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gh-74690: typing: Call _get_protocol_attrs and _callable_members_only at protocol class creation time, not during isinstance() checks #103160

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Merged
AlexWaygood merged 9 commits into from
Apr 5, 2023
Merged

gh-74690: typing: Call _get_protocol_attrs and _callable_members_only at protocol class creation time, not during isinstance() checks #103160

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f15c64a
gh-74690: typing: Cache results of `_get_protocol_attrs` and `_callab…
AlexWaygood Mar 31, 2023
f3ec3db
Simplify and inline
AlexWaygood Apr 1, 2023
8e3c6f7
Use a metaclass instance method, not a classmethod on the class
AlexWaygood Apr 1, 2023
a34f700
Move to class-creation time
AlexWaygood Apr 1, 2023
4d05074
Use `__init__`, not `__new__`
AlexWaygood Apr 1, 2023
d3f9ebe
Add missing `super().__init__()` call, to be safe
AlexWaygood Apr 1, 2023
e8a6304
Merge branch 'main' into protocol-attrs-cache-1
AlexWaygood Apr 2, 2023
4816afa
Merge branch 'main' into protocol-attrs-cache-1
AlexWaygood Apr 5, 2023
08af615
Update Lib/typing.py
AlexWaygood Apr 5, 2023
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Use __init__, not __new__
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@AlexWaygood
AlexWaygood committed Apr 1, 2023
commit 4d050749087ea86a7cacc9ce96b867057e655788
4 changes: 1 addition & 3 deletions Lib/typing.py
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Expand Up @@ -1997,15 +1997,13 @@ def _allow_reckless_class_checks(depth=3):
class _ProtocolMeta(ABCMeta):
# This metaclass is really unfortunate and exists only because of
# the lack of __instancehook__.
def __new__(metacls, name, bases, namespace, **kwargs):
cls = super().__new__(metacls, name, bases, namespace, **kwargs)
def __init__(cls, *args, **kwargs):
cls.__protocol_attrs__ = _get_protocol_attrs(cls)
# PEP 544 prohibits using issubclass()
# with protocols that have non-method members.
cls.__callable_proto_members_only__ = all(
callable(getattr(cls, attr, None)) for attr in cls.__protocol_attrs__
)
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@AlexWaygood AlexWaygood Apr 1, 2023 •
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The only reason I'm adding this method here rather than doing this work in Protocol.__init_subclass__ is that it seems slightly more backwards-compatible. With this PR, _ProtcolMeta.__instancecheck__ assumes that all classes with _ProtocolMeta as their metaclass will have a __protocol_attrs__ attribute. Since _ProtocolMeta is an undocumented implementation detail, it should only be Protocol and Protocol subclasses using _ProtocolMeta as their metaclass, and if we could count on that, then it would be safe to do this work in Protocol.__init_subclass__. But it's possible users might have been reaching into the internals of typing.py and creating other classes that use _ProtocolMeta as their metaclass, and I don't want to risk breaking their code unnecessarily.

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@AlexWaygood AlexWaygood Apr 1, 2023

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Confirmed that there's at least a few uses of _ProtocolMeta out in the wild:

https://github.com/antonagestam/phantom-types/blob/68ee857af4452a90a48669a8dbc7d1d7110b31d0/src/phantom/sized.py#L71-L82

https://github.com/protolambda/remerkleable/blob/91ed092d08ef0ba5ab076f0a34b0b371623db728/remerkleable/core.py#L18-L22

https://github.com/InterStella0/stella_bot/blob/bf5f5632bcd88670df90be67b888c282c6e83d99/utils/useful.py#L337-L343

return cls

def __instancecheck__(cls, instance):
# We need this method for situations where attributes are
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