list.append只是暫時改變列表

list_1 = [1,2,3,4]
list_1.append(5)
list_1.insert(1,9)
print(list_1)

[1, 9, 2, 3, 4, 5]

list_1 = [1,2,3,4]
list_1.append(6)
list_1.insert(1,9)
print(list_1)

[1, 9, 2, 3, 4, 6]