Remove `filter_backend`.

Closes #1775.

rest_framework/generics.py

@@ -83,7 +83,6 @@ class GenericAPIView(views.APIView):
     slug_url_kwarg = 'slug'
     slug_field = 'slug'
     allow_empty = True
-    filter_backend = api_settings.FILTER_BACKEND
 
     def get_serializer_context(self):
         """
@@ -191,24 +190,7 @@ class GenericAPIView(views.APIView):
         """
         Returns the list of filter backends that this view requires.
         """
-        if self.filter_backends is None:
-            filter_backends = []
-        else:
-            # Note that we are returning a *copy* of the class attribute,
-            # so that it is safe for the view to mutate it if needed.
-            filter_backends = list(self.filter_backends)
-
-        if not filter_backends and self.filter_backend:
-            warnings.warn(
-                'The `filter_backend` attribute and `FILTER_BACKEND` setting '
-                'are deprecated in favor of a `filter_backends` '
-                'attribute and `DEFAULT_FILTER_BACKENDS` setting, that take '
-                'a *list* of filter backend classes.',
-                DeprecationWarning, stacklevel=2
-            )
-            filter_backends = [self.filter_backend]
-
-        return filter_backends
+        return list(self.filter_backends)
 
     # The following methods provide default implementations
     # that you may want to override for more complex cases.

rest_framework/settings.py

@@ -111,9 +111,6 @@ DEFAULTS = {
     ),
     'TIME_FORMAT': None,
 
-    # Pending deprecation
-    'FILTER_BACKEND': None,
-
 }
 
 
@@ -129,7 +126,6 @@ IMPORT_STRINGS = (
     'DEFAULT_PAGINATION_SERIALIZER_CLASS',
     'DEFAULT_FILTER_BACKENDS',
     'EXCEPTION_HANDLER',
-    'FILTER_BACKEND',
     'TEST_REQUEST_RENDERER_CLASSES',
     'UNAUTHENTICATED_USER',
     'UNAUTHENTICATED_TOKEN',
@@ -196,15 +192,9 @@ class APISettings(object):
         if val and attr in self.import_strings:
             val = perform_import(val, attr)
 
-        self.validate_setting(attr, val)
-
         # Cache the result
         setattr(self, attr, val)
         return val
 
-    def validate_setting(self, attr, val):
-        if attr == 'FILTER_BACKEND' and val is not None:
-            # Make sure we can initialize the class
-            val()
 
 api_settings = APISettings(USER_SETTINGS, DEFAULTS, IMPORT_STRINGS)