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 Py

Answer 1

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.