def pi(N):

    ' 计算pi的值 '

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

    odd_sequence = [2*i + 1 for i in range(N)]

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

    sequence_N = odd_sequence[:N]

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

    terms = [(-1)**i * 4/num for i, num in enumerate(sequence_N)]

    # step 4: 求和:

    return sum(terms)