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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Unneeded type widening of inner function call when using explicit return type","articleBody":"Using an explicit return type unnecessarily widens the types in `filter()`:\r\n\r\n```python\r\ndef return_a(strings: list[str]) -\u003e Optional[str]:\r\n    # Item \"None\" of \"Optional[str]\" has no attribute \"strip\"\r\n    return next(filter(lambda s: s.strip() == \"a\", strings), None)\r\n```\r\nand a comparable error when the filter function is explicitly typed:\r\n```python\r\ndef matches_a(s: str) -\u003e bool:\r\n    return s.strip() == \"a\"\r\n\r\ndef return_a(strings: list[str]) -\u003e Optional[str]:\r\n    # Argument 1 to \"filter\" has incompatible type \"Callable[[str], Any]\"; expected \"Callable[[Optional[str]], Any]\"\r\n    return next(filter(matches_a, strings), None)\r\n```\r\n\r\nNo error occurs when the explicit return type is removed:\r\n```python\r\n# return_a is inferred to return str | None\r\ndef return_a(strings: list[str]):\r\n    # No error\r\n    return next(filter(lambda s: s.strip() == \"a\", strings), None)\r\n```\r\n\r\nMypy 0.931, Python 3.8.6.","author":{"url":"https://github.com/knbk","@type":"Person","name":"knbk"},"datePublished":"2022-01-28T10:53:21.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/12092/mypy/issues/12092"}