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[unittest] Allow assertIsNone() and assertIsNotNone() to narrow types #16767

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[unittest] Allow assertIsNone() and assertIsNotNone() to narrow types #16767

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11 changes: 10 additions & 1 deletion stdlib/unittest/case.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -102,8 +102,17 @@ class TestCase:
def assertFalse(self, expr: Any, msg: Any = None) -> None: ...
def assertIs(self, expr1: object, expr2: object, msg: Any = None) -> None: ...
def assertIsNot(self, expr1: object, expr2: object, msg: Any = None) -> None: ...
def assertIsNone(self, obj: object, msg: Any = None) -> None: ...

@overload
def assertIsNone(self, obj: None, msg: Any = None) -> None: ...
@overload
def assertIsNone(self, obj: object, msg: Any = None) -> Never: ...
Comment on lines +106 to +109

@AlexWaygood AlexWaygood Nov 11, 2026 •
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Hmm, I don't think this is correct. None is a subtype of object. If you pass an object of type object, Any, Any | None, or similar into the function, then a type checker will take the second overload and infer Never, but the passed-in object could still easily be None. Here's a demo using ty with your typeshed branch checked out at ../typeshed locally:

% cat foo.py
import unittest

def any_object() -> object:
    return None

reveal_type(unittest.TestCase().assertIsNone(any_object()))
% uvx ty check foo.py --custom-typeshed-dir ../typeshed --ignore=undefined-reveal
info[revealed-type]: Revealed type
 --> foo.py:6:13
  |
6 | reveal_type(unittest.TestCase().assertIsNone(any_object()))
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Never`

Found 1 diagnostic

@AlexWaygood AlexWaygood Nov 11, 2026

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Also note that while it might lead to a type checker making inferences about reachability and control flow if you have an overload returning Never, it won't lead to the type checker successfully narrowing the type. Here, ty infers the type as Never after the call because it deems everything after the call unreachable, rather than narrowing the type to None. Meanwhile, mypy doesn't even see the reveal_type call, because it doesn't emit any diagnostics (including reveal_type diagnostics) in regions of code it deems to be unreachable:

% cat foo.py
import unittest

def any_object() -> object:
    return None

def narrow_please(x: object):
    unittest.TestCase().assertIsNone(x)
    reveal_type(x)
% uvx ty check foo.py --custom-typeshed-dir ../typeshed --ignore=undefined-reveal
info[revealed-type]: Revealed type
 --> foo.py:8:17
  |
8 |     reveal_type(x)
  |                 ^ `Never`

Found 1 diagnostic
% uvx mypy foo.py --custom-typeshed-dir ../typeshed
Success: no issues found in 1 source file


@overload
def assertIsNotNone(self, obj: None, msg: Any = None) -> Never: ...
@overload
def assertIsNotNone(self, obj: object, msg: Any = None) -> None: ...

def assertIn(self, member: Any, container: Iterable[Any] | Container[Any], msg: Any = None) -> None: ...
def assertNotIn(self, member: Any, container: Iterable[Any] | Container[Any], msg: Any = None) -> None: ...
def assertIsInstance(self, obj: object, cls: _ClassInfo, msg: Any = None) -> None: ...
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