def pi(N): ' 计算pi的值 '

# step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
jishu = itertools.count(1,2)
# step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
jiequ = itertools.takewhile(lambda x:x<=2*N-1,jishu)
# step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...
sum = 0
for i,n in enumerate(jiequ):
    sum = sum + pow(-1,i)*4/n
# step 4: 求和:
return sum