def pi(N): ' 计算pi的值 ' n1 = itertools.count(1,2)

# step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
n2 = itertools.takewhile(lambda x : x < 2*N, n1)
# step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
sum = 0
ax = 0
for n in n2:
    if ax == 0:
        sum = sum + 4 / n
        ax = 1
    else:
        sum = sum + -4 / n
        ax = 0
return sum


# step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...

# step 4: 求和:
return 3.14