filter()和map一樣，接收一個函式和一個序列。和map不同的是，filter()把傳入的函式依次作用於每個元素，然後根據返回值是True還是False決定保留還是丟棄該元素
過濾出奇數：
def is_odd(n):
return n%2==1
print filter(is_odd,[1,2,3,4,5]) # ==>[1, 3, 5]
過濾掉空字串：
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]