Wrong "Overloaded function implementation does not accept all possible arguments"

Bug Report

The following example (reduced from psycopg 3) seems to be a false positive:

To Reproduce

from typing import Any, Callable, Generic, overload, Optional, TypeVar, Tuple, Union

Row = TypeVar("Row")

class Connection(Generic[Row]):
    @overload
    def connect(
        self, *, row_factory: Callable[[], Row], **kwargs: Union[None, int, str]
    ) -> "Connection[Row]":
        ...

    @overload
    def connect(
        self, **kwargs: Union[None, int, str],
    ) -> "Connection[Tuple[Any, ...]]":
        ...

    def connect(
        self, *, row_factory: Optional[Callable[[], Row]] = None, **kwargs: Any,
    ) -> "Connection[Any]":
        raise NotImplementedError

The file is flagged with the error:

bug_mypy.py:18: error: Overloaded function implementation does not accept all possible arguments of signature 2  [misc]

Your Environment

Mypy version used: 0.910
Python version used: 3.8
Operating system and version: Linux

You may have already figured this out, but there is a simple workaround. If you change the second overload to include an explicit row_factory parameter, it will type check without errors.

    @overload
    def connect(
        self,
        *,
        row_factory: None = None,
        **kwargs: Union[None, int, str],
    ) -> "Connection[Tuple[Any, ...]]":
        ...

@erictraut

@erictraut No, I didn't notice that, thank you. It is interesting, however I prefer to use a type: ignore rather than a dummy parameter.

Another thing that makes the error go is to use **kwargs: Any in the second signature.

I think that's the issue I tried to resolve in #10390.

@dlax

Amazing @dlax, didn't know you had looked into it so much to propose a MR! 🙂

@dlax

@dlax Do you plan to merge the change soon?

@adam-grant-hendry

@adam-grant-hendry, the PR is still pending review; and then possible merge by a project member.

This isn't a bug? As currently written, the second overload accepts Connection().connect(row_factory=10) and the implementation signature does not, so mypy warns about a real problem.

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Wrong "Overloaded function implementation does not accept all possible arguments" #11004

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Wrong "Overloaded function implementation does not accept all possible arguments" #11004

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erictraut commented on Aug 21, 2021

dvarrazzo commented on Aug 21, 2021

dlax commented on Aug 23, 2021

dvarrazzo commented on Aug 23, 2021

adam-grant-hendry commented on Jul 5, 2022

dlax commented on Jul 6, 2022

sterliakov commented on Mar 26, 2025

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