9.6. Cloning Lists

If we want to modify a list and also keep a copy of the original, we need to be able to make a copy of the list itself, not just the reference. This process is sometimes called cloning, to avoid the ambiguity of the word copy.

The easiest way to clone a list is to use the slice operator.

Taking any slice of a creates a new list. In this case the slice happens to consist of the whole list.

a = [81,82,83] b = a[:] # make a clone using slice print(a == b) print(a is b) b[0] = 5 print(a) print(b)

Now we are free to make changes to b without worrying about a. Again, we can clearly see in codelens that a and b are entirely different list objects.

Check your understanding

What is printed by the following statements?

alist = [4,2,8,6,5]
blist = alist * 2
blist[3] = 999
print(alist)

• [4,2,8,999,5,4,2,8,6,5]
• print alist not print blist
• [4,2,8,999,5]
• blist is changed, not alist.
• [4,2,8,6,5]
• Yes, alist was unchanged by the assignment statement. blist was a copy of the references in alist.

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