Python | a += b is not always a = a + b

In python a += b doesn’t always behave the same way as a = a + b, same operands may give the different results under different conditions.

Consider these examples for list manipulation:
Example 1

Output:

[5, 4, 3, 2, 1, 1, 2, 3, 4]
[5, 4, 3, 2, 1, 1, 2, 3, 4]

Example 2

Output:

[5, 4, 3, 2, 1, 1, 2, 3, 4]
[5, 4, 3, 2, 1]

• expression list1 += [1, 2, 3, 4] modifies the list in-place, means it extends the list such that “list1” and “list2” still have the reference to the same list.
• expression list1 = list1 + [1, 2, 3, 4] creates a new list and changes “list1” reference to that new list and “list2” still refer to the old list.