Add dataclass_compatible flag

docs/api.rst

@@ -202,7 +202,7 @@ Helpers
       ... class CInspect:
       ...     pass
       >>> attrs.inspect(CInspect)  # doctest: +ELLIPSIS
-      ClassProps(is_exception=False, is_slotted=True, has_weakref_slot=True, is_frozen=False, kw_only=<KeywordOnly.NO: 'no'>, collected_fields_by_mro=True, added_init=True, added_repr=True, added_eq=True, added_ordering=False, hashability=<Hashability.UNHASHABLE: 'unhashable'>, added_match_args=True, added_str=False, added_pickling=True, on_setattr_hook=<function pipe.<locals>.wrapped_pipe at ...>, field_transformer=None)
+      ClassProps(is_exception=False, is_slotted=True, has_weakref_slot=True, is_frozen=False, kw_only=<KeywordOnly.NO: 'no'>, collected_fields_by_mro=True, added_init=True, added_repr=True, added_eq=True, added_ordering=False, hashability=<Hashability.UNHASHABLE: 'unhashable'>, added_match_args=True, added_str=False, added_pickling=True, added_dataclass_fields=False, on_setattr_hook=<function pipe.<locals>.wrapped_pipe at ...>, field_transformer=None)
 
 .. autoclass:: attrs.ClassProps
 .. autoclass:: attrs.ClassProps.Hashability

src/attr/_make.py

@@ -1022,6 +1022,32 @@ def __str__(self):
         self._cls_dict["__str__"] = self._add_method_dunders(__str__)
         return self
 
+    def add_dataclass_fields(self):
+        import dataclasses
+
+        dataclass_fields = {}
+        for attribute in self._attrs:
+            field = dataclasses.field()
+            field._field_type = dataclasses._FIELD
+
+            field.name = attribute.name
+            if attribute.type is None:
+                msg = "__dataclass_fields__ can only be generated if all attributes are annotated."
+                raise ValueError(msg)
+            field.type = attribute.type
+            field.kw_only = attribute.kw_only
+            field.metadata = attribute.metadata
+            if attribute.default is not NOTHING:
+                if isinstance(attribute.default, Factory):
+                    field.default_factory = attribute.default.factory
+                else:
+                    field.default = attribute.default
+
+            dataclass_fields[field.name] = field
+
+        self._cls_dict["__dataclass_fields__"] = dataclass_fields
+        return self
+
     def _make_getstate_setstate(self):
         """
         Create custom __setstate__ and __getstate__ methods.
@@ -1374,6 +1400,7 @@ def attrs(
     on_setattr=None,
     field_transformer=None,
     match_args=True,
+    dataclass_compatible=False,
     unsafe_hash=None,
     force_kw_only=True,
 ):
@@ -1557,6 +1584,7 @@ def wrap(cls):
                 ("__getstate__", "__setstate__"),
                 default=slots,
             ),
+            added_dataclass_fields=dataclass_compatible,
             on_setattr_hook=on_setattr,
             field_transformer=field_transformer,
         )
@@ -1606,6 +1634,9 @@ def wrap(cls):
         if match_args and not _has_own_attribute(cls, "__match_args__"):
             builder.add_match_args()
 
+        if props.added_dataclass_fields:
+            builder.add_dataclass_fields()
+
         return builder.build_class()
 
     # maybe_cls's type depends on the usage of the decorator.  It's a class
@@ -2959,12 +2990,17 @@ class ClassProps:
             Whether the class has *attrs*-generated ``__getstate__`` and
             ``__setstate__`` methods for `pickle`.
 
+        added_dataclass_fields (bool):
+            Whether the class has an *attrs*-generated ``__dataclass_fields__``
+            attribute.
+
         on_setattr_hook (Callable[[Any, Attribute[Any], Any], Any] | None):
             The class's ``__setattr__`` hook.
 
         field_transformer (Callable[[Attribute[Any]], Attribute[Any]] | None):
             The class's `field transformers <transform-fields>`.
 
