系统内建函数不改变列表内容，列表自己调用函数则内容改变

>>> mylist = [3,5,2,1,4]
>>> sorted(mylist) #不改变列表值
[1, 2, 3, 4, 5]
>>> mylist
[3, 5, 2, 1, 4]
>>> mylist.reverse() #会改变列表值
>>> mylist
[4, 1, 2, 5, 3]
>>> reversed(mylist) #不改变列表值
<list_reverseiterator object at 0x7f698b42ac88>
>>> for var in reversed(mylist):
...     print (var)
... 
3
5
2
1
4
>>> mylist
[4, 1, 2, 5, 3]
>>>