score=int(input('请输入一个数字'))
grade= ' '
if score>100 or score<0:
        score = int(input('输入错误！！！请重新输入一个在0-100之间数字'))
else:
        if score>=90:
                grade = 'A'
        elif score>=80:
                grade = 'B'
        elif score>=70:
                grade = 'C'
        elif score>=60:
                grade = 'D'
        elif score<60:
                grade = 'E'
print('分数{0}，等级{1}'.format(score,grade))

选择结构嵌套
score = int(input('请输入一个0-100之间的数字'))
degree='ABCDE'
num = 0
if score>100 or score<0:
        score = int(input('输入错误！！请重新输入一个0-100之间数字'))
else:
        num = score//10
        if num<6:num=5
print('分数是{0},等级是{1}'.format(score,degree[9-num]))