在python实现函数的多态性，具体方法如下：
def remove_user(self,user_or_username):
try:
#If user_or_username is a User object
username = user_or_username.name
except AttributeError: #Oops -- didn't works. ask forgiveness ;-)
#If user_or_username is a string
username = user_or_username
remote.remove(username)
