typeerror字符串索引必须是整数，而不是str python

Question

I have defined a function get_class_average() having one argument "students". students is a list of 3 students: lloyd, alice, tyler, which are indirectly dictionaries. get_average() is a function that calculates average and it works fine. When I try to call the function get_class_average() I'm getting error:

typeerror string indices must be integers not str

My code

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!
from math import *
def average(numbers):
    total = sum(numbers)
    total = float(total)
    ans = total / len(numbers)
    return ans
num = [1,2,4,5,6]
a = average(num)
print a

def get_average(student):
    homework = average(student["homework"])
    quizzes = average(student["quizzes"])
    tests = average(student["tests"])
    sum = (0.1*homework) + (0.3 * quizzes) + (0.6 * tests)
    return sum

student = [lloyd, alice, tyler]
for key in student:   
    a = get_average(key)
    print a

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"

avg = get_average(lloyd)
t = get_letter_grade(avg)
print t


def get_class_average(students):
    results = []
    students = [lloyd, alice, tyler]
    a= get_average(students)
    r= results.append(a)
    return r

student = [lloyd, alice, tyler]
for key in student:   
    a = get_class_average(key)

print a

Answer 1

Your get_class_average() function expects to be passed the whole list of students, not each individual student.

Remove your for key in student: loop:

students = [lloyd, alice, tyler]
a = get_class_average(students)

Within the get_class_average() function you appear to be returning too early ; unindent the return to not be part of the loop. results.append() returns None as it alters results in-place :

def get_class_average(students):
    results = []
    for key in students:   
        a= get_average(key)
        results.append(a)
    return results

This still doesn't calculate the average for the whole class, however; it just collects the averages for each student into a list.