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.
nodd =list(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, ...
n = 0
while n < len(nodd):
    if n % 2 == 1:
        nodd[n] = -4 /nodd[n]
    else:
        nodd[n] = 4/nodd[n]
    n +=1   
# step 4: 求和:
noddSum = sum(nodd)
return noddSum