+
     .. versionadded:: 25.4.0
     """
 
@@ -3013,6 +3049,7 @@ class KeywordOnly(enum.Enum):
         "added_match_args",
         "added_str",
         "added_pickling",
+        "added_dataclass_fields",
         "on_setattr_hook",
         "field_transformer",
     )
@@ -3033,6 +3070,7 @@ def __init__(
         added_match_args,
         added_str,
         added_pickling,
+        added_dataclass_fields,
         on_setattr_hook,
         field_transformer,
     ):
@@ -3050,6 +3088,7 @@ def __init__(
         self.added_match_args = added_match_args
         self.added_str = added_str
         self.added_pickling = added_pickling
+        self.added_dataclass_fields = added_dataclass_fields
         self.on_setattr_hook = on_setattr_hook
         self.field_transformer = field_transformer
 

src/attr/_next_gen.py

@@ -43,6 +43,7 @@ def define(
     on_setattr=None,
     field_transformer=None,
     match_args=True,
+    dataclass_compatible=False,
     force_kw_only=False,
 ):
     r"""
@@ -243,6 +244,11 @@ def define(
             :pep:`634` (*Structural Pattern Matching*). It is a tuple of all
             non-keyword-only ``__init__`` parameter names.
 
+        dataclass_compatible (bool):
+            If True, add ``__dataclass_fields__`` to the class. This enables
+            compatibility with various dataclass functions, notably
+            `dataclasses.fields`. Only works if all of the fields are annotated.
+
         force_kw_only (bool):
             A back-compat flag for restoring pre-25.4.0 behavior.  If True and
             ``kw_only=True``, all attributes are made keyword-only, including
@@ -384,6 +390,7 @@ def do_it(cls, auto_attribs):
             field_transformer=field_transformer,
             match_args=match_args,
             force_kw_only=force_kw_only,
+            dataclass_compatible=dataclass_compatible,
         )
 
     def wrap(cls):

src/attrs/__init__.pyi

@@ -183,6 +183,7 @@ def define(
     on_setattr: _OnSetAttrArgType | None = ...,
     field_transformer: _FieldTransformer | None = ...,
     match_args: bool = ...,
+    dataclass_compatible: bool = ...,
 ) -> _C: ...
 @overload
 @dataclass_transform(field_specifiers=(attrib, field))
@@ -209,6 +210,7 @@ def define(
     on_setattr: _OnSetAttrArgType | None = ...,
     field_transformer: _FieldTransformer | None = ...,
     match_args: bool = ...,
+    dataclass_compatible: bool = ...,
 ) -> Callable[[_C], _C]: ...
 
 mutable = define
@@ -288,6 +290,7 @@ class ClassProps:
     added_match_args: bool
     added_str: bool
     added_pickling: bool
+    added_dataclass_fields: bool
     on_setattr_hook: _OnSetAttrType | None
     field_transformer: Callable[[Attribute[Any]], Attribute[Any]] | None
 
@@ -309,6 +312,7 @@ class ClassProps:
         added_match_args: bool,
         added_str: bool,
         added_pickling: bool,
+        added_dataclass_fields: bool,
         on_setattr_hook: _OnSetAttrType,
         field_transformer: Callable[[Attribute[Any]], Attribute[Any]],
     ) -> None: ...

