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Cover common isinstance calling patterns with some overloads #15954

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jonathandung Jun 27, 2026
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jonathandung Jun 27, 2026
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pre-commit-ci[bot] Jun 27, 2026
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jonathandung Oct 10, 2026
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[pre-commit.ci] auto fixes from pre-commit.com hooks
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jonathandung Oct 11, 2026
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9 changes: 9 additions & 0 deletions stdlib/@tests/test_cases/builtins/check_isinstance.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
from typing import Any, cast
from typing_extensions import TypeIs, assert_type

o = cast(Any, "")
assert_type(isinstance(o, int), TypeIs[int])

Check failure on line 5 in stdlib/@tests/test_cases/builtins/check_isinstance.py

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GitHub Actions / pyright: regression tests (Linux, 3.13) 

"assert_type" mismatch: expected "TypeIs[int]" but received "TypeIs[int]" (reportAssertTypeFailure)

@JelleZijlstra JelleZijlstra Oct 10, 2026

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These tests are failing and they do not prove that the revised stubs help with reasonable usage patterns. The issue talks about patterns like .filter(lambda x: isinstance(x, int) where presumably there's some TypeIs on the .filter(), that's what we need to prove is actually improved by this.

@jonathandung jonathandung Oct 11, 2026

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Understood. I will continue to work on the tests later.

assert_type(isinstance(o, object), TypeIs[object])

Check failure on line 6 in stdlib/@tests/test_cases/builtins/check_isinstance.py

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GitHub Actions / pyright: regression tests (Linux, 3.13) 

"assert_type" mismatch: expected "TypeIs[object]" but received "TypeIs[object]" (reportAssertTypeFailure)
assert_type(isinstance(o, str), TypeIs[str])

Check failure on line 7 in stdlib/@tests/test_cases/builtins/check_isinstance.py

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GitHub Actions / pyright: regression tests (Linux, 3.13) 

"assert_type" mismatch: expected "TypeIs[str]" but received "TypeIs[str]" (reportAssertTypeFailure)
assert_type(isinstance(o, (bytes, memoryview)), TypeIs[bytes | memoryview[int]])

Check failure on line 8 in stdlib/@tests/test_cases/builtins/check_isinstance.py

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GitHub Actions / pyright: regression tests (Linux, 3.13) 

"assert_type" mismatch: expected "TypeIs[bytes | memoryview[int]]" but received "TypeIs[bytes | memoryview[int]]" (reportAssertTypeFailure)
assert_type(isinstance(o, (set, type[dict])), TypeIs[set[Any] | type[dict[Any, Any]]])

Check failure on line 9 in stdlib/@tests/test_cases/builtins/check_isinstance.py

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GitHub Actions / pyright: regression tests (Linux, 3.13) 

Second argument to "isinstance" must be a class or tuple of classes
  Generic type with type arguments not allowed for instance or class checks (reportArgumentType)

Check failure on line 9 in stdlib/@tests/test_cases/builtins/check_isinstance.py

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GitHub Actions / pyright: regression tests (Linux, 3.13) 

"assert_type" mismatch: expected "TypeIs[set[Any] | type[dict[Any, Any]]]" but received "TypeIs[set[Unknown] | type[dict[Unknown, Unknown]]]" (reportAssertTypeFailure)
20 changes: 20 additions & 0 deletions stdlib/builtins.pyi
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Expand Up @@ -1882,7 +1882,27 @@ def iter(object: Callable[[], _T], sentinel: object, /) -> Iterator[_T]: ...

_ClassInfo: TypeAlias = type | types.UnionType | tuple[_ClassInfo, ...]

@overload
def isinstance(obj: object, class_or_tuple: type[_T] | tuple[type[_T]], /) -> TypeIs[_T]: ...
@overload
def isinstance(obj: object, class_or_tuple: tuple[type[_T], type[_T1]], /) -> TypeIs[_T | _T1]: ...
@overload
def isinstance(obj: object, class_or_tuple: tuple[type[_T], type[_T1], type[_T2]], /) -> TypeIs[_T | _T1 | _T2]: ...
@overload
def isinstance(
obj: object, class_or_tuple: tuple[type[_T], type[_T1], type[_T2], type[_T3]], /
) -> TypeIs[_T | _T1 | _T2 | _T3]: ...
@overload
def isinstance(
obj: object, class_or_tuple: tuple[type[_T], type[_T1], type[_T2], type[_T3], type[_T4]], /
) -> TypeIs[_T | _T1 | _T2 | _T3 | _T4]: ...
@overload
def isinstance(
obj: object, class_or_tuple: tuple[type[_T], type[_T1], type[_T2], type[_T3], type[_T4], type[_T5]], /
) -> TypeIs[_T | _T1 | _T2 | _T3 | _T4 | _T5]: ...
@overload
def isinstance(obj: object, class_or_tuple: _ClassInfo, /) -> bool: ...

def issubclass(cls: type, class_or_tuple: _ClassInfo, /) -> bool: ...
def len(obj: Sized, /) -> int: ...

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