#- class test: #- var1=1 #- var2=2 #- res=var1+var2 #- #- t=test() #- print t.res #- >> 3 So far OK. #- t.var1=6 #- print t.res #- >> 3 Because you can not change a class attribute changing the instance attribute. Just google for "class variables in python". . Facundo