Limited inference with union of callables and assignment to a union

Bug Report

Apologies if this was raised already. The only related issue I could find is #15887.

In Pydantic 2.10, we changed the definition of our field function to be:

from typing import Callable, TypeVar

_T = TypeVar('_T')

def Field(
    *,
    default_factory: Callable[[], _T] | Callable[[dict[str, Any]], _T],
) -> _T: ...

While this type checks correctly for some common use cases, we get errors when the Field function is assigned to a union of types, or when the default factory signature isn't trivial (e.g. contains overloads). Here are some examples failing:

(playground)

EDIT: Seems like the issue was fixed since 1.16.0. The issue with Class.b remains:

cvar = ContextVar[None]("session_id", default=None)


class Class:
    # error: Argument "default_factory" to "Field" has incompatible type "type[list[Any]]"; expected "Callable[[], Never] | Callable[[dict[str, Any]], Never]"  [arg-type]:
    a: list | None = Field(default_factory=list)

    # error: Argument "default_factory" to "Field" has incompatible type overloaded function; expected "Callable[[], Never] | Callable[[dict[str, Any]], Never]"  [arg-type]:
    b: None = Field(default_factory=cvar.get)  # note that `cvar.get` is overloaded. The first overload (`() -> None`) should match.


    # fine:
    fine: list = Field(default_factory=list)

Note that (at least for a), this worked as expected with the old type inference.

I first wanted to know if this can be considered a mypy limitation, or is the code really unsafe in some way? Or maybe is it because the typing spec does not provide any info regarding these use cases?

In comparison, pyright and pyre accepts the provided examples. We will currently recommend users to add a type: ignore comment and redirect to this issue. However, feel free to close if there's already an issue tracking this.

Thanks in advance

Your Environment

Mypy version used: 1.13
Mypy command-line flags: None
Mypy configuration options from mypy.ini (and other config files): None
Python version used: 3.13

fix: Require `instructlab-sdg>=0.6.2` to resolve dependency issues + …

I am not 100% sure is related, but the following code produces a similar error. The idea is to use the outer function as a parameter for a decorator, similar to what functools.wraps do. From my limited testing, it seems the error arises from State being a union type and nested Callables being involved.

from typing import Callable

class Failure[F]: error: F
class Success[S]: value: S
type State[F, S] = Failure[F] | Success[S]

type InnerFn[F, S] = Callable[[], State[F, S]]
type WrapperFn[F, S] = Callable[[InnerFn[F, S]], InnerFn[F, S]]
type CallerFn[F, S] = Callable[..., InnerFn[F, S]]

def decorator_with_argument[F, S](caller: CallerFn[F, S]) -> WrapperFn[F, S]:
    def wrapper(fn: InnerFn[F, S]) -> InnerFn[F, S]:
        return fn
    return wrapper

def outer_function1[F, S](state: State[F, S]) -> InnerFn[F, S]:
    @decorator_with_argument(outer_function1)
    def inner_function() -> State[F, S]:
        return state

    return inner_function

MyPy produces the following errors:

main.py:17: error: Argument 1 has incompatible type "Callable[[], Failure[F] | Success[S]]"; expected "Callable[[], Failure[Never] | Success[Never]]"  [arg-type]
main.py:24: error: Argument 1 has incompatible type "Callable[[], Failure[str] | Success[str]]"; expected "Callable[[], Failure[Never] | Success[Never]]"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Playground link: https://mypy-play.net/?mypy=latest&python=3.12&gist=6a93abe464faa4f20505341e99a3dd49.

Update mypy :)

Seems like the issue for Class.a got fixed in 1.16.0. Great! Class.b still raises an error on latest though.

@marek-elisity

@marek-elisity, @Viicos My example still fails in 1.18.2 in python 3.12, 3.13 and 3.14.

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Limited inference with union of callables and assignment to a union #18191

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Limited inference with union of callables and assignment to a union #18191

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pcastellazzi commented on May 17, 2025

marek-elisity commented on Sep 23, 2025

Viicos commented on Sep 23, 2025

pcastellazzi commented on Sep 23, 2025

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