x = [1, 2, 3] y = x print(x) # y = y + [3, 2, 1] # y + [3, 2, 1] creates a new list before being assigned to y y += [3, 2, 1] # preserves original object y, which is also pointed to by x # appends operate directly on y, so preserves original object y, which is also pointed to by x # y.append(3) # y.append(2) # y.append(1) print(x)