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Fix type narrowing with not isinstance(x, cls) in classmethods #21312

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Fix type narrowing with not isinstance(x, cls) in classmethods #21312

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1c65fb6
Fix type narrowing with not isinstance(x, cls) in classmethods
sanyam-brudite Apr 24, 2026
777bb73
Fix redundant get_proper_type call in checker.py
sanyam-brudite Apr 24, 2026
09dbe18
Merge branch 'master' into fix-issue-21271
sanyamk23 Apr 24, 2026
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7 changes: 5 additions & 2 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8173,10 +8173,13 @@ def get_type_range_of_type(self, typ: Type) -> TypeRange | None:
# Type[A] means "any type that is a subtype of A" rather than "precisely type A"
# we indicate this by setting is_upper_bound flag
is_upper_bound = True
if isinstance(typ.item, NoneType):
item = typ.item
if isinstance(item, TypeVarType) and item.id.is_self():
is_upper_bound = False
elif isinstance(item, NoneType):
# except for Type[None], because "'NoneType' is not an acceptable base type"
is_upper_bound = False
if isinstance(typ.item, Instance) and typ.item.type.is_final:
elif isinstance(item, Instance) and item.type.is_final:
is_upper_bound = False
return TypeRange(typ.item, is_upper_bound=is_upper_bound)
if isinstance(typ, AnyType):
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13 changes: 13 additions & 0 deletions test-data/unit/check-isinstance.test
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Expand Up @@ -3322,3 +3322,16 @@ def f2(x: A | None, t: type[A]):
else:
reveal_type(x) # N: Revealed type is "None"
[builtins fixtures/isinstancelist.pyi]

[case testNarrowingSelfInClassMethod]
# flags: --warn-unreachable
from typing import Self, Union

class Foo:
@classmethod
def foo(cls, x: Union[Self, float]) -> None:
if isinstance(x, cls):
reveal_type(x) # N: Revealed type is "Self`0"
else:
reveal_type(x) # N: Revealed type is "builtins.float"
[builtins fixtures/isinstancelist.pyi]
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