René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[1.16 regression] a cast is both needed and redundant at the same time","articleBody":"**Bug Report**\n\nSince 1.16, [_structlog_](https://github.com/hynek/structlog#)'s package type check fails with:\n\n```\nsrc/structlog/stdlib.py:1160: error: Redundant cast to \"MutableMapping[str, Any]\"  [redundant-cast]\n                ed = p(logger, meth_name, cast(EventDict, ed))\n                                          ^\nFound 1 error in 1 file (checked 1 source file)\n```\n\nIf I remove the cast, I get:\n\n```\nsrc/structlog/stdlib.py:1160: error: Argument 3 has incompatible type \"Mapping[str, Any] | str | bytes | bytearray | tuple[Any, ...]\"; expected\n\"MutableMapping[str, Any]\"  [arg-type]\n                ed = p(logger, meth_name, ed)\n                                          ^~\nFound 1 error in 1 file (checked 1 source file)\n```\n\nIf I add a reveal_type, I get on both 1.15 \u0026 1.16:\n\n```\nsrc/structlog/stdlib.py:1160: note: Revealed type is \"typing.MutableMapping[builtins.str, Any]\"\nsrc/structlog/stdlib.py:1160: note: Revealed type is \"Union[typing.Mapping[builtins.str, Any], builtins.str, builtins.bytes, builtins.bytearray, builtins.tuple[Any, ...]]\"\n```\n\nSo I guess Mypy got somehow smarter/confused about that?\n\n**To Reproduce**\n\n- `git clone git@github.com:hynek/structlog.git`\n- `git checkout d948027`\n- run `tox -e mypy-pkg`\n\nI really tried to create a minimal reproducer, but given that I don't quite understand what's going on, I've failed.\n\nThe relevant code is here: https://github.com/hynek/structlog/blob/e941e337d312eaf256595d9d09da4542c5e94d0b/src/structlog/stdlib.py#L1160\n\nThe types are defined here: https://github.com/hynek/structlog/blob/e941e337d312eaf256595d9d09da4542c5e94d0b/src/structlog/typing.py#L61-L76\n\n**Expected Behavior**\n\nI would expect it to keep working, or at least decide whether or not I need the cast. 🤓\n\n**Actual Behavior**\n\nI don't seem to find a way to make the code pass except using Any all over the place I guess.\n\n**Your Environment**\n\n- Mypy version used: 1.16\n- Mypy command-line flags: none\n- Mypy configuration options from `mypy.ini` (and other config files): https://github.com/hynek/structlog/blob/e941e337d312eaf256595d9d09da4542c5e94d0b/pyproject.toml#L175-L191\n- Python version used: 3.13\n\n**edit** I managed to [work around it](https://github.com/hynek/structlog/commit/57e165bfacb79eb5942389c20eb9d883d88bfcb6) with the help of @Tinche but it still looks like I managed to break poor little Mypy.","author":{"url":"https://github.com/hynek","@type":"Person","name":"hynek"},"datePublished":"2025-05-30T06:19:19.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/19170/mypy/issues/19170"}