列表-合并+ .append() .extend()

>>> a=[1,2,3]
>>> b=[3,6,9]

>>> a+b #a b 都不发生改变
[1, 2, 3, 3, 6, 9]

>>> a.extend(b)#无[ ] 等同于+ 且 a改变
>>> a
[1, 2, 3, 3, 6, 9]

>>> a.append(b)#带有[ ]
>>> a
[1, 2, 3, [3, 6, 9]]