Fixed #18963 -- Used a subclass-friendly pattern

for Python 2 object model compatibility methods.

django/core/files/base.py

@@ -28,7 +28,9 @@ class File(FileProxyMixin):
 
     def __bool__(self):
         return bool(self.name)
-    __nonzero__ = __bool__ # Python 2
+
+    def __nonzero__(self):      # Python 2 compatibility
+        return type(self).__bool__(self)
 
     def __len__(self):
         return self.size
@@ -142,7 +144,9 @@ class ContentFile(File):
 
     def __bool__(self):
         return True
-    __nonzero__ = __bool__ # Python 2
+
+    def __nonzero__(self):      # Python 2 compatibility
+        return type(self).__bool__(self)
 
     def open(self, mode=None):
         self.seek(0)

django/core/serializers/base.py

@@ -112,7 +112,7 @@ class Serializer(object):
         if callable(getattr(self.stream, 'getvalue', None)):
             return self.stream.getvalue()
 
-class Deserializer(object):
+class Deserializer(six.Iterator):
     """
     Abstract base deserializer class.
     """
@@ -138,8 +138,6 @@ class Deserializer(object):
         """Iteration iterface -- return the next item in the stream"""
         raise NotImplementedError
 
-    next = __next__             # Python 2 compatibility
-
 class DeserializedObject(object):
     """
     A deserialized model.

django/core/serializers/xml_serializer.py

@@ -161,8 +161,6 @@ class Deserializer(base.Deserializer):
                 return self._handle_object(node)
         raise StopIteration
 
-    next = __next__             # Python 2 compatibility
-
     def _handle_object(self, node):
         """
         Convert an <object> node to a DeserializedObject.