1,取出列表中为偶数的值，形成新的列表。
传统做法：
numbers=[1,2,3,4,5,6]
even=[]
for i in numbers:
if i%2==0:
even.append(i)
print even
列表推导：
numbers=[1,2,3,4,5,6]
even =[i for i in numbers if i%2==0]
print even
对列表每个元素进行乘2操作
even =[i*2 for i in numbers ]
原文：http://www.cnblogs.com/liuye-micky/p/6938152.html