Python 如何使用类型提示指定“可为空”返回类型
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How to specify "nullable" return type with type hints
提问by exfizik
Suppose I have a function:
假设我有一个函数：
def get_some_date(some_argument: int=None) -> %datetime_or_None%:
if some_argument is not None and some_argument == 1:
return datetime.utcnow()
else:
return None
How do I specify the return type for something that can be None?
我如何为可以是的东西指定返回类型None？
回答by Dimitris Fasarakis Hilliard
You're looking for Optional.
您正在寻找Optional.
Since your return type can either be datetime(as returned from datetime.utcnow()) or Noneyou should use Optional[datetime]:
由于您的返回类型可以是datetime（从 返回datetime.utcnow()），或者None您应该使用Optional[datetime]：
from typing import Optional
def get_some_date(some_argument: int=None) -> Optional[datetime]:
# as defined
From the documentation on typing, Optionalis shorthand for:
从打字文档中，Optional是简写：
Optional[X]is equivalent toUnion[X, None].
Optional[X]相当于Union[X, None]。
where Union[X, Y]means a value of type Xor Y.
whereUnion[X, Y]表示类型为Xor的值Y。
If you want to be explicit due to concerns that others might stumble on Optionaland not realize it's meaning, you could always use Union:
如果您因为担心其他人可能会偶然发现Optional而没有意识到它的含义而想要明确，您可以始终使用Union：
from typing import Union
def get_some_date(some_argument: int=None) -> Union[datetime, None]:
But I doubt this is a good idea, Optionalis an indicative name and it does save a couple of keystrokes.
但我怀疑这是一个好主意，Optional是一个指示性名称，它确实节省了几次击键。
As pointed out in the comments by @Michael0x2a Union[T, None]is tranformed to Union[T, type(None)]so no need to use typehere.
正如@Michael0x2a 的评论中指出的那样，Union[T, None]已转换为Union[T, type(None)]因此无需在type此处使用。
Visually these might differ but programatically, in both cases, the result is exactly the same; Union[datetime.datetime, NoneType]will be the type stored in get_some_date.__annotations__*:
从视觉上看，这些可能不同，但在编程上，在这两种情况下，结果完全相同；Union[datetime.datetime, NoneType]将是存储在get_some_date.__annotations__* 中的类型：
>>> from typing import get_type_hints
>>> print(get_type_hints(get_some_date))
{'return': typing.Union[datetime.datetime, NoneType],
'some_argument': typing.Union[int, NoneType]}
*Use typing.get_type_hintsto grab the objects' __annotations__attribute instead of accessing it directly.
*用typing.get_type_hints抢的对象的__annotations__直接访问它的属性来代替。
