def pi(N):

    ' 计算pi的值 '

    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...

    a=itertools.count(1,2)

    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.

    a1=itertools.takewhile(lambda x: x<=2*N-1,a)

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

    he=0

    zhengfu=1

    for n in a1:

      he=he+(4*zhengfu/n)

      zhengfu=zhengfu*(-1)

    # step 4: 求和:

    return he