class myList(list):
    def myAppend(self, item):
        if isinstance(item, list):
            print 'Appending a list'
            self.append(item)
        elif isinstance(item, str):
            print 'Appending a string item'
            self.append(item)
        else:
            raise Exception

L = myList()
L.myAppend([1,2,3])
L.myAppend('one two three')
print L

#Output:
#Appending a list
#Appending a string item
#[[1, 2, 3], 'one two three']