partial plugin does not handle overloads

Bug Report

it appears the new partial plugin added in mypy 1.11 doesn't properly handle overloads (and just picks the first one)

To Reproduce

import functools
from typing import assert_type, overload

@overload
def identity(x: int, *, p: str) -> int: ...
@overload
def identity(x: str, *, p: str) -> str: ...
def identity(x: int | str, *, p: str) -> int | str: return x


p = functools.partial(identity, p='hi')
assert_type(p(1), int)
assert_type(p('hi'), str)

Expected Behavior

(no errors)

Actual Behavior

$ mypy t.py 
t.py:13: error: Expression is of type "int", not "str"  [assert-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy version used: 1.11.0
Mypy command-line flags: none
Mypy configuration options from mypy.ini (and other config files): none
Python version used: 3.12.2

an alternative spelling with TypeVars seems to not work either:

import functools
from typing import assert_type, overload, TypeVar

T = TypeVar("T", int, str)

def identity(x: T, *, p: str) -> T: return x


p = functools.partial(identity, p='hi')
assert_type(p(1), int)
assert_type(p('hi'), str)

$ ./venv/bin/mypy t.py 
t.py:10: error: Expression is of type "Any", not "int"  [assert-type]
t.py:11: error: Expression is of type "Any", not "str"  [assert-type]
Found 2 errors in 1 file (checked 1 source file)

ref: work around partial missing overload support

ref: work around partial missing overload support (#74976)

Yes, the behaviour with overloads is unchanged from mypy 1.10. Shouldn't be too bad to add support for it.

mypy/mypy/plugins/functools.py

Lines 131 to 132 in d1d3c78

    
           # TODO: handle overloads, just fall back to whatever the non-plugin code does 
        
           return ctx.default_return_type

The constrained type var example you have is a duplicate of #17411. Note that unconstrained type vars or bounded type vars should work correctly. In general, mypy's handling of constrained type vars is a little weird.

Improve support for functools.partial of overloaded callable protocol (…

ref: work around partial missing overload support (getsentry#74976)

I think I'm hitting this as well in mypy 1.18.2. I won't post the actual code, but the gist is that the overload should be chosen based on an enum choice:

Have an enum "CommandsEnum" that describes a set of functions a function commands_core can perform
Have @overloads of that function with the command set to something like Literal[CommandsEnum.Command3]
Create a partial with do_comm3 = partial(commands_core, command=CommandsEnum.Command3)

It seems to always pick the "first" overload.

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`partial` plugin does not handle overloads #17585

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	# TODO: handle overloads, just fall back to whatever the non-plugin code does
	return ctx.default_return_type

partial plugin does not handle overloads #17585

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asottile-sentry commented on Jul 25, 2024

hauntsaninja commented on Jul 26, 2024

AaronDMarasco commented on Dec 10, 2025

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`partial` plugin does not handle overloads #17585