def pi(N):
    ' 计算pi的值 '    
    # step 1: 创建一个奇数序列: 1, 3, 5, 7, 9, ...    
    odd = itertools.count(1, 2)
    # step 2: 取该序列的前N项: 1, 3, 5, 7, 9, ..., 2*N-1.    
    odd_n = itertools.takewhile(lambda x: x <= 2 * N - 1, odd)
    # step 3: 添加正负符号并用4除: 4/1, -4/3, 4/5, -4/7, 4/9, ...    
    tmp = map(lambda x: 4 / x if x % 4 == 1 else -4 / x, odd_n)
    # step 4: 求和:    
    return sum(tmp)