generics inferred differently when assigning to variable even though the variable's type is the same #11455

DetachHead · 2021-11-04T08:02:55Z

from typing import Callable, TypeVar

A = TypeVar("A")
B = TypeVar("B")


def func(a: A, b: B, fn: Callable[[A], A]) -> A | B:
    ...


inferred = func(
    1,
    None,
    lambda value: reveal_type(value),  # Revealed type is "builtins.int"
)
reveal_type(inferred)  # "Union[builtins.int, None]"

explicit: int | None
explicit = func(
    1,
    None,
    lambda value: reveal_type(value), # Revealed type is "Union[builtins.int, None]"
)
reveal_type(explicit)  # "Union[builtins.int, None]"

https://mypy-play.net/?mypy=latest&python=3.10&gist=7c2af9e77a6246ee6fb3e5052722eec1

DetachHead · 2022-01-10T04:12:09Z

a simpler example

from typing import TypeVar

T = TypeVar("T")

def foo(value: list[T]) -> T | int:
    ...


def bar(value: list[T]) -> None:
    # error: Argument 1 to "foo" has incompatible type "List[T]"; expected "List[Union[T, int]]"  [arg-type]
    baz: T | int = foo(value)
    
    qux = foo(value)
    quxx: T | int = qux # no error

https://mypy-play.net/?mypy=latest&python=3.10&flags=show-error-context%2Cshow-error-codes%2Cstrict%2Ccheck-untyped-defs%2Cdisallow-any-decorated%2Cdisallow-any-expr%2Cdisallow-any-explicit%2Cdisallow-any-generics%2Cdisallow-any-unimported%2Cdisallow-incomplete-defs%2Cdisallow-subclassing-any%2Cdisallow-untyped-calls%2Cdisallow-untyped-decorators%2Cdisallow-untyped-defs%2Cno-implicit-optional%2Cno-implicit-reexport%2Clocal-partial-types%2Cstrict-equality%2Cwarn-incomplete-stub%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable%2Cwarn-unused-configs%2Cwarn-unused-ignores&gist=ea9fa865399112b3d9b5e2c7f2c861e4

…ions in return position (#18976) * Fixes #17221. * Fixes #17654. * Fixes #17553. * Fixes #17536. * Fixes #16659. * Fixes #16267. * Fixes #15755. * Fixes #15150. * Fixes #14664. * Incidentally improves error message in #12156. * Fixes #12092. * Fixes #11985. * Improves #11455 (but the problem with union `TypeVar | SomeFixedType` reported in comments there remains). * Fixes #10426. When using context, we can perform some overly optimistic inference when return type is `T1 | T2`. This breaks in important case of `builtins.min` when `default` and `key` are passed, essentially making them always incompatible. This is not the most principled approach, but let's see the primer results. This resolves quite a few issues (some of them duplicates, but some - substantially different), `min` problem was a very popular one... Diff run: sterliakov/mypy-issues#30

DetachHead added the bug mypy got something wrong label Nov 4, 2021

DetachHead mentioned this issue Jan 10, 2022

generic inferred incorrectly when assigning to union with the generic #11954

Closed

sterliakov mentioned this issue Apr 26, 2025

Extend special case for context-based typevar inference to typevar unions in return position #18976

Merged

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generics inferred differently when assigning to variable even though the variable's type is the same #11455

generics inferred differently when assigning to variable even though the variable's type is the same #11455

generics inferred differently when assigning to variable even though the variable's type is the same #11455

generics inferred differently when assigning to variable even though the variable's type is the same #11455

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