diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 7a24f8a9e5ccee..75861ee908593b 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1009,6 +1009,10 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
# Otherwise it's a field of some type.
cls_fields.append(_get_field(cls, name, type, kw_only))
+ # Test whether '__init__' is to be auto-generated or if
+ # it is provided explicitly by the user.
+ has_init_method = init or '__init__' in cls.__dict__
+
for f in cls_fields:
fields[f.name] = f
@@ -1018,6 +1022,15 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
# sees a real default value, not a Field.
if isinstance(getattr(cls, f.name, None), Field):
if f.default is MISSING:
+ # https://github.com/python/cpython/issues/89529
+ if f.default_factory is not MISSING and not has_init_method:
+ raise ValueError(
+ f'specifying default_factory for {f.name!r}'
+ f' requires the @dataclass decorator to be'
+ f' called with init=True or to implement'
+ f' an __init__ method'
+ )
+
# If there's no default, delete the class attribute.
# This happens if we specify field(repr=False), for
# example (that is, we specified a field object, but
diff --git a/Lib/test/test_dataclasses/__init__.py b/Lib/test/test_dataclasses/__init__.py
index 2e6c49e29ce828..48e95f3cb8df1d 100644
--- a/Lib/test/test_dataclasses/__init__.py
+++ b/Lib/test/test_dataclasses/__init__.py
@@ -9,6 +9,7 @@
import pickle
import inspect
import builtins
+import re
import types
import weakref
import traceback
@@ -18,6 +19,7 @@
from typing import get_type_hints
from collections import deque, OrderedDict, namedtuple, defaultdict
from copy import deepcopy
+from itertools import product
from functools import total_ordering, wraps
import typing # Needed for the string "typing.ClassVar[int]" to work as an annotation.
@@ -1413,6 +1415,71 @@ class C:
C().x
self.assertEqual(factory.call_count, 2)
+ def test_default_factory_and_init_method_interaction(self):
+ # See https://github.com/python/cpython/issues/89529.
+
+ @dataclass
+ class BaseWithInit:
+ x: list
+
+ @dataclass(slots=True)
+ class BaseWithSlots:
+ x: list
+
+ @dataclass(init=False)
+ class BaseWithOutInit:
+ x: list
+
+ @dataclass(init=False, slots=True)
+ class BaseWithOutInitWithSlots:
+ x: list
+
+ err = re.escape(
+ "specifying default_factory for 'x' requires the "
+ "@dataclass decorator to be called with init=True "
+ "or to implement an __init__ method"
+ )
+
+ for base_class, slots, field_init in product(
+ (object, BaseWithInit, BaseWithSlots,
+ BaseWithOutInit, BaseWithOutInitWithSlots),
+ (True, False),
+ (True, False),
+ ):
+ with self.subTest('generated __init__', base_class=base_class,
+ init=True, slots=slots, field_init=field_init):
+ @dataclass(init=True, slots=slots)
+ class C(base_class):
+ x: list = field(init=field_init, default_factory=list)
+ self.assertListEqual(C().x, [])
+
+ with self.subTest('user-defined __init__', base_class=base_class,
+ init=True, slots=slots, field_init=field_init):
+ @dataclass(init=True, slots=slots)
+ class C(base_class):
+ x: list = field(init=field_init, default_factory=list)
+ def __init__(self, *a, **kw):
+ # deliberately use something else
+ self.x = 'hello'
+ self.assertEqual(C().x, 'hello')
+
+ with self.subTest('no generated __init__', base_class=base_class,
+ init=False, slots=slots, field_init=field_init):
+ with self.assertRaisesRegex(ValueError, err):
+ @dataclass(init=False, slots=slots)
+ class C(base_class):
+ x: list = field(init=field_init, default_factory=list)
+
+ with self.subTest('user-defined __init__', base_class=base_class,
+ init=False, slots=slots, field_init=field_init):
+ @dataclass(init=False, slots=slots)
+ class C(base_class):
+ x: list = field(init=field_init, default_factory=list)
+ def __init__(self, *a, **kw):
+ # deliberately use something else
+ self.x = 'world'
+ self.assertEqual(C().x, 'world')
+
def test_default_factory_not_called_if_value_given(self):
# We need a factory that we can test if it's been called.
factory = Mock()
diff --git a/Misc/NEWS.d/next/Library/2024-08-16-15-35-57.gh-issue-89529.ayfZ1n.rst b/Misc/NEWS.d/next/Library/2024-08-16-15-35-57.gh-issue-89529.ayfZ1n.rst
new file mode 100644
index 00000000000000..e34859d1cf26f3
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-08-16-15-35-57.gh-issue-89529.ayfZ1n.rst
@@ -0,0 +1,2 @@
+Disallow ``default_factory`` for dataclass fields if the dataclass does not
+have an ``__init__`` method. Patch by Bénédikt Tran.
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