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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Typing: Document performance pitfalls of protocols decorated with `@runtime_checkable`","articleBody":"# Documentation\r\n\r\nThe following two calls look similar, but the second is dramatically slow compared to the first:\r\n\r\n```python\r\nfrom typing import SupportsInt\r\n\r\nisinstance(3, int)\r\nisinstance(3, SupportsInt)\r\n```\r\n\r\nThe reason for the slowness is because of [`_ProtocolMeta.__instancecheck__`](https://github.com/python/cpython/blob/d3ca042c99b2e86a2bb927a877fdfcbacdc22f89/Lib/typing.py#L1977-L1999), which makes calling `isinstance()` against any runtime-checkable protocol pretty slow. It's possible we can find ways of optimising `_ProtocolMeta.__instancecheck__`, but whatever we do, it is likely to remain substantially slower to call `isinstance()` against runtime-checkable protocols than it is to call `isinstance()` against \"normal\" classes. This is often pretty surprising for users, so one of the consensus points in https://github.com/python/typing/issues/1363 was that this should be better documented.\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-102937\n* gh-102963\n* gh-102964\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-03-22T22:48:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/102936/cpython/issues/102936"}