gh-148941: Skip generating __init__ when class already defines it (GH-148941)

When @DataClass(init=True) 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 for inherited fields
with defaults followed by fields without.  The docs state "If the class
already defines __init__(), this parameter is ignored."  Align the
implementation with that documented behaviour by checking
'__init__' not in cls.__dict__ before calling _init_fn.

Author: nahcmon

Lib/dataclasses.py

@@ -520,7 +520,7 @@ def add_fns_to_class(self, cls):
                 if already_exists and (msg_extra := self.overwrite_errors.get(name)):
                     error_msg = (f'Cannot overwrite attribute {fn.__name__} '
                                  f'in class {cls.__name__}')
-                    if not msg_extra is True:
+                    if msg_extra is not True:
                         error_msg = f'{error_msg} {msg_extra}'
 
                     raise TypeError(error_msg)
@@ -729,14 +729,14 @@ def _frozen_set_del_attr(cls, fields, func_builder):
                         ('self', 'name', 'value'),
                         (f'  if {condition}:',
                           '   raise FrozenInstanceError(f"cannot assign to field {name!r}")',
-                         f'  super(__class__, self).__setattr__(name, value)'),
+                         '  super(__class__, self).__setattr__(name, value)'),
                         locals=locals,
                         overwrite_error=True)
     func_builder.add_fn('__delattr__',
                         ('self', 'name'),
                         (f'  if {condition}:',
                           '   raise FrozenInstanceError(f"cannot delete field {name!r}")',
-                         f'  super(__class__, self).__delattr__(name)'),
+                         '  super(__class__, self).__delattr__(name)'),
                         locals=locals,
                         overwrite_error=True)
 
@@ -1099,7 +1099,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
 
     # Do we have any Field members that don't also have annotations?
     for name, value in cls.__dict__.items():
-        if isinstance(value, Field) and not name in cls_annotations:
+        if isinstance(value, Field) and name not in cls_annotations:
             raise TypeError(f'{name!r} is a field but has no type annotation')
 
     # Check rules that apply if we are derived from any dataclasses.
@@ -1142,7 +1142,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
 
     func_builder = _FuncBuilder(globals)
 
-    if init:
+    if init and '__init__' not in cls.__dict__:
         # Does this class have a post-init function?
         has_post_init = hasattr(cls, _POST_INIT_NAME)
 

Lib/test/test_dataclasses/__init__.py

@@ -2502,6 +2502,26 @@ def __init__(self, x):
                 self.x = 2 * x
         self.assertEqual(C(5).x, 10)
 
+    def test_overwriting_init_with_field_ordering_conflict(self):
+        # gh-148941: When a class defines its own __init__, @dataclass must
+        # not raise TypeError due to field ordering issues in the generated
+        # __init__ signature — it would be discarded anyway.
+        @dataclass
+        class Base:
+            x: int = 0  # has a default
+
+        # Without a user-defined __init__, this would raise TypeError
+        # ("non-default argument 'y' follows default argument 'x'").
+        @dataclass
+        class Child(Base):
+            y: int  # no default — would produce invalid signature
+            def __init__(self, y):
+                self.y = y
+
+        self.assertEqual(Child(5).y, 5)
+        # Verify it's Child's own __init__, not a generated one.
+        self.assertNotIn('x', vars(Child(5)))
+
     def test_inherit_from_protocol(self):
         # Dataclasses inheriting from protocol should preserve their own `__init__`.
         # See bpo-45081.

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

@@ -0,0 +1,3 @@
+Fix :func:`dataclasses.dataclass` raising :exc:`TypeError` about field
+ordering when a subclass defines its own :meth:`~object.__init__` and
+inherits from a dataclass that has fields with default values.