Again, the problem is not what it appears. You're keeping a reference to an existing item rather than making a copy. The results would be just as bad if you pas

Again, the problem is not what it appears. You're keeping a reference to an existing item rather than making a copy. The results would be just as bad if you passed in an initial list rather than taking the default. initial = ['first item'] bag3 = Bag(initial) bag3.add_item('second item') print(initial) I think the surprising thing to most people is that you don't automatically get a copy when you do the assignment. That's how it works in older languages like C and C++, and how it appears to behave when you use immutable objects.