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, ...
odd_n = map(lambda x: (-1)**(x//2) * 4 / x, odd_n)
# step 4: 求和:
return sum(odd_n)