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No-field class derived from dataclass_transform base class uses base class's constructor signature #19452
Description
Bug Report
When a class derives from a base class marked with dataclass_transform and the derived class has no fields, mypy incorrectly infers that the derived class does not provide its own constructor. This issue does not happen if the derived class has any fields.
Pyright infers the correct constructor signature here.
To Reproduce
from typing import dataclass_transform, reveal_type
@dataclass_transform()
class Base:
def __init__( self, a: int ) -> None:
pass
class DerivedNoFields( Base ):
pass
class DerivedWithFields( Base ):
b: str
reveal_type( DerivedNoFields.__init__ )
reveal_type( DerivedWithFields.__init__ )Expected Behavior
main.py:14: note: Revealed type is "def (self: __main__.DerivedNoFields)"
main.py:15: note: Revealed type is "def (self: __main__.DerivedWithFields, b: builtins.str)"
Actual Behavior
main.py:14: note: Revealed type is "def (self: __main__.Base, a: builtins.int)"
main.py:15: note: Revealed type is "def (self: __main__.DerivedWithFields, b: builtins.str)"
Your Environment
- Mypy version used: 3.16.1
- Python version used: 3.13
- No command-line flags or configuration options
Activity
[-]No-field class derived from `dataclass_transform` base uses base class's constructor signature[/-][+]No-field class derived from `dataclass_transform` base class uses base class's constructor signature[/+]sterliakov commentedon Jul 15, 2025
mypy/mypy/plugins/dataclasses.py
Lines 254 to 262 in a79e85e
There's a wrong premise in this comment, default
__init__is not always applicable. It predates@dataclass_transformand just wasn't noticed before.