python3-切片

取一个list或tuple的部分元素是非常常见的操作。比如,一个list如下：

L = ["Michael", "Sarah", "Tracy", "Bob", "Jack"]

取前3个元素,应该怎么做/

笨方法：

   

>>> L[0],L[1],L[2]
["Michael", "Sarah", "Tracy"]
["Michael","Sarah","Tracy"] 

之所以是笨方法是因为要是取n个数我们就没辙了。

取前N个元素,也就是索引为0-(N-1)的元素,可以用循环

>>> L = ["Mercedes","nationality","hand","coat"]
>>> r = []
>>> n = 3
>>> for i in range(n):
...  r.append(L[i])
...
>>> r
['Mercedes', 'nationality', 'hand']