Index: pyyaml/trunk/lib/yaml/serializer.py
===================================================================
--- pyyaml/trunk/lib/yaml/serializer.py	(revision 139)
+++ pyyaml/trunk/lib/yaml/serializer.py	(revision 222)
@@ -9,5 +9,5 @@
     pass
 
-class Serializer:
+class Serializer(object):
 
     ANCHOR_TEMPLATE = u'id%03d'
@@ -68,12 +68,7 @@
                     self.anchor_node(item)
             elif isinstance(node, MappingNode):
-                if hasattr(node.value, 'keys'):
-                    for key in node.value.keys():
-                        self.anchor_node(key)
-                        self.anchor_node(node.value[key])
-                else:
-                    for key, value in node.value:
-                        self.anchor_node(key)
-                        self.anchor_node(value)
+                for key, value in node.value:
+                    self.anchor_node(key)
+                    self.anchor_node(value)
 
     def generate_anchor(self, node):
@@ -109,12 +104,7 @@
                 self.emit(MappingStartEvent(alias, node.tag, implicit,
                     flow_style=node.flow_style))
-                if hasattr(node.value, 'keys'):
-                    for key in node.value.keys():
-                        self.serialize_node(key, node, None)
-                        self.serialize_node(node.value[key], node, key)
-                else:
-                    for key, value in node.value:
-                        self.serialize_node(key, node, None)
-                        self.serialize_node(value, node, key)
+                for key, value in node.value:
+                    self.serialize_node(key, node, None)
+                    self.serialize_node(value, node, key)
                 self.emit(MappingEndEvent())
             self.ascend_resolver()