s = 'asdfghjklqwertyuiopzxcvbnm'

num = 1

for h in s:

    num = num +1

    if num >20:

         break

    print(h)

## 若要输出前二十位,注意print位置在if一级

s = 'asdfghjklqwertyuiopzxcvbnm'

num = 1

for h in s:

    num = num +1

    if num >20:

         break

         print(h)

输出print(h)要在循环里面。