将文件a.txt中的成绩读出，统计其平均值，方差，并将结果写到b.txt中

每一行读出时是以字符串格式读出，而姓名和成绩之间是用“，”分隔的

with open('a.txt', 'r') as f:
    data = f.read().split('\n')

t = [x.split(',') for x in data]
score = [int(x[1]) for x in t]
avg = sum(score)/len(score)
t = sum(map(lambda x:(avg-x)**2, score))
variance = (t/len(score))**0.5

with open('b.txt', 'w') as f:
    f.write('\n'.join([str(avg),str(variance)]))