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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Clarify the valid use locations of `typing.Concatenate`","articleBody":"\u003c!-- Please describe the problem or your idea or suggestion below. Please include the URL of the page this issue is about, if applicable. --\u003e\n\nIn python/cpython#142965, it was reported that the documentation of `typing.Concatenate` is \"incorrect\" (documentation that originates back to python/cpython#24000).\n\n@A5rocks, in your example, `Concatenate[int, P_2]` somewhat finally lands as a first argument to `Callable`, just indirectly.\nThis snippet previously defined:\n\n```py\nP_2 = ParamSpec(\"P_2\")\n\nclass X(Generic[T, P]):\n  f: Callable[P, int]\n  x: T\n```\n\nSimilar example is included in the typing spec:\n\nhttps://typing.python.org/en/latest/spec/generics.html#user-defined-generic-classes\n\n```py\nclass X[T, **P]:\n    f: Callable[P, int]\n    x: T\n\n# (...)\ndef accept_concatenate[**P](x: X[int, Concatenate[int, P]]) -\u003e str: ...  # Accepted\n```\n\nHowever, it seems that the current situation (confirmed with mypy, pyright and ty) is that you can use `Concatenate` in all valid locations of `ParamSpec` except directly in a `Concatenate`. I.e., you can't do `Concatenate[int, Concatenate[str, P]]`), but you can (besides passing `Concatenate` form as the first argument to `Callable`):\n\n1) Accumulate `Concatenate`s as `ParamSpec`s\n\n```py\nfrom collections.abc import Callable\nfrom typing import Concatenate\n\ntype Y[**P] = Callable[Concatenate[int, P], None]\ntype X[**P] = Y[Concatenate[int, P]]\n\ndef foo(f: X[str]) -\u003e None:\n    reveal_type(f)\n    # mypy: def (builtins.int, builtins.int, builtins.str)\n    # pyright: (int, int, str) -\u003e None\n    # ty: (...) -\u003e None\n```\n\n2) Bind `Concatenate`s to user-defined generics as `ParamSpec`s\n\n```py\nclass X[T, **P]:\n    f: Callable[P, int]\n    x: T\n\ndef accept_concatenate[**P](x: X[int, Concatenate[int, P]]) -\u003e str: ...\n```\n\n3) Use `Concatenate` as a type argument to `tuple` -- is this correct?\n\n```py\nfrom typing import Concatenate\n\ndef c(t: tuple[Concatenate[int, ...]]) -\u003e None:\n    reveal_type(c)\n    # mypy: def (t: tuple[[builtins.int, *Any, **Any]])\n    # pyright: (t: tuple[Concatenate[int, ...]]) -\u003e None\n    # ty: def c(t: tuple[@Todo]) -\u003e None\n```\n\nI've found this mostly by poking around -- I haven't analyzed the actual implementations (yet).\nI'll continue to investigate this from [these searches](https://github.com/johnslavik/python-type-checkers/blob/8aa628e36a8074be853284019eb3f7365f243f3b/searches/concatenate.md).\n\nI think that the valid use locations of `Concatenate` should be clarified in the typing spec and then in the CPython docs.\n\nIs there anything else I overlooked? CC @JelleZijlstra @AlexWaygood \n","author":{"url":"https://github.com/johnslavik","@type":"Person","name":"johnslavik"},"datePublished":"2025-12-20T16:47:17.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/2140/typing/issues/2140"}