I did not know that max() works with a list of strings :/

that's because in your init the list is probably pointing towards the base list, so when you delete in the object you also delete in the base list (what happens: a[0] returns 1 and deletes 1 from the a list and from the base list so base[0] returns 2 instead of 1). To avoid this, you need to do a "copy" of the base list without reference by doing something like this : self.list = baselist[:]