René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"False positive unreachable with redefined variable in loop","articleBody":"When checking the following code:\r\n\r\n```py\r\nfrom typing import AbstractSet, Iterable, Optional, Set, TypeVar\r\n\r\nT = TypeVar('T')\r\n\r\ndef intersection(seq: Iterable[AbstractSet[T]]) -\u003e Optional[Set[T]]:\r\n    ret = None\r\n    for elem in seq:\r\n        if ret is None:\r\n            ret = set(elem)\r\n        else:\r\n            ret \u0026= elem\r\n    return ret\r\n```\r\n\r\nmypy reports:\r\n\r\n```\r\n$ mypy --warn-unreachable intersection.py\r\nintersection.py:11: error: Statement is unreachable\r\n```\r\n\r\nThis statement is the `ret \u0026= elem` inside the `else` block.\r\n\r\nIt seems that mypy is using the initial type of `ret` to conclude that `ret is None` is always true, ignoring the fact that inside the loop `ret` is redefined to a different type.\r\n\r\nIf I place a `reveal_type` just before the `return`, mypy does report ``Union[builtins.set[T`-1], None]`` as the type, so it figures out the redefinition eventually.\r\n\r\nI'm using mypy 0.770 on Pyton 3.8.2, with the `--warn-unreachable` flag.\r\n","author":{"url":"https://github.com/mthuurne","@type":"Person","name":"mthuurne"},"datePublished":"2020-05-20T21:51:15.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/8865/mypy/issues/8865"}