René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Runtime-checkable protocols are broken on py312 (the sequel)","articleBody":"# Bug report\r\n\r\nOn 3.8-3.11, all subclasses of `typing.Generic` were guaranteed to have an `_is_protocol` class attribute, which is used as an internal marker:\r\n\r\nhttps://github.com/python/cpython/blob/76873ca6b1ad1a1eb9518f0ff7fc594ec96d0a65/Lib/typing.py#L1790\r\n\r\nTwo places in `typing.py` rely on all subclasses of `Generic` having this marker:\r\n\r\nhttps://github.com/python/cpython/blob/dbc8216f4c00ea40b0c2d3ca487e5afeb4b0e0b1/Lib/typing.py#L1895-L1897\r\n\r\nhttps://github.com/python/cpython/blob/dbc8216f4c00ea40b0c2d3ca487e5afeb4b0e0b1/Lib/typing.py#L2062-L2064\r\n\r\nHowever, on Python 3.12 (due to the implementation of PEP-695), subclasses of `Generic` no longer have this marker:\r\n\r\n```pycon\r\n\u003e\u003e\u003e class Foo[T]: ...\r\n...\r\n\u003e\u003e\u003e Foo._is_protocol\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\nAttributeError: type object 'Foo' has no attribute '_is_protocol'\r\n```\r\n\r\nThis leads to `AttributeError` being raised in two situations where it shouldn't be:\r\n\r\n```pycon\r\nPython 3.13.0a0 (heads/main:1080c4386d, May 25 2023, 13:11:38) [MSC v.1932 64 bit (AMD64)] on win32\r\nType \"help\", \"copyright\", \"credits\" or \"license\" for more information.\r\n\u003e\u003e\u003e from typing import Protocol, runtime_checkable\r\n\u003e\u003e\u003e @runtime_checkable\r\n... class Foo[T]: ...\r\n...\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 2062, in runtime_checkable\r\n    if not issubclass(cls, Generic) or not cls._is_protocol:\r\n                                           ^^^^^^^^^^^^^^^^\r\nAttributeError: type object 'Foo' has no attribute '_is_protocol'\r\n\u003e\u003e\u003e @runtime_checkable\r\n... class HasX(Protocol):\r\n...     x: int\r\n...\r\n\u003e\u003e\u003e class Bar[T]:\r\n...     x: T\r\n...     def __init__(self, x):\r\n...         self.x = x\r\n...\r\n\u003e\u003e\u003e isinstance(Bar(42), HasX)\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1810, in __instancecheck__\r\n    if super().__instancecheck__(instance):\r\n       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\abc.py\", line 119, in __instancecheck__\r\n    return _abc_instancecheck(cls, instance)\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1794, in __subclasscheck__\r\n    return super().__subclasscheck__(other)\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\abc.py\", line 123, in __subclasscheck__\r\n    return _abc_subclasscheck(cls, subclass)\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"C:\\Users\\alexw\\coding\\cpython\\Lib\\typing.py\", line 1897, in _proto_hook\r\n    issubclass(other, Generic) and other._is_protocol):\r\n                                   ^^^^^^^^^^^^^^^^^^\r\nAttributeError: type object 'Bar' has no attribute '_is_protocol'\r\n```\r\n\r\nCc. @JelleZijlstra for PEP-695\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-104939\n* gh-104941\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-05-25T15:20:50.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/104935/cpython/issues/104935"}