isinstance(x, NoneType) doesn't get checked if the type annotation is None

[strny0]

Bug Report

isinstance check of mypy doesn't detect instances of NoneType (actual None), doesn't visit/check branches checked of isinstance() conditions.

Expected Behavior

from types import NoneType

def f(x: str | None):
    if isinstance(x, NoneType):
        reveal_type(x) # N: Revealed type is "None"
    elif isinstance(x, str):
        reveal_type(x) # N: Revealed type is "builtins.str"
    reveal_type(x) # N: Revealed type is "Union[builtins.str, None]"

Actual Behavior

from types import NoneType

def f(x: str | None):
    if isinstance(x, NoneType):
        reveal_type(x)
    elif isinstance(x, str):
        reveal_type(x) # N: Revealed type is "builtins.str"
    reveal_type(x) # N: Revealed type is "Union[builtins.str, None]"

Your Environment

• Mypy version used: Any version that implements PR Treating NoneType as None and warning against using NoneType. #13153
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10
• Operating system and version: Linux 5.15.0-41-generic 20.04.1-Ubuntu x86_64

[erictraut]

This is a pretty atypical way to check for None. The more idiomatic (and easier-to-read) approach takes advantage of the fact that None is a singleton and cannot be subclassed.

def f(x: str | None):
    if x is None:
        ...

[KotlinIsland]

• This would fix it: Use _typeshed.NoneType for the type of None #11287

[tyralla]

@strny0 We implemented this in Mypy 1.9, but I wasn't aware of this issue. I think it can be closed.

[JelleZijlstra]

Confirmed mypy's behavior is now the expected behavior in the OP.