pythongh-102699: Add dataclasses.DataclassLike

Author: AlexWaygood

Doc/library/dataclasses.rst

@@ -468,6 +468,30 @@ Module contents
      def is_dataclass_instance(obj):
          return is_dataclass(obj) and not isinstance(obj, type)
 
+.. class:: DataclassLike
+
+   An abstract base class for all dataclasses. Mainly useful for type-checking.
+
+   All classes created using the :func:`@dataclass <dataclass>` decorator are
+   considered subclasses of this class; all dataclass instances are considered
+   instances of this class:
+
+      >>> from dataclasses import dataclass, DataclassLike
+      >>> @dataclass
+      ... class Foo:
+      ...     x: int
+      ...
+      >>> issubclass(Foo, DataclassLike)
+      True
+      >>> isinstance(Foo(), DataclassLike)
+      True
+
+   ``DataclassLike`` is an abstract class that cannot be instantiated. It is
+   also a "final" class that cannot be subclassed: use the
+   :func:`@dataclass <dataclass>` decorator to create new dataclasses.
+
+   .. versionadded:: 3.12
+
 .. data:: MISSING
 
    A sentinel value signifying a missing default or default_factory.

Lib/dataclasses.py

@@ -26,6 +26,7 @@
            'make_dataclass',
            'replace',
            'is_dataclass',
+           'DataclassLike',
            ]
 
 # Conditions for adding methods.  The boxes indicate what action the
@@ -1267,6 +1268,32 @@ def is_dataclass(obj):
     return hasattr(cls, _FIELDS)
 
 
+class DataclassLike(metaclass=abc.ABCMeta):
+    """Abstract base class for all dataclass types.
+
+    Mainly useful for type-checking.
+    """
+    # __dataclass_fields__ here is really an "abstract class variable",
+    # but there's no good way of expressing that at runtime,
+    # so just make it a regular class variable with a dummy value
+    __dataclass_fields__ = {}
+
+    def __init_subclass__(cls):
+        raise TypeError(
+            "Use the @dataclass decorator to create dataclasses, "
+            "rather than subclassing dataclasses.DataclassLike"
+        )
+
+    def __new__(cls):
+        raise TypeError(
+            "dataclasses.DataclassLike is an abstract class that cannot be instantiated"
+        )
+
+    @classmethod
+    def __subclasshook__(cls, other):
+        return hasattr(other, _FIELDS)
+
+
 def asdict(obj, *, dict_factory=dict):
     """Return the fields of a dataclass instance as a new dictionary mapping
     field names to field values.

Lib/test/test_dataclasses.py

@@ -1503,6 +1503,58 @@ class A(types.GenericAlias):
         self.assertTrue(is_dataclass(type(a)))
         self.assertTrue(is_dataclass(a))
 
+    def test_DataclassLike(self):
+        with self.assertRaises(TypeError):
+            DataclassLike()
+
+        with self.assertRaises(TypeError):
+            class Foo(DataclassLike): pass
+
+        @dataclass
+        class Dataclass:
+            x: int
+
+        self.assertTrue(issubclass(Dataclass, DataclassLike))
+        self.assertIsInstance(Dataclass(42), DataclassLike)
+
+        with self.assertRaises(TypeError):
+            issubclass(Dataclass(42), DataclassLike)
+
+        class NotADataclass:
+            def __init__(self):
+                self.x = 42
+
+        self.assertFalse(issubclass(NotADataclass, DataclassLike))
+        self.assertNotIsInstance(NotADataclass(), DataclassLike)
+
+        class NotADataclassButDataclassLike:
+            """A class from an outside library (attrs?) with dataclass-like behaviour"""
+            __dataclass_fields__ = {}
+
+        self.assertTrue(issubclass(NotADataclassButDataclassLike, DataclassLike))
+        self.assertIsInstance(NotADataclassButDataclassLike(), DataclassLike)
+
+        class HasInstanceDataclassFieldsAttribute:
+            def __init__(self):
+                self.__dataclass_fields__ = {}
+
+        self.assertFalse(issubclass(HasInstanceDataclassFieldsAttribute, DataclassLike))
+        self.assertNotIsInstance(HasInstanceDataclassFieldsAttribute(), DataclassLike)
+
+        class HasAllAttributes:
+            def __getattr__(self, name):
+                return {}
+
+        self.assertFalse(issubclass(HasAllAttributes, DataclassLike))
+        self.assertNotIsInstance(HasAllAttributes(), DataclassLike)
+
+        @dataclass
+        class GenericAliasSubclass(types.GenericAlias):
+            origin: type
+            args: type
+
+        self.assertTrue(issubclass(GenericAliasSubclass, DataclassLike))
+        self.assertIsInstance(GenericAliasSubclass(int, str), DataclassLike)
 
     def test_helper_fields_with_class_instance(self):
         # Check that we can call fields() on either a class or instance,

Misc/NEWS.d/next/Library/2023-03-22-21-05-34.gh-issue-102699.48uE4z.rst

@@ -0,0 +1,2 @@
+Add :class:`dataclasses.DataclassLike`, an abstract base class for all
+dataclasses. Patch by Alex Waygood.