Suppose I have a function:
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
?
You're looking for Optional
.
Since your return type can either be datetime
(as returned from datetime.utcnow()
) or None
you should use Optional[datetime]
:
from typing import Optional
def get_some_date(some_argument: int=None) -> Optional[datetime]:
# as defined
From the documentation on typing, Optional
is shorthand for:
Optional[X]
is equivalent toUnion[X, None]
.
where Union[X, Y]
means a value of type X
or Y
.
If you want to be explicit due to concerns that others might stumble on Optional
and not realize it's meaning, you could always use Union
:
from typing import Union
def get_some_date(some_argument: int=None) -> Union[datetime, None]:
But I doubt this is a good idea, Optional
is an indicative name and it does save a couple of keystrokes.
As pointed out in the comments by @Michael0x2a Union[T, None]
is tranformed to Union[T, type(None)]
so no need to use type
here.
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__
*:
>>> 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_hints
to grab the objects' __annotations__
attribute instead of accessing it directly.