How to specify “nullable” return type with type hints

Question

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?

Answer 1

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 to Union[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.