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False positive: Protocol with EXCLUDED_ATTRIBUTES cannot be used with issubclass() #5614
Description
Describe the Bug
from typing import runtime_checkable, Protocol
from abc import abstractmethod
# >>> Current
@runtime_checkable
class Buffer(Protocol):
__slots__ = ()
@abstractmethod
def __buffer__(self, flags: int, /) -> memoryview: ...
x = issubclass(type(bytes()), Buffer) # error, expected good
# >>> Comparison
@runtime_checkable
class BufferExt(Protocol):
@abstractmethod
def __buffer__(self, flags: int, /) -> memoryview: ...
x = issubclass(type(bytes()), BufferExt) # goodPython: 3.12
Sandbox Link
(Only applicable for extension issues) IDE Information
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Activity
amas127 commentedon Nov 11, 2026
Description
When an "
runtime_checkable"Protocoldefines attributes defined inEXCLUDED_ATTRIBUTES(e.g.,__slots__), Pyrefly incorrectly treats them as non-callable / data members of the protocol. As a result, using this protocol withissubclass()triggers a false-positive error.Steps to Reproduce
Shown as in Sandbox Link.
Related
cpython/Lib/typing.py
python/typing #1833
microsoft/pyright #11834
"Should Protocols with empty
__slots__count as Data Protocols?"python/cpython #113320
python/cpython #106363
Expected Behavior
Pyrefly should exclude attribute names included in
EXCLUDED_ATTRIBUTESwhen checking whether a protocol contains non-method members.issubclass(bytes, Buffer)should be accepted without diagnostic errors.Current Behavior
Pyrefly flags
issubclass(bytes, Buffer)with:Additional Context & Behavior
In CPython's
typing.py,__slots__is explicitly listed inEXCLUDED_ATTRIBUTESand filtered out by_get_protocol_attrs(). Consequently,Buffer.__non_callable_proto_members__evaluates toset(), andissubclass(bytes, Buffer)executes cleanly at runtime, returningTrue.Many
@runtime_checkableprotocols in the standard library (such astyping.SupportsInt,typing.SupportsFloat, andcollections.abc.Buffer) define__slots__ = ()by convention, and checkingissubclass(..., SupportsInt)is valid both at runtime and in typeshed.As noted by Guido van Rossum on Discuss Python "Should Protocols with empty
__slots__count as Data Protocols?":Control Cases and Comparisons
__slots__ = ()plus a methodissubclass()succeedstyping.SupportsInt(__slots__ = ()in typeshed)issubclass()succeeds__slots__ = ("value",)plus a methodTypeError: Protocols with non-method members don't support issubclass()value: intTypeError: Protocols with non-method members don't support issubclass()__slots__ = ()plusvalue: intTypeError: Protocols with non-method members don't support issubclass()A correct checker should accept A and B, and report an error for C, D, and E.
The following results were produced with each checker's default settings, except where noted.
TypeErrorbelow refers to the CPython runtime result, not to the checker. A checker never raisesTypeErrorin this comparison; it either emits a diagnostic or does not.none: the checker emits no diagnostic; CPython succeeds.FP: the checker emits a diagnostic; CPython succeeds, so the diagnostic is a false positive.diagnostic: the checker emits a diagnostic; CPython raisesTypeError, so the diagnostic is correct.missed: the checker emits no diagnostic; CPython raisesTypeError, so the checker missed an error.mypy --strict --python-version 3.13 case.py pyright --pythonversion 3.13 --level error case.py basedpyright --pythonversion 3.13 --level error case.py pytype -V 3.12 case.py zuban check --strict case.py ty check case.py pyre check # with source_directories = ["."] pyrefly check case.py --preset strictSupportsIntTypeErrorTypeErrorTypeError--strict)-V 3.12)--strict)--preset strict)amas127 commentedon Nov 11, 2026
#claim
github-actions commentedon Nov 11, 2026
@amas127 you've claimed this issue — it's now assigned to you. Thanks for picking it up! 🎉