if flag:
return 'success'
else:
return None
如果要注释该函数可能返回多个类型,例如 str |bool
,使用 Union
:
If you want to annotate that the function can potentially return multiple types, e.g. str | bool
, use Union
:
from typing import Union
def test(flag: bool) -> Union[str, bool]:
if flag:
return 'success'
else:
return False
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