filter()和map一样，接收一个函数和一个序列。和map不同的是，filter()把传入的函数依次作用于每个元素，然后根据返回值是True还是False决定保留还是丢弃该元素
过滤出奇数：
def is_odd(n):
return n%2==1
print filter(is_odd,[1,2,3,4,5]) # ==>[1, 3, 5]
过滤掉空字符串：
def not_empty(s):
return s and s.strip()
print filter(not_empty,['A','','B',None,'C',' ']) # ==>['A', 'B', 'C']
L=range(1,101)
def iseven(x):
return x%2==0
print filter(iseven,L)
#[2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100]