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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"False \"Invalid index type\" for min derived from return type","articleBody":"\u003c!--\r\nIf you're not sure whether what you're experiencing is a mypy bug, please see the \"Question and Help\" form instead.\r\nPlease consider:\r\n\r\n- checking our common issues page: https://mypy.readthedocs.io/en/stable/common_issues.html\r\n- searching our issue tracker: https://github.com/python/mypy/issues to see if it's already been reported\r\n- asking on gitter chat: https://gitter.im/python/typing\r\n--\u003e\r\n\r\n**Bug Report**\r\n\r\n\u003c!--\r\nIf you're reporting a problem with a specific library function, the typeshed tracker is better suited for this report: https://github.com/python/typeshed/issues\r\n\r\nIf the project you encountered the issue in is open source, please provide a link to the project.\r\n--\u003e\r\n\r\nMypy gives `[index]` error for `key` when calling `min` from function that is declared to return `Optional`. \r\n\r\n**To Reproduce**\r\n\r\nThis pretty valid\r\n\r\n```python\r\ndef func() -\u003e Optional[int]:\r\n    d = {1: 2, 3: 4}\r\n    return min([1, 2], key=lambda k: d[k], default=None)\r\n```\r\nreturns error\r\n```\r\nInvalid index type \"Optional[int]\" for \"Dict[int, int]\"; expected type \"int\"  [index]\r\n```\r\n\r\nChanging default to `0` doesn't fix the problem, but changing return type of `func` to `int` does. So, the following works fine:\r\n\r\n```python\r\ndef func() -\u003e int:\r\n    d = {1: 2, 3: 4}\r\n    return min([1, 2], key=lambda k: d[k], default=0)\r\n```\r\n\r\n**Expected Behavior**\r\n\r\n\u003c!--\r\nHow did you expect mypy to behave? It’s fine if you’re not sure your understanding is correct.\r\nWrite down what you thought would happen. If you expected no errors, delete this section.\r\n--\u003e\r\n\r\nNo error for the first example, as `default` is never compared to objects in collection.\r\n\r\n**Actual Behavior**\r\n\r\n\u003c!-- What went wrong? Paste mypy's output. --\u003e\r\n\r\n```\r\nInvalid index type \"Optional[int]\" for \"Dict[int, int]\"; expected type \"int\"  [index]\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: 1.0.0\r\n- Mypy command-line flags: no\r\n- Mypy configuration options from `mypy.ini` (and other config files): no\r\n- Python version used: 3.10\r\n\r\n\u003c!-- You can freely edit this text, please remove all the lines you believe are unnecessary. --\u003e\r\n","author":{"url":"https://github.com/ods","@type":"Person","name":"ods"},"datePublished":"2023-02-09T17:25:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/14664/mypy/issues/14664"}