builtins.sum: Spurious error for operands having __add__ defined using partialmethod

Here's a minimal reproducer:

from __future__ import annotations
from functools import partialmethod

class A:
    def my_add(self, other: A, reverse: bool) -> A:
        return self

    __add__ = partialmethod(my_add, reverse=False)

a = A()
aa = A()
aaa = A()
result = sum([aa, a], start=aaa)

And the commands to reproduce it:

$ mypy --version
mypy 0.950 (compiled: yes)

$ mypy --show-error-codes --strict  mypy_bug_report.py 
mypy_bug_report.py:15: error: No overload variant of "sum" matches argument types "List[A]", "A"  [call-overload]
mypy_bug_report.py:15: note: Possible overload variants:
mypy_bug_report.py:15: note:     def [_SumT <: _SupportsSum] sum(Iterable[_SumT]) -> Union[_SumT, Literal[0]]
mypy_bug_report.py:15: note:     def [_SumT <: _SupportsSum, _SumS <: _SupportsSum] sum(Iterable[_SumT], start: _SumS) -> Union[_SumT, _SumS]
Found 1 error in 1 file (checked 1 source file)

Related #7735, #7578

It appears that this is a regression. mypy 0.942 does not object to the code in the OP.

This still (unsurprisingly) reproduces after the recent changes to the sum stub. Here's the current error message:

(.venv) C:\Users\alexw\coding\typeshed>mypy test.py --strict --show-error-codes --custom-typeshed-dir .
test.py:13: error: No overload variant of "sum" matches argument types "List[A]", "A"  [call-overload]
test.py:13: note: Possible overload variants:
test.py:13: note:     def sum(__iterable, Iterable[bool], start: int = ...) -> int
test.py:13: note:     def [_SupportsSumNoDefaultT <: _SupportsSumWithNoDefaultGiven] sum(__iterable, Iterable[_SupportsSumNoDefaultT]) -> Union[_SupportsSumNoDefaultT, Literal[0]]
test.py:13: note:     def [_AddableT1 <: SupportsAdd[Any, Any], _AddableT2 <: SupportsAdd[Any, Any]] sum(__iterable, Iterable[_AddableT1], start: _AddableT2) -> Union[_AddableT1, _AddableT2]
Found 1 error in 1 file (checked 1 source file)

The reason why there's now an error where previously there was none is almost certainly due to changes made in typeshed between 0.942 and 0.950 that made the stub forsum` more complicated. Nonetheless, I feel like this is sort-of a bug in mypy's understanding of higher-order callables rather than a bug in the stub. In an ideal world, mypy should be able to understand that the arguments being passed in here match the third overload here:

typeshed/stdlib/builtins.pyi

Lines 1664 to 1665 in 1828ba2

    
           @overload 
        
           def sum(__iterable: Iterable[_AddableT1], start: _AddableT2) -> _AddableT1 | _AddableT2: ...

@AlexWaygood

I agree with @AlexWaygood. The output hasn't changed much with mypy 1.11.2. A reveal_type on A.__add__ reveals def (*Any, **Any) -> foo.A. This should match sum's third overload, but doesn't for some reason. This looks like a problem with mypy to me.

Navigation Menu

Repository navigation

`builtins.sum`: Spurious error for operands having `add` defined using `partialmethod` #7739

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

	@overload
	def sum(__iterable: Iterable[_AddableT1], start: _AddableT2) -> _AddableT1 \| _AddableT2: ...

builtins.sum: Spurious error for operands having __add__ defined using partialmethod #7739

Description

Activity

inducer commented on Apr 29, 2022

AlexWaygood commented on Jun 13, 2022

srittau commented on Sep 2, 2024

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

`builtins.sum`: Spurious error for operands having `add` defined using `partialmethod` #7739