def pi(N):
    ' 计算pi的值 '
    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
    l = []
    for n in itertools.count(1,step=2):
        l.extend([n,])
        if len(l) == N:
            break
    # step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...
    for i,j in enumerate(l):
        # print (i,j)
        if i%2 == 0:
            l[i] = 4/j
        else:
            l[i] = -4/j
    # step 4: 求和:
    return sum(l)