diff --git a/odoo-rpc.py b/odoo-rpc.py
index 4244e2b..b689f74 100755
--- a/odoo-rpc.py
+++ b/odoo-rpc.py
@@ -339,14 +339,14 @@ def flatten_stack( data ):
 	def flatten( obj ):
 		if type(obj) == dict:
 			key = obj.keys()[0]
-			flat += [key]
+			flat.append( key )
 			for field in obj[key].keys():
 				flatten( obj[key][field] )
 			
 		if type(obj) == list:
 			for item in obj:
 				if type(item) == str:
-					flat += [item]
+					flat.append( item )
 				if type(item) == dict:
 					flatten( item )