求解呀，为啥这个老报错呀
源代码：
#计算班级的低分高分平均分
#key:grade,value:grade list
course_grades={}
with open(r'班级成绩.txt') as fin:
for line in fin:
line=line[:-1]
course,sno,sname,grade=line.split(',')
if course not in course_grades:
course_grades[course]=[]
course_grades[course].append(int(grade))
print(course_grades)
