Python 3 type hinting for None?

Question

def foo(
        hello: str='world', bar: str=None,
        another_string_or_None: str|????=None):
    pass

I'm trying to set a type hint in Python in a function, you can add more than one type hint with something: str|bool='default value', but, what are the type hinting for None? :/

Answer 1

From your example:

def foo(
        hello: str='world', bar: str=None,
        another_string_or_None: str|????=None):
    ...

I've noticed that your use case is "something or None".

Since version 3.5, Python supports type annotations via typing module. And in your case, the recommended way of annotating is by using typing.Optional[something] hint. This has exact meaning you're looking for.

Therefore the hint for another_string_or_None would be:

import typing

def foo(
        hello: str='world', bar: str=None,
        another_string_or_None: typing.Optional[str]=None):
    ...

Answer 2

It's just None!

>>> def nothing(nun: None) -> None:
...     return nun
... 
>>> nothing(None)
>>> 

Or at least, it can be.

Since these annotations are meaningless to Python beyond being in/correct syntax, it's sort of up to the tools.

If you use typecheck-decorator for example, then you'll need to use type(None):

>>> import typecheck as tc
>>>
>>> @tc.typecheck
>>> def nothing(nun: type(None)) -> type(None):
...     return nun
... 
>>> nothing(None)
>>> nothing(0)
typecheck.framework.InputParameterError: nothing() has got an incompatible value for nun: 0
>>> nothing(False)
typecheck.framework.InputParameterError: nothing() has got an incompatible value for nun: False

Typecheck also allows you to somewhat more clearly "add more than one type hint with" with tc.any() (OR), tc.all() (AND), and far more besides.

Beware that tc.none() is a NAND-like predicate; not what you are looking for - with no arguments it will accept any type, equivalent to tc.all() or the more apt tc.anything.

Answer 3

Python 3.10 will support your original desired notation: str | None.

Source

Answer 4

I know this question is considered answered thanks to @mbdevpl, however, I've wanted to add that type(None) is how you get the actual for None type, this can be useful for example in an if statement check like:

if isinstance(x_var, type(None)):
    pass

and since python3.5, you can also use do Union of a bunch of types with None as shown here:

x_var: typing.Union[str, None]
y_var: typing.Union[Dict, List, None]

this is equivalent to:

x_var: typing.Optional[str]
y_var: typing.Optional[typing.Union[Dict, List]]