Make ReturnDict support dict union operators on Python 3.9 and later (#8302)

Fixes issue #8301

rest_framework/utils/serializer_helpers.py

@@ -1,3 +1,4 @@
+import sys
 from collections import OrderedDict
 from collections.abc import Mapping, MutableMapping
 
@@ -28,6 +29,22 @@ class ReturnDict(OrderedDict):
         # but preserve the raw data.
         return (dict, (dict(self),))
 
+    if sys.version_info >= (3, 9):
+        # These are basically copied from OrderedDict, with `serializer` added.
+        def __or__(self, other):
+            if not isinstance(other, dict):
+                return NotImplemented
+            new = self.__class__(self, serializer=self.serializer)
+            new.update(other)
+            return new
+
+        def __ror__(self, other):
+            if not isinstance(other, dict):
+                return NotImplemented
+            new = self.__class__(other, serializer=self.serializer)
+            new.update(self)
+            return new
+
 
 class ReturnList(list):
     """

tests/test_serializer.py

@@ -740,3 +740,25 @@ class TestDeclaredFieldInheritance:
             'f4': serializers.CharField,
             'f5': serializers.CharField,
         }
+
+
+class Test8301Regression:
+    @pytest.mark.skipif(
+        sys.version_info < (3, 9),
+        reason="dictionary union operator requires Python 3.9 or higher",
+    )
+    def test_ReturnDict_merging(self):
+        # Serializer.data returns ReturnDict, this is essentially a test for that.
+
+        class TestSerializer(serializers.Serializer):
+            char = serializers.CharField()
+
+        s = TestSerializer(data={'char': 'x'})
+        assert s.is_valid()
+        assert s.data | {} == {'char': 'x'}
+        assert s.data | {'other': 'y'} == {'char': 'x', 'other': 'y'}
+        assert {} | s.data == {'char': 'x'}
+        assert {'other': 'y'} | s.data == {'char': 'x', 'other': 'y'}
+
+        assert (s.data | {}).__class__ == s.data.__class__
+        assert ({} | s.data).__class__ == s.data.__class__