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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"wrongly reported typing error in `min` with `key` lambda function and default value when result is Optional","articleBody":"**Bug Report**\r\n\r\nWhen using `key` function in `min` builtin function, default None value and returning it from a function which has an `Optional` type, mypy reports a problem where I believe there is none.\r\n\r\n**To Reproduce**\r\n\r\n```python\r\nfrom dataclasses import dataclass\r\nfrom typing import List\r\n\r\n@dataclass\r\nclass X:\r\n    x: int\r\n\r\ndef get_min(vals: List[X]) -\u003e X | None:\r\n    return min(vals, key = lambda tsync: tsync.x, default=None)\r\n\r\nresult = get_min([X(1), X(2), X(3)])\r\n```\r\n\r\n**Expected Behavior**\r\n\r\nNo errors reported.\r\n\r\n**Actual Behavior**\r\n\r\n```\r\n$ mypy b.py --pretty\r\nb.py:11: error: Item \"None\" of \"X | None\" has no attribute \"x\"  [union-attr]\r\n        return min(vals, key = lambda tsync: tsync.x, default=None)\r\n                                             ^~~~~~~\r\nFound 1 error in 1 file (checked 1 source file)\r\n```\r\n\r\nThe error goes away if I remove the `default=None` or when the `min` is not in a function with `X | None` return type:\r\n```python\r\nfrom dataclasses import dataclass\r\nfrom typing import List\r\n\r\n@dataclass\r\nclass X:\r\n    x: int\r\n\r\ndef get_min_1(vals: List[X]) -\u003e X | None:\r\n    return min(vals, key = lambda tsync: tsync.x, default=None)\r\n\r\ndef get_min_2(vals: List[X]) -\u003e X | None:\r\n    return min(vals, key = lambda tsync: tsync.x) # \u003c- this is fine\r\n\r\nvals = [X(1), X(2), X(3)]\r\n\r\nresult1 = get_min_1(vals)\r\nresult2 = get_min_2(vals)\r\nresult3 = min(vals, key = lambda tsync: tsync.x, default=None) # \u003c- this is fine\r\n```\r\n\r\nHere, mypy complains only about `get_min_1`:\r\n```\r\n$ mypy a.py --pretty\r\na.py:11: error: Item \"None\" of \"X | None\" has no attribute \"x\"  [union-attr]\r\n        return min(vals, key = lambda tsync: tsync.x, default=None)\r\n                                             ^~~~~~~\r\nFound 1 error in 1 file (checked 1 source file)\r\n```\r\n\r\n**Your Environment**\r\n\r\n\u003c!-- Include as many relevant details about the environment you experienced the bug in --\u003e\r\n\r\n- Mypy version used:  1.8.0\r\n- Mypy command-line flags: none required for reproducting the bug\r\n- Mypy configuration options from `mypy.ini` (and other config files): (no config)\r\n- Python version used: 3.11\r\n","author":{"url":"https://github.com/jan-spurny","@type":"Person","name":"jan-spurny"},"datePublished":"2024-05-07T09:14:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/17221/mypy/issues/17221"}