Add `assert` statement to `.save()` method in Serializer:

- Asserts that `_data` does not exist when calling `.save()`

rest_framework/serializers.py

@@ -166,6 +166,12 @@ class BaseSerializer(Field):
             "For example: 'serializer.save(owner=request.user)'.'"
         )
 
+        assert not hasattr(self, '_data'), (
+            "You cannot call `.save()` after accessing `serializer.data`."
+            "If you need to access data before committing to the database then "
+            "inspect 'serializer.validated_data' instead. "
+        )
+
         validated_data = dict(
             list(self.validated_data.items()) +
             list(kwargs.items())

tests/test_serializer.py

@@ -61,6 +61,7 @@ class TestSerializer:
         with pytest.raises(AssertionError):
             serializer.save()
 
+
 class TestValidateMethod:
     def test_non_field_error_validate_method(self):
         class ExampleSerializer(serializers.Serializer):