def pi(N): ' 计算pi的值 '

# step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
ns = itertools.count(1, 2)
# step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
ns = itertools.takewhile(lambda x : x<2*N, ns)
# step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...
chushu = itertools.cycle((4, -4))
nt = []
for i in ns:
    nt.append(next(chushu)/i)
# step 4: 求和:
return sum(nt)