python / python/mypy

`@overload` breaks with named tuple and some overloaded functions

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bug topic-overloads
Dominant language
Python
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Description

Bug Report

Mypy incorrectly reports Any as the resolved type (quite possibly due this https://github.com/python/typing/issues/253#issuecomment-235358878 )

To Reproduce

from collections import namedtuple
from typing import Any, Callable, cast, Dict, List, NamedTuple, overload, TypeVar, Union

F = TypeVar("F", bound=Callable[..., Any])
Arg: NamedTuple = namedtuple("Arg", ["arg"])


class Test:
    @overload
    def __call__(self, fn: F) -> F:
        ...

    @overload
    def __call__(self, *args: Any, **kwargs: Any) -> "Test":
        ...

    def __call__(self, *args: Any, **kwargs: Any) -> Union[F, "Test"]:
        if len(args) == 1:
            return cast(F, args[0])

        return self

reveal_type(Test()(Test))



def broken(arg: Arg) -> Arg:
    return arg

reveal_type(broken)
reveal_type(Test()(broken))



def working(arg: int) -> int:
    return arg

reveal_type(Test()(working))



@overload
def overloaded_broken(arg: F) -> F:
    ...

@overload
def overloaded_broken(*args: Any, **kwargs: Any) -> Dict[str, int]:
    ...

def overloaded_broken(*args: Any, **kwargs: Any) -> Union[F, Dict[str, int]]:
    if len(args) == 1:
        return cast(F, args[0])

    return dict(args=len(args), kwargs=len(kwargs))


reveal_type(overloaded_broken)
reveal_type(Test()(overloaded_broken))


@overload
def overloaded_working(arg: str) -> str:
    ...

@overload
def overloaded_working(arg: bytes) -> bytes:
    ...

def overloaded_working(arg: Union[bytes, str]) -> Union[bytes, str]:
    return arg


reveal_type(Test()(overloaded_working))

Expected Behavior

All revealed types are printed as non Any

Actual Behavior

# a class is OK
test.py:23:13: note: Revealed type is 'def () -> test.Test'

# this is what is expected when using a named tuple, but it resolves as any
test.py:30:13: note: Revealed type is 'def (arg: Tuple[Any, fallback=test.Arg]) -> Tuple[Any, fallback=test.Arg]'
test.py:31:13: note: Revealed type is 'Any'

# this is not using a named tuple and it works as expected
test.py:38:13: note: Revealed type is 'def (arg: builtins.int) -> builtins.int'

# this is what is expected for a complex overloaded function, but it resolves as any
test.py:57:13: note: Revealed type is 'Overload(def [F <: def (*Any, **Any) -> Any] (arg: F`-1) -> F`-1, def (*args: Any, **kwargs: Any) -> builtins.dict[builtins.str, builtins.int])'
test.py:58:13: note: Revealed type is 'Any'

# passing a simpler overloaded function
test.py:73:13: note: Revealed type is 'Overload(def (arg: builtins.str) -> builtins.str, def (arg: builtins.bytes) -> builtins.bytes)'

Your Environment

mypy --version: mypy 0.812 (default config)
python --version: Python 3.8.9

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 reproduction in test.py with mypy 0.812 and compare the revealed types for the named-tuple and overloaded-function cases. Trace overload resolution and the handling of named-tuple types, then verify that the affected reveals resolve to their expected callable types rather than Any.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
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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