René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[arg-type] wrong argument type in generic function with union","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\nI have a generic class and function, which return Union containing TypeVar as in generic. Mypy will give error [arg-type] if I pass function result in another function which gets a object. But if i save the result in a variable and then pass it in function, mypy won't show error. It's very counterintuitive\r\n\r\n**To Reproduce**\r\n\r\n```python\r\nfrom typing import *\r\n\r\nT = TypeVar(\"T\")\r\nD = TypeVar(\"D\")\r\n\r\n\r\nclass Key: ...\r\n\r\n\r\nclass FancyKey(Generic[T]): ...\r\n\r\n\r\ndef get(key: FancyKey[T], default: D) -\u003e D | T:\r\n    ...\r\n    return default\r\n\r\n\r\ndef foo(value: object) -\u003e Any: ...\r\n\r\n\r\nkey = FancyKey[Key]()\r\nfoo(\r\n    get(key, default=None)\r\n)  # mypy: Argument 1 to \"bar\" of \"Bar\" has incompatible type \"Foo[Lab]\"; expected \"Foo[object]\" [arg-type]\r\n\r\ntreasure = get(key, default=None)  # no error\r\nfoo(treasure)  # no error\r\n```\r\n[Playground](https://mypy-play.net/?mypy=latest\u0026python=3.12\u0026gist=ed6b7f7a1d748e1b5091ba3c3e972a76)\r\n\r\n**Actual Behavior**\r\n\r\nmypy: Argument 1 to \"bar\" of \"Bar\" has incompatible type \"Foo[Lab]\"; expected \"Foo[object]\" [arg-type]\r\n\r\nIf a return-type function won't be generic or function will return not a Union - no error will be received\r\nIf type of `value` change to Any - no error will be received\r\n\r\nMaybe this issue related with [another](https://github.com/python/mypy/issues/17025)\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: 1.11.1\r\n- Mypy command-line flags: no flags\r\n- Mypy configuration options from `mypy.ini` (and other config files): no config\r\n- Python version used: 3.12\r\n\r\n","author":{"url":"https://github.com/RoketFlame","@type":"Person","name":"RoketFlame"},"datePublished":"2024-08-09T09:29:52.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/17654/mypy/issues/17654"}