Python Codecademy Student Becomes the Teacher

#三个学生的信息
lloyd = {
    "name": "Lloyd",
    "homework": [90.0, 97.0, 75.0, 92.0],
    "quizzes": [88.0, 40.0, 94.0],
    "tests": [75.0, 90.0]
}
alice = {
    "name": "Alice",
    "homework": [100.0, 92.0, 98.0, 100.0],
    "quizzes": [82.0, 83.0, 91.0],
    "tests": [89.0, 97.0]
}
tyler = {
    "name": "Tyler",
    "homework": [0.0, 87.0, 75.0, 22.0],
    "quizzes": [0.0, 75.0, 78.0],
    "tests": [100.0, 100.0]
}

# Add your function below!
#求一个list的平均值
def average(number):
    total=sum(number)
    total=float(total)
    return total/len(number)
#求一个学生所有成绩的加权平均
def get_average(student):
    homework=average(student["homework"])
    quizzes=average(student["quizzes"])
    tests=average(student["tests"])
    return homework*0.1+quizzes*0.3+tests*0.6
#根据分数分等级
def get_letter_grade(score):
    if score>=90:
        return ‘A‘
    elif score>=80:
        return ‘B‘
    elif score>=70:
        return ‘C‘
    elif score>=60:
        return ‘D‘
    else:
        return ‘F‘
#获取lloyd的等级
get_letter_grade(get_average(lloyd))
#将所有学生的成绩统计到新的列表中并求平均值
def get_class_average(students):
    results=[]
    for student in students:
        results.append(get_average(student))
    return average(results)
#学生信息
students=[lloyd,alice,tyler]
#打印班级总平均分和等级
print get_class_average(students)
print get_letter_grade(get_class_average(students))