Fix FilterSet warnings. (#4660)

rest_framework/filters.py

@@ -38,31 +38,19 @@ class BaseFilterBackend(object):
 
 
 if django_filters:
-    from django_filters.filterset import FilterSetMetaclass as DFFilterSetMetaclass
     from django_filters.rest_framework.filterset import FilterSet as DFFilterSet
 
-    class FilterSetMetaclass(DFFilterSetMetaclass):
-        def __new__(cls, name, bases, attrs):
+    class FilterSet(DFFilterSet):
+        def __init__(self, *args, **kwargs):
             warnings.warn(
                 "The built in 'rest_framework.filters.FilterSet' is pending deprecation. "
                 "You should use 'django_filters.rest_framework.FilterSet' instead.",
                 PendingDeprecationWarning
             )
-            return super(FilterSetMetaclass, cls).__new__(cls, name, bases, attrs)
-    _BaseFilterSet = DFFilterSet
+            return super(FilterSet, self).__init__(*args, **kwargs)
 else:
-    # Dummy metaclass just so we can give a user-friendly error message.
-    class FilterSetMetaclass(type):
-        def __init__(self, name, bases, attrs):
-            # Assert only on subclasses, so we can define FilterSet below.
-            if bases != (object,):
-                assert False, 'django-filter must be installed to use the `FilterSet` class'
-            super(FilterSetMetaclass, self).__init__(name, bases, attrs)
-    _BaseFilterSet = object
-
-
-class FilterSet(six.with_metaclass(FilterSetMetaclass, _BaseFilterSet)):
-    pass
+    def FilterSet():
+        assert False, 'django-filter must be installed to use the `FilterSet` class'
 
 
 class DjangoFilterBackend(BaseFilterBackend):