[3.10] bpo-45663: Fix is_dataclass() for dataclasses which are subclasses of types.GenericAlias (GH-29294)

Lib/dataclasses.py

@@ -1211,7 +1211,7 @@ def _is_dataclass_instance(obj):
 def is_dataclass(obj):
     """Returns True if obj is a dataclass or an instance of a
     dataclass."""
-    cls = obj if isinstance(obj, type) else type(obj)
+    cls = obj if isinstance(obj, type) and not isinstance(obj, GenericAlias) else type(obj)
     return hasattr(cls, _FIELDS)
 
 

Lib/test/test_dataclasses.py

@@ -8,6 +8,7 @@
 import pickle
 import inspect
 import builtins
+import types
 import unittest
 from unittest.mock import Mock
 from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional, Protocol
@@ -1350,6 +1351,17 @@ class B:
                 with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'):
                     replace(obj, x=0)
 
+    def test_is_dataclass_genericalias(self):
+        @dataclass
+        class A(types.GenericAlias):
+            origin: type
+            args: type
+        self.assertTrue(is_dataclass(A))
+        a = A(list, int)
+        self.assertTrue(is_dataclass(type(a)))
+        self.assertTrue(is_dataclass(a))
+
+
     def test_helper_fields_with_class_instance(self):
         # Check that we can call fields() on either a class or instance,
         #  and get back the same thing.

Misc/NEWS.d/next/Library/2021-10-28-23-11-59.bpo-45663.J90N5R.rst

@@ -0,0 +1,2 @@
+Fix :func:`dataclasses.is_dataclass` for dataclasses which are subclasses of
+:class:`types.GenericAlias`.