René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Type inference for `max()` \"tainted\" by `default`","articleBody":"Consider the following code:\r\n\r\n```python\r\nfrom __future__ import annotations\r\nfrom collections.abc import Iterable\r\nfrom packaging.version import Version\r\n\r\ndef best_str(strs: Iterable[str]) -\u003e str | None:\r\n    return max(strs, key=keyfunc, default=None)\r\n\r\n\r\ndef keyfunc(s: str) -\u003e str:\r\n    return s[::-1]\r\n```\r\n\r\nI believe this should type-check without issue; in particular, by my reading of [`max()`'s type annotations in typeshed](https://github.com/python/typeshed/blob/4a854366e03dee700109f8e758a08b2457ea2f51/stdlib/builtins.pyi#L1543), the `key` callable should be expected to take only those types contained in the iterable.  However, mypy disagrees:\r\n\r\n```text\r\nmax-arg2.py:6: error: Argument \"key\" to \"max\" has incompatible type \"Callable[[str], str]\"; expected \"Callable[[str | None], SupportsDunderLT[Any] | SupportsDunderGT[Any]]\"  [arg-type]\r\n```\r\n\r\nNote that mypy expects the `key` callable to accept `str | None` even though `strs` is an `Iterable[str]`.  Moreover, if the `default` argument is omitted and the return type of `best_str()` is changed to `str`, then mypy accepts the code.\r\n\r\n**Your Environment**\r\n\r\n- Mypy version used: 1.6.0\r\n- Mypy command-line flags: None\r\n- Mypy configuration options from `mypy.ini` (and other config files): None\r\n- Python version used: 3.11.6","author":{"url":"https://github.com/jwodder","@type":"Person","name":"jwodder"},"datePublished":"2023-10-15T01:50:45.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/16267/mypy/issues/16267"}