9. Repetition and References¶

We have already seen the repetition operator working on strings as well as lists. For example,

orig_list = [45, 76, 34, 55] print(orig_list * 3)

With a list, the repetition operator creates copies of the references. Although this may seem simple enough, when we allow a list to refer to another list, a subtle problem can arise.

Consider the following extension on the previous example.

orig_list = [45, 76, 34, 55] print(orig_list * 3) new_list = [orig_list] * 3 print(new_list)

new_list is a list of three references to orig_list that were created by the repetition operator. The reference diagram is shown below.

Now, what happens if we modify a value in orig_list.

orig_list = [45, 76, 34, 55] new_list = [orig_list] * 3 print(new_list) orig_list[1] = 99 print(new_list)

new_list shows the change in three places. This can easily be seen by noting that in the reference diagram, there is only one orig_list, so any changes to it appear in all three references from new_list.

Here is the same example in codelens. Step through the code paying particular attention to the result of executing the assignment statement orig_list[1] = 99.

It is worth noting that there is a difference between new_list = [orig_list] * 3 and an assignment statement where orig_list is not in brackets: another_list = orig_list * 3. The former creates a list of three references to orig_list whereas the latter creates a new object using the values in orig_list. Note in the example below how new_list will change when a value in orig_list changes, but another_list does not change because it is no longer bound to orig_list.

orig_list = [45, 76, 34, 55] new_list = [orig_list] * 3 another_list = orig_list * 3 print(new_list) print(another_list) orig_list[1] = 99 print(new_list) # Note the change print(another_list) # Note the lack of change

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.

What is printed by the following statements?

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

• [4, 2, 8, 999, 5, 4, 2, 8, 999, 5]
• [alist] * 2 creates a list containing two lists: alist repeated 2 times
• [[4, 2, 8, 999, 5], [4, 2, 8, 999, 5]]
• Yes, blist is a list containing two references to alist, so changes to alist appear in both.
• [4, 2, 8, 6, 5]
• print(blist)
• [[4, 2, 8, 999, 5], [4, 2, 8, 6, 5]]
• blist contains two references, both to alist, so changes to alist appear both times.