使用 range：

#!/usr/bin/python
# -*- coding: UTF-8 -*-

def k(x):
    if x in range(60):
        print('C')
    elif x in range(60,90):
        print('B')
    else:
        print('A')
score = int(raw_input('输入分数:\n'))
k(score)

参考方法：

#!/usr/bin/python
# -*- coding: UTF-8 -*-

i= int(input('请输入成绩：'))
ar= [90,60,0]
res= ['A','B','C']
for idx in range (0,3):
    if i >=ar[idx]:
        print(res[idx])
        break

参考方法，兼容Python3.x与Python2.x：

# -*- coding: UTF-8 -*-

score = [80,78,90,48,62]    #5个同学的学习成绩
grade = []                  #用来存放评级

for i in range(5):
    s = (score[i]>=90)and 'A' or ((score[i]>=60)and 'B' or 'C')
    grade.append(s)

print (grade)