Make sure Pydantic dataclasses with slots and validate_assignment a…

…re unpickable
pydantic · Viicos · Apr 29, 2025 · Apr 17, 2025 · Apr 26, 2025 · Apr 17, 2025
commit d174f8b8f7c1775e3f3ca42a08ec2c6f8fd1f835
diff --git a/pydantic/_internal/_dataclasses.py b/pydantic/_internal/_dataclasses.py
@@ -5,7 +5,7 @@
 import dataclasses
 import typing
 import warnings
-from functools import partial, wraps
+from functools import partial
 from typing import Any, ClassVar
 
 from pydantic_core import (
@@ -178,22 +178,12 @@ def __init__(__dataclass_self__: PydanticDataclass, *args: Any, **kwargs: Any) -
     # We are about to set all the remaining required properties expected for this cast;
     # __pydantic_decorators__ and __pydantic_fields__ should already be set
     cls = typing.cast('type[PydanticDataclass]', cls)
-    # debug(schema)
 
     cls.__pydantic_core_schema__ = schema
-    cls.__pydantic_validator__ = validator = create_schema_validator(
+    cls.__pydantic_validator__ = create_schema_validator(
         schema, cls, cls.__module__, cls.__qualname__, 'dataclass', core_config, config_wrapper.plugin_settings
     )
     cls.__pydantic_serializer__ = SchemaSerializer(schema, core_config)
-
-    if config_wrapper.validate_assignment:
-
-        @wraps(cls.__setattr__)
-        def validated_setattr(instance: Any, field: str, value: str, /) -> None:
-            validator.validate_assignment(instance, field, value)
-
-        cls.__setattr__ = validated_setattr.__get__(None, cls)  # type: ignore
-
     cls.__pydantic_complete__ = True
     return True
 

diff --git a/pydantic/dataclasses.py b/pydantic/dataclasses.py
@@ -3,6 +3,7 @@
 from __future__ import annotations as _annotations
 
 import dataclasses
+import functools
 import sys
 import types
 from typing import TYPE_CHECKING, Any, Callable, Generic, Literal, NoReturn, TypeVar, overload
@@ -264,6 +265,35 @@ def create_dataclass(cls: type[Any]) -> type[PydanticDataclass]:
             **kwargs,
         )
 
+        if config_wrapper.validate_assignment:
+
+            @functools.wraps(cls.__setattr__)
+            def validated_setattr(instance: Any, field: str, value: str, /) -> None:
+                type(instance).__pydantic_validator__.validate_assignment(instance, field, value)
+
+            cls.__setattr__ = validated_setattr.__get__(None, cls)  # type: ignore
+
+            if slots and not hasattr(cls, '__setstate__'):
+                # If slots is set, `pickle` (relied on by `copy.copy()`) will use
+                # `__setattr__()` to reconstruct the dataclass. However, the custom
+                # `__setattr__()` set above relies on `validate_assignment()`, which
+                # in turn excepts all the field values to be already present on the
+                # instance, resulting in attribute errors.
+                # As such, we make use of `object.__setattr__()` instead.
+                # Note that we do so only if `__setstate__()` isn't already set (this is the
+                # case if on top of `slots`, `frozen` is used).
+
+                # Taken from `dataclasses._dataclass_get/setstate()`:
+                def _dataclass_getstate(self: Any) -> list[Any]:
+                    return [getattr(self, f.name) for f in dataclasses.fields(self)]
+
+                def _dataclass_setstate(self: Any, state: list[Any]) -> None:
+                    for field, value in zip(dataclasses.fields(self), state):
+                        object.__setattr__(self, field.name, value)
+
+                cls.__getstate__ = _dataclass_getstate  # pyright: ignore[reportAttributeAccessIssue]
+                cls.__setstate__ = _dataclass_setstate  # pyright: ignore[reportAttributeAccessIssue]
+
         # This is an undocumented attribute to distinguish stdlib/Pydantic dataclasses.
         # It should be set as early as possible:
         cls.__is_pydantic_dataclass__ = True

diff --git a/tests/test_dataclasses.py b/tests/test_dataclasses.py
@@ -2430,6 +2430,23 @@ class Model:
     assert dc.b == 'bar'
 
 
+# Must be defined at the module level to be pickable:
+@pydantic.dataclasses.dataclass(slots=True, config={'validate_assignment': True})
+class DataclassSlotsValidateAssignment:
+    a: int
+
+
+@pytest.mark.skipif(sys.version_info < (3, 10), reason='slots are only supported for dataclasses in Python >= 3.10')
+def test_dataclass_slots_validate_assignment():
+    """https://github.com/pydantic/pydantic/issues/11768"""
+
+    m = DataclassSlotsValidateAssignment(1)
+    m_pickle = pickle.loads(pickle.dumps(m))
+    assert m_pickle.a == 1
+    with pytest.raises(ValidationError):
+        m.a = 'not_an_int'
+
+
 @pytest.mark.parametrize(
     'dataclass_decorator',
     [