Python生成不重复随机值的方法

import random
total = 100
li = [i for i in range(total)]
res = []
num = 20
for i in range(num):
  t = random.randint(i,total-1)
  res.append(li[t])
  li[t], li[i] = li[i], li[t]
print res

print random.sample(li, num)