Q.Consider a list:

list1 = [6,7,8,9]  

What is the difference between the following operations on list1:
a. list1 * 2
b. list1 *= 2
c. list1 = list1 * 2

All three produce the same value [6, 7, 8, 9, 6, 7, 8, 9]; they differ in what happens to list1.

• a. list1 * 2 — just an expression: builds the repeated list, but it is discarded; list1 is unchanged.
• b. list1 *= 2 — in-place repetition: the same list object is extended to [6, 7, 8, 9, 6, 7, 8, 9]; any other variable referring to it sees the change.
• c. list1 = list1 * 2 — builds a new list object and rebinds the name list1 to it; other references still point to the old [6, 7, 8, 9].

list1 = [6,7,8,9]; alias = list1
list1 *= 2          # alias also becomes length 8   (same object)
list1 = [6,7,8,9]; alias = list1 …

The idea. The * operator on a list means repetition. The question is really about the difference between an expression, an in-place (augmented) assignment, and a rebinding assignment — i.e. about object identity and mutation, which matter as soon as two names refer to the same list.

a. list1 * 2 — a bare expression.

list1 = [6,7,8,9]
print(list1 * 2)
print(list1)

[6, 7, 8, 9, 6, 7, 8, 9]
[6, 7, 8, 9]

The repeated list is created, (here) printed, and thrown away. list1 itself is never modified — unless you assign the result somewhere.

b. list1 *= 2 — in-place repetition. For mutable types, the augmented assignment extends the same list object:

list1 = [6,7,8,9]
alias = list1                # second name for the SAME object
list1 *= 2
print(list1)
print(alias)                 # alias sees the change
print(list1 is alias)

[6, 7, 8, 9, 6, 7, 8, 9]
[6, 7, 8, 9, 6, 7, 8, 9]
True

c. list1 = list1 * 2 — new object, rebound name.

list1 = [6,7,8,9]
alias = list1
list1 = list1 * 2            # builds a NEW list, name re-points to it
print(list1)
print(alias)                 # alias still refers to the OLD object
print(list1 is alias)

[6, 7, 8, 9, 6, 7, 8, 9]
[6, 7, 8, 9]
False

Summary table:

| Operation | Value of list1 afterwards | Same object as before? | Aliases affected? |

|---|---|---|---| …