PAT甲级 1001 A+B Format python

虽说python可以用format函数的{:,}直接搞定，但这肯定不是出题者的原意，那样写就没意思了。

a,b = map(int, input().split(' '))
out = [i for i in str(a + b)]
len0 = len(out); k = 1
while len0-1-k*3 >= 0 :
    out.insert(len0-k*3,',')
    k += 1
for i in range(len(out)):
    if out[0] == '-' and out[1] == ',':
            if i != 1:
                print(out[i],end='')
    else:
        print(out[i],end='')