python / python/mypy

mypy failed to return correct overloaded func type when decorate func accepts more than one parameter

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bug topic-overloads topic-paramspec
Dominant language
Python
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Description

Bug Report

mypy failed to return the correct overloaded func type when decorate func accepts more than one parameter.

To Reproduce

from functools import wraps
from typing import overload, Any, TypeVar, Callable, Type

from typing_extensions import ParamSpec, reveal_type


@overload
def foo(a: str, /) -> str:
    pass


@overload
def foo(a: int, /) -> int:
    pass


def foo(a: Any) -> Any:
    return a


P = ParamSpec("P")
T = TypeVar("T")
R = TypeVar("R")


def forbid_return_type(
    func: Callable[P, T],
    tp: Type[Any],
    /,
) -> Callable[P, T]:
    @wraps(func)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
        result = func(*args, **kwargs)

        if isinstance(result, tp):
            raise TypeError(f"Return type {tp} is forbidden")

        return result

    return wrapper


foo_as_int = forbid_return_type(foo, str)

reveal_type(foo_as_int(1))
reveal_type(foo_as_int("1"))

Expected Behavior

note: Revealed type is "builtins.int"
note: Revealed type is "builtins.str"

Actual Behavior

note: Revealed type is "builtins.str"
error: Argument 1 has incompatible type "int"; expected "str"  [arg-type]
note: Revealed type is "builtins.str"

Your Environment

  • Mypy version used: mypy 1.14.0+dev
  • Mypy command-line flags: -
  • Mypy configuration options from mypy.ini (and other config files): -
  • Python version used: 3.8.19

Both codes bellow work as expected:

from functools import wraps
from typing import overload, Any, TypeVar, Callable

from typing_extensions import ParamSpec, reveal_type


@overload
def foo(a: str, /) -> str:
    pass


@overload
def foo(a: int, /) -> int:
    pass


def foo(a: Any) -> Any:
    return a


P = ParamSpec("P")
T = TypeVar("T")
R = TypeVar("R")


def forbid_return_type(
    func: Callable[P, T],
) -> Callable[P, T]:
    @wraps(func)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
        result = func(*args, **kwargs)
        return result

    return wrapper


foo_as_int = forbid_return_type(foo)

reveal_type(foo_as_int(1))
reveal_type(foo_as_int("1"))
from functools import wraps
from typing import Any, TypeVar, Callable, Type, Union

from typing_extensions import ParamSpec, reveal_type


def foo(a: Union[int, str]) -> Union[int, str]:
    return a


P = ParamSpec("P")
T = TypeVar("T")
R = TypeVar("R")


def forbid_return_type(
    func: Callable[P, T],
    tp: Type[Any],
    /,
) -> Callable[P, T]:
    @wraps(func)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
        result = func(*args, **kwargs)

        if isinstance(result, tp):
            raise TypeError(f"Return type {tp} is forbidden")

        return result

    return wrapper


foo_as_int = forbid_return_type(foo, str)

reveal_type(foo_as_int(1))
reveal_type(foo_as_int("1"))

So it's not working only when the function accepts an overloaded function and another argument.

I checked with pyright and it correctly handles code.

  /Users/yuriikarabas/projects/mypy/temp.py:45:13 - information: Type of "foo_as_int(1)" is "int"
  /Users/yuriikarabas/projects/mypy/temp.py:46:13 - information: Type of "foo_as_int("1")" is "str"
0 errors, 0 warnings, 2 informations 

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the minimal reproducer with mypy and compare the revealed types with the expected output. Trace overload and ParamSpec inference for the decorated function when the decorator receives its second argument. Done means the reproducer reveals int for foo_as_int(1) and str for foo_as_int("1") without an argument-type error.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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