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.
    b = 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, ...
    c = [1 / x if i % 2 == 0 else -1 / x for i, x in enumerate(b)]
    # step 4: 求和:
    return 4 * reduce(lambda x, y: x + y, c)