Bug

When @dataclass(init=True) (the default) is applied to a class that already defines __init__ in its own __dict__, the generated __init__ is always discarded by _set_new_attribute. However, _init_fn was still called unconditionally, and its field-ordering validation raised TypeError when inherited fields with defaults were followed by fields without defaults:

from dataclasses import dataclass

@dataclass
class Base:
    x: int = 0  # has a default

@dataclass
class Child(Base):
    y: int          # no default — would produce invalid generated signature
    def __init__(self, y):
        self.y = y
# TypeError: non-default argument 'y' follows default argument 'x'

The docs state: "If the class already defines __init__(), this parameter is ignored." The design table in dataclasses.py itself also documents this as the intended behaviour (the init=True / class-has-__init__ cell is intentionally blank/no-op).

Fix

Add and '__init__' not in cls.__dict__ to the guard before calling _init_fn in _process_class. This skips the generation (and its validation) entirely when the class's own __init__ would have been preserved anyway.

Tests

test_overwriting_init_with_field_ordering_conflict is added to TestInit in Lib/test/test_dataclasses/__init__.py:

• Fails before the fix: TypeError: non-default argument 'y' follows default argument 'x'
• Passes after the fix: Child(5).y == 5, class's own __init__ is used

All 282 dataclasses tests pass.

NEWS

Misc/NEWS.d/next/Library/2026-06-09-16-41-45.gh-issue-148941.aB3kLm.rst added.

CLA

⚠️ The GitHub account nahcmon may not have a signed PSF CLA on file. If the CLA bot flags this PR, the contributor will need to sign at https://www.python.org/psf/contrib/contrib-form/ before it can be merged.

• Issue: dataclass tries to generate __init__() even though the class already defines one #148941