tests/test_dataclass_compatible.py

@@ -0,0 +1,88 @@
+# SPDX-License-Identifier: MIT
+
+"""
+Tests for types with dataclass compatibility added.
+"""
+
+import dataclasses
+
+from typing import Annotated, Any
+
+import pytest
+
+import attrs
+
+
+def _fields_as_tuple(o: Any) -> list[tuple[object, ...]]:
+    return [
+        (
+            f._field_type,
+            f.name,
+            f.type,
+            f.kw_only,
+            f.metadata,
+            f.default,
+            f.default_factory,
+        )
+        for f in dataclasses.fields(o)
+    ]
+
+
+class TestDataclassCompatible:
+    """
+    Tests for types with dataclass compatibility added.
+    """
+
+    def test_dataclass_compatible_fields(self):
+        """
+        Check that setting `dataclass_compatible` makes a dataclass-y type.
+        """
+
+        @attrs.define(dataclass_compatible=True)
+        class C:
+            x: Annotated[int, "some-annotation"]
+            y: "float" = 3.14
+            z: str = attrs.field(metadata={"foo": "bar"}, default="baz")
+            my_list: list[int] = attrs.field(factory=list)
+
+            def __attrs_post_init__(self) -> None:
+                self.x += 1
+
+        @dataclasses.dataclass
+        class D:
+            x: Annotated[int, "some-annotation"]
+            y: "float" = 3.14
+            z: str = dataclasses.field(metadata={"foo": "bar"}, default="baz")
+            my_list: list[int] = dataclasses.field(default_factory=list)
+
+            def __post_init__(self) -> None:
+                self.x += 1
+
+        # Assert at the class level
+        assert _fields_as_tuple(C) == _fields_as_tuple(D)
+
+        # Assert at the instance level
+        assert _fields_as_tuple(C(1)) == _fields_as_tuple(D(1))
+
+        # Check high level dataclasses functions
+        assert dataclasses.is_dataclass(C) == dataclasses.is_dataclass(D)
+        assert dataclasses.is_dataclass(C(1)) == dataclasses.is_dataclass(D(1))
+        assert dataclasses.asdict(C(1)) == dataclasses.asdict(D(1))
+        assert dataclasses.astuple(C(1)) == dataclasses.astuple(D(1))
+        assert dataclasses.asdict(
+            dataclasses.replace(C(1), x=2)
+        ) == dataclasses.asdict(dataclasses.replace(D(1), x=2))
+
+    def test_raises_on_missing_type(self):
+        """
+        Raises ValueError if type is missing.
+        """
+        with pytest.raises(ValueError) as e:
+
+            @attrs.define(dataclass_compatible=True)
+            class C:
+                x = attrs.field()
+
+        assert (
+            "__dataclass_fields__ can only be generated if all attributes are annotated.",
+        ) == e.value.args

tests/test_make.py

@@ -591,6 +591,7 @@ class C:
             collected_fields_by_mro=False,
             added_str=True,
             added_pickling=True,
+            added_dataclass_fields=False,
             on_setattr_hook=None,
             field_transformer=None,
         ) == attrs.inspect(C)
@@ -620,6 +621,7 @@ class CDef:
             collected_fields_by_mro=False,
             added_str=False,
             added_pickling=False,
+            added_dataclass_fields=False,
             on_setattr_hook=None,
             field_transformer=None,
         ) == attrs.inspect(CDef)
@@ -2090,6 +2092,7 @@ class C:
                 collected_fields_by_mro=True,
                 added_str=False,
                 added_pickling=True,
+                added_dataclass_fields=False,
                 on_setattr_hook=None,
                 field_transformer=None,
             ),
@@ -2125,6 +2128,7 @@ class C:
                 collected_fields_by_mro=True,
                 added_str=False,
                 added_pickling=True,
+                added_dataclass_fields=False,
                 on_setattr_hook=None,
                 field_transformer=None,
             ),
@@ -2226,6 +2230,7 @@ def our_hasattr(obj, name, /) -> bool:
                 collected_fields_by_mro=True,
                 added_str=False,
                 added_pickling=True,
+                added_dataclass_fields=False,
                 on_setattr_hook=None,
                 field_transformer=None,
             ),

tests/test_next_gen.py

@@ -493,6 +493,7 @@ class C:
                 collected_fields_by_mro=True,
                 added_str=True,
                 added_pickling=False,  # because slots=False
+                added_dataclass_fields=False,
                 on_setattr_hook=None,
                 field_transformer=None,
             )
@@ -524,6 +525,7 @@ class C:
                 collected_fields_by_mro=True,
                 added_str=False,
                 added_pickling=True,
+                added_dataclass_fields=False,
                 on_setattr_hook=None,
                 field_transformer=None,
             )