列表撤消中的Python列表

Question

I'm making a program that will do some operations like add, insert, pop on a list of numbers. I have to also make an "undo" function and for that I thought about making a list in list to save up my list every step of the way but my tests are not working. Here's the code:

def backup(l):
        lbackup.append(list(l))

<-- here I save the list every step of the way by calling this function

> def undo(l):
>     l=lbackup[len(lbackup)-1] # here I give my list the previous list
>     del lbackup[len(lbackup)-1]  # here I delete the sublist from the history

My test:

def testUndo():
    add(lista, 1)
    backup(lista)
    add(lista, 2)
    backup(lista)
    add(lista,111111)
    print(lbackup)
    print(lista)
    undo(lista)
    print(lista)
    print(lbackup)

So I checked the inside of the Undo function and it's working! It actually changed the value of my list to what I wanted but once it left the function and got to the print after undo in the test... it had the same values again. I know lists are mutable and that might be a problem but is there anything I could do? Sorry if I wasnt clean enough.

Answer 1

When you call, say, undo(lista) , the l inside of undo may be the same list as lista , but assigning to l does not change lista . If instead you had undo return l , and assign the value returned by undo back to list , that would make the change you seem to want.