[spec] Formalize excluded Protocol members
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 35/100
- Issue type
- Documentation
- Clarity
- Mostly clear
- Activity status
- Stale
- Tech stack
- python
- Domain
- documentation
Research direction
Start with the EXCLUDED_ATTRIBUTES definition in Lib/typing.py and compare it with PEP 544, the typing specification, and the linked pyright and mypy examples. Determine which Protocol members require formal specification and document the intended behavior so the differing checker results have a clear resolution.
Written by the indexing model from the issue text.
Description
At runtime, the following members are currently excluded by Protocol: (source)
_TYPING_INTERNALS = frozenset({
'__parameters__', '__orig_bases__', '__orig_class__',
'_is_protocol', '_is_runtime_protocol', '__protocol_attrs__',
'__non_callable_proto_members__', '__type_params__',
})
_SPECIAL_NAMES = frozenset({
'__abstractmethods__', '__annotations__', '__dict__', '__doc__',
'__init__', '__module__', '__new__', '__slots__',
'__subclasshook__', '__weakref__', '__class_getitem__',
'__match_args__', '__static_attributes__', '__firstlineno__',
'__annotate__',
})
# These special attributes will be not collected as protocol members.
EXCLUDED_ATTRIBUTES = _TYPING_INTERNALS | _SPECIAL_NAMES | {'_MutableMapping__marker'}
However, neither PEP 544 nor the typing spec formalize this list, which leads to diverging behavior between different type checkers:
Code sample in pyright playground, mypy playground
from typing import TypeIs, Protocol
class MyProtocol(Protocol):
@classmethod
def __subclasshook__(cls, other: type, /) -> TypeIs[type["MyProtocol"]]:
...
x: MyProtocol = int(1) # mypy: ✅ pyright: ❌
Code sample in pyright playground, mypy playground
from typing import Protocol
class MyProtocol(Protocol):
__abstractmethods__: frozenset[str]
x: MyProtocol = int(1) # mypy: ✅ pyright: ❌
A real-world example where this matters is for instance a Protocol for NamedTuple Instances, that checks whether types.get_original_bases(cls) includes typing.NamedTuple inside __subclasshook__.
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- Python
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