Add validation for decorator order with @api_view (#9821)

* Add validation for decorator order with @api_view

Raise TypeError when API policy decorators (@permission_classes,
@renderer_classes, etc.) are applied after @api_view instead of before it.

Fixes #9596

* Address PR review feedback: update error message wording and example

- Change 'must be applied before' to 'must come after (below) the' to match DRF docs language
- Fix decorator order in example to show @api_view first, then policy decorator below
- Remove unnecessary f-string prefixes from non-interpolated lines
- Update all test assertions to match new error message wording

Addresses feedback from @browniebroke in PR #9821

Author: kernelshard

rest_framework/decorators.py

@@ -87,64 +87,90 @@ def handler(self, *args, **kwargs):
     return decorator
 
 
+def _check_decorator_order(func, decorator_name):
+    """
+    Check if an API policy decorator is being applied after @api_view.
+    """
+    # Check if func is actually a view function (result of APIView.as_view())
+    if hasattr(func, 'cls') and issubclass(func.cls, APIView):
+        raise TypeError(
+            f"@{decorator_name} must come after (below) the @api_view decorator. "
+            "The correct order is:\n\n"
+            "    @api_view(['GET'])\n"
+            f"    @{decorator_name}(...)\n"
+            "    def my_view(request):\n"
+            "        ...\n\n"
+            "See https://www.django-rest-framework.org/api-guide/views/#api-policy-decorators"
+        )
+
+
 def renderer_classes(renderer_classes):
     def decorator(func):
+        _check_decorator_order(func, 'renderer_classes')
         func.renderer_classes = renderer_classes
         return func
     return decorator
 
 
 def parser_classes(parser_classes):
     def decorator(func):
+        _check_decorator_order(func, 'parser_classes')
         func.parser_classes = parser_classes
         return func
     return decorator
 
 
 def authentication_classes(authentication_classes):
     def decorator(func):
+        _check_decorator_order(func, 'authentication_classes')
         func.authentication_classes = authentication_classes
         return func
     return decorator
 
 
 def throttle_classes(throttle_classes):
     def decorator(func):
+        _check_decorator_order(func, 'throttle_classes')
         func.throttle_classes = throttle_classes
         return func
     return decorator
 
 
 def permission_classes(permission_classes):
     def decorator(func):
+        _check_decorator_order(func, 'permission_classes')
         func.permission_classes = permission_classes
         return func
     return decorator
 
 
 def content_negotiation_class(content_negotiation_class):
     def decorator(func):
+        _check_decorator_order(func, 'content_negotiation_class')
         func.content_negotiation_class = content_negotiation_class
         return func
     return decorator
 
 
 def metadata_class(metadata_class):
     def decorator(func):
+        _check_decorator_order(func, 'metadata_class')
         func.metadata_class = metadata_class
         return func
     return decorator
 
 
 def versioning_class(versioning_class):
     def decorator(func):
+        _check_decorator_order(func, 'versioning_class')
         func.versioning_class = versioning_class
         return func
     return decorator
 
 
 def schema(view_inspector):
     def decorator(func):
+        _check_decorator_order(func, 'schema')
         func.schema = view_inspector
         return func
     return decorator

tests/test_decorators.py

@@ -204,6 +204,124 @@ def view(request):
 
         assert isinstance(view.cls.schema, CustomSchema)
 
+    def test_incorrect_decorator_order_permission_classes(self):
+        """
+        If @permission_classes is applied after @api_view, we should raise a TypeError.
+        """
+        with self.assertRaises(TypeError) as cm:
+            @permission_classes([IsAuthenticated])
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@permission_classes must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_renderer_classes(self):
+        """
+        If @renderer_classes is applied after @api_view, we should raise a TypeError.
+        """
+        with self.assertRaises(TypeError) as cm:
+            @renderer_classes([JSONRenderer])
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@renderer_classes must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_parser_classes(self):
+        """
+        If @parser_classes is applied after @api_view, we should raise a TypeError.
+        """
+        with self.assertRaises(TypeError) as cm:
+            @parser_classes([JSONParser])
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@parser_classes must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_authentication_classes(self):
+        """
+        If @authentication_classes is applied after @api_view, we should raise a TypeError.
+        """
+        with self.assertRaises(TypeError) as cm:
+            @authentication_classes([BasicAuthentication])
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@authentication_classes must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_throttle_classes(self):
+        """
+        If @throttle_classes is applied after @api_view, we should raise a TypeError.
+        """
+        class OncePerDayUserThrottle(UserRateThrottle):
+            rate = '1/day'
+
+        with self.assertRaises(TypeError) as cm:
+            @throttle_classes([OncePerDayUserThrottle])
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@throttle_classes must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_versioning_class(self):
+        """
+        If @versioning_class is applied after @api_view, we should raise a TypeError.
+        """
+        with self.assertRaises(TypeError) as cm:
+            @versioning_class(QueryParameterVersioning)
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@versioning_class must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_metadata_class(self):
+        """
+        If @metadata_class is applied after @api_view, we should raise a TypeError.
+        """
+        with self.assertRaises(TypeError) as cm:
+            @metadata_class(None)
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@metadata_class must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_content_negotiation_class(self):
+        """
+        If @content_negotiation_class is applied after @api_view, we should raise a TypeError.
+        """
+        class CustomContentNegotiation(BaseContentNegotiation):
+            def select_renderer(self, request, renderers, format_suffix):
+                return (renderers[0], renderers[0].media_type)
+
+        with self.assertRaises(TypeError) as cm:
+            @content_negotiation_class(CustomContentNegotiation)
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@content_negotiation_class must come after (below) the @api_view decorator' in str(cm.exception)
+
+    def test_incorrect_decorator_order_schema(self):
+        """
+        If @schema is applied after @api_view, we should raise a TypeError.
+        """
+        class CustomSchema(AutoSchema):
+            pass
+
+        with self.assertRaises(TypeError) as cm:
+            @schema(CustomSchema())
+            @api_view(['GET'])
+            def view(request):
+                return Response({})
+
+        assert '@schema must come after (below) the @api_view decorator' in str(cm.exception)
+
 
 class ActionDecoratorTestCase(TestCase):