[Python] Cannot use None as TypeVar constraint?

Using TypeVar one can annotate functions like:

def f[T: (str, int)](x:T) -> T:
if isinstance(x, int):
return 0
return x


and type checkers will be happy. However it does not work for a None constraint.

def f[T: (str, None)](x:T) -> T:
if x is None:
return None
return x


gives the error

Type "None" is not assignable to type "T@f".
​

Why is this not possible? Is there a way to achieve this with TypeVars or does one have to use overloads?

The examples used to say return 'a', which would raise a valid error from the type checker. This was an oversight I made while trimming down the example for this question.

Also it is indeed something that I ran into with pyright, not mypy (mypy doesn't complain).

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