def pi(N):
    """计算pi的值"""
    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...
    odd_list = itertools.count(1, 2)
    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.
    item = list(itertools.takewhile(lambda x: x <= 2 * N - 1, odd_list))
    result = []
    # step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...
    for n in range(N):
        result.append(4 / (item[n] * ((-1) ** n)))
    # step 4: 求和:
    return sum(result)