Incorrect argument names for dataclass with "private" (mangled) members

Bug Report

Mypy infers dataclass arguments incorrectly when a class has private (__name) members.

To Reproduce

from dataclasses import dataclass


@dataclass(kw_only=True)
class Data:
    __mangled: int


Data(_Data__mangled=1)  # Runs correctly, but mypy complains
Data(__mangled=1)       # Fails, but mypy approves

https://mypy-play.net/?mypy=latest&python=3.13&gist=f585904f6f5652ee86cd11d6502de13f

Expected Behavior

I expect mypy's behaviour to match the execution and requrie the _Data__mangled argument.
Additionally, I expect it to reject the __mangled argument name as unexpected.

Actual Behavior

main.py:9: error: Unexpected keyword argument "_Data__mangled" for "Data"; did you mean "__mangled"? [call-arg]
Found 1 error in 1 file (checked 1 source file)

Your Environment

I reproduced this on mypy-play.net with Python 3.13

Mypy version used: 1.17.1
Mypy command-line flags: default mypy-play.net settings
Python version used: Python 3.13

There's also a similar issue with attrs:

import attrs


@attrs.define(kw_only=True)
class Attr:
    a: int
    _b: int
    __c: int
    __x__: int


Attr(a=1, b=2, Attr__c=3, x__=4)  # Valid, but mypy complains
Attr(a=1, b=2, c=3, x__=4)        # Invalid, but mypy approves

This is basically #8267 but specifically supporting this for attrs might require more work so I'll keep this open. (e.g. only trimming a single underscore).

Both problems would be fixed by merging #16715.

[case testNameManglingAttrs]
import attr

@attr.define(kw_only=True)
class X:
    a: int
    _b: int
    __c: int
    __d__: int

X(a=1, b=2, X__c=3, d__=4)
X(a=1, b=2, c=3, d__=4)   # E: Unexpected keyword argument "c" for "X"

x: X
reveal_type(x.a)  # N: Revealed type is "builtins.int"
reveal_type(x._b)  # N: Revealed type is "builtins.int"
reveal_type(x._X__c)  # N: Revealed type is "builtins.int"
reveal_type(x.__d__)  # N: Revealed type is "builtins.int"

[builtins fixtures/list.pyi]

[case testNameManglingDataclasses]
from dataclasses import dataclass

@dataclass(kw_only=True)
class X:
    a: int
    _b: int
    __c: int
    __d__: int

X(a=1, _b=2, _X__c=3, __d__=4)
X(a=1, _b=2, __c=3, __d__=4)   # E: Unexpected keyword argument "__c" for "X"

x: X
reveal_type(x.a)  # N: Revealed type is "builtins.int"
reveal_type(x._b)  # N: Revealed type is "builtins.int"
reveal_type(x._X__c)  # N: Revealed type is "builtins.int"
reveal_type(x.__d__)  # N: Revealed type is "builtins.int"

[builtins fixtures/dataclasses.pyi]
[typing fixtures/typing-medium.pyi]

I will include them as additional test cases. However, the PR has been sleeping for a long time now. A review from someone who has the time to read my lengthy monologue and help fine-tune the different assumptions would be greatly appreciated.

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Incorrect argument names for dataclass with "private" (mangled) members #19755

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Incorrect argument names for dataclass with "private" (mangled) members #19755

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tmr232 commented on Aug 29, 2025

A5rocks commented on Aug 30, 2025

tyralla commented on Sep 1, 2025

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