René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"polymorphic overloads on `list.__add__` and `dict.__or__` lead to divergences in type checkers.","articleBody":"The overloads on `list.__add__` lead to divergent behavior between `mypy` and `pyright` when doing something as simple as a list concatenation\n\n```python\n    # Overloading looks unnecessary, but is needed to work around complex mypy problems\n    @overload\n    def __add__(self, value: list[_T], /) -\u003e list[_T]: ...\n    @overload\n    def __add__(self, value: list[_S], /) -\u003e list[_S | _T]: ...\n```\n\nCode sample in [pyright playground](https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCCKcANFGAG4CmIANmAIYAmAUCwMa0MDO3UAMkm4wCAbQAqAXQBcUKAGIoVCAFcuMKnwBGKpLRio%2BtITBZy5AAUo16zM%2BaZVgUAPovmTNwApuVWsDIwGAALGlkAIkFhMSlwsgB6AEooAFoAPihIkxjJcNkQKhgVEBQBbK9E%2B0trOkZWcyhHZzcPN1EAZUkfPwDyELDMqJEO3ITk9MHsiSgAHyhOvKgCopKy6IqqqE2OLl41mAAhCRk5RQB3JBCwFXwa2yZuTabXdyZPFxHu-0D%2BkAiho4LMapDJZYRHcSzea5fKFYqlAEbTYNFGotHojEKKAQRBwTENOYIOAgJBoYL4fGUlEsArUBi0FzwBBULxQYzCUTCEBdZIAajZJk5MG5FSgyXMinZMCF3IxcylMskmMAoOQ0qh0hlMlkCjlcnlQfkK1AwfXi0466VcqHGpVyeWCq1zG0NVW0qj0xmIbUKvWiw2CohwU0NSUB4i2tH23XCiNowAy5Gw3R6tayhmJfXz9umY6LxYo04r0XMC3qMa71e7NV7U1MMwas6IbbnzSXhdaUCa7Q3HZgOxHyxrPcya9FFX6G4Hg1iC5Oi92YxiE2rByn9kc6-yAWPknm14XI3vS%2BiB5Wh9qtxu902dy2TOu206%2B12Lw-e525Cfk9XDznM1vJ82053jKUKzlCL4gKB4bmKqQA), https://mypy-play.net/?mypy=latest\u0026python=3.12\u0026gist=abf6a8834020af17a16bd8cfb44b2f10\n\n```python\nfrom typing import Any, overload\n\nclass ListA[T]:  # emulates builtins list\n    @overload\n    def __add__(self, other: \"ListA[T]\", /) -\u003e \"ListA[T]\": return ListA()\n    @overload\n    def __add__[S](self, other: \"ListA[S]\", /) -\u003e \"ListA[T | S]\": return ListA()\n    \n    \nclass ListB[T]:  # without overloads\n    def __add__[S](self, other: \"ListB[S]\", /) -\u003e \"ListB[T | S]\": return ListB()\n\n                                            # mypy              | pyright                             \nreveal_type( list[str]() + list[str]() )    # list[str]         | list[str]          ✅️\nreveal_type( list[str]() + list[int]() )    # list[str | int]   | list[str | int]    ✅️\nreveal_type( list[str]() + list[Any]() )    # list[Any]         | list[str]          ❌️\n\nreveal_type( ListA[str]() + ListA[str]() )  # ListA[str]        | ListA[str]         ✅️\nreveal_type( ListA[str]() + ListA[int]() )  # ListA[str | int]  | ListA[str | int]   ✅️\nreveal_type( ListA[str]() + ListA[Any]() )  # ListA[Any]        | ListA[str]         ❌️\n\nreveal_type( ListB[str]() + ListB[str]() )  # ListB[str]        | ListB[str]         ✅️\nreveal_type( ListB[str]() + ListB[int]() )  # ListB[str | int]  | ListB[str | int]   ✅️\nreveal_type( ListB[str]() + ListB[Any]() )  # ListB[str | Any]  | ListB[str | Any]   ✅️\n\n```\n\nThis ultimately causes some very annoying type errors when checking wrapper functions in `pyright`.\n\n\u003cdetails\u003e\n\nCode sample in [pyright playground](https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCyAhgsugDRQCCKcAUHQMRTACuKAxjEmClAO5IYACygAmKGATdeRADZQiINKwgBTFDADOdACZrgLMGAAUStAC5JAIwBWarpQD0lAFSUpMAIxWtMEFAAvFAAdGEe0mJWqPjBYSEAlFAAtAB8UAByvGpW8QzM-CAkCGoBIkT4wLj8Srpaisr1AG5IRFC6SFx6BkZgAPqFxaVmylZgdg4wlFX9ANY1jVbEpKhoANp%2BIJQ0cAC6SWmZ2RZ0UGdQzMVycFAoavztBkSsctqn5zN98%2Bb1wQDeACJPF4AVYAXd%2BH19MBnq8AQBfKAAH3anRgJk%2B30aCXeZxmIzQblcmIWaC0SQuUABmwBUA4RBQKDA%2BGsakUWi0SDQd10sDAUAQSiI6hgpSpnjEALoQA)\n\n```python\nfrom typing import Mapping, Any\n\n# function with 2 optional arguments\ndef foo(arg: object, /, *, opt1: str = ..., opt2: int = ...) -\u003e None: ...\n\n# wrapper that forwards args via dict\ndef foo_wrapper(arg: object, foo_kwargs: Mapping[str, Any]) -\u003e None:\n    # apply new defaults\n    foo_kwargs = {\"opt1\": \"new_default\"} | dict(foo_kwargs)\n    foo(arg, **foo_kwargs)  # \"str\" cannot be assigned to parameter \"opt2\"\n\n```\n\n\u003c/details\u003e\n\nPR #14282 and #14284 show mypy-primer results of simplifying the overloads away from `list.__add__` and `dict.__or__`.\n\n\n\n\n","author":{"url":"https://github.com/randolf-scholz","@type":"Person","name":"randolf-scholz"},"datePublished":"2025-06-16T11:32:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/14283/typeshed/issues/14283"}