如何使用 MergeSort 根据属性对 pickle 对象进行排序？

Question

我正在创建一个数据库程序，我正在尝试合并一个排序系统。我有多个对象，都是Students。这些学生具有三个属性：name、grade和average。

我正在尝试让程序对学生的平均分进行排序，然后按平均分从最高到最低的顺序显示学生。我已经能够取消选择存储对象的外部文件，然后将每个学生的平均值存储到列表中。获得列表后，我可以使用 MergeSort 对列表进行排序。但是，我不知道如何使用排序列表对每个对象进行排序和显示。我尝试使用搜索来查找平均值在对象列表中的位置，这样我就可以显示该对象，但我无法使其正常工作。

我的程序如下：

import pickle

class Student():
    def __init__(self,nam,grd,avg):
        self.name = nam
        self.grade = grd
        self.average = avg

    def get_details(self):
        print(self.name, self.grade, self.average)

    def create_item():
        new_student = Student(input("Enter name: "),input("Enter grade: "), input("Enter average: "))
        save_object(new_student, 'student_data.pkl')

def clear_database():
    file = open('student_data.pkl', 'w')
    file.close()

def save_object(obj, filename):
    with open(filename, 'ab') as output:
        pickle.dump(obj, output, pickle.HIGHEST_PROTOCOL)

def unpickle_database(filename):
    with open(filename, 'rb') as f:
        while True:
            try:
                yield pickle.load(f)
            except EOFError:
                break

def display_database():
    print("\nName:               Grade:              Average:            ")
    for student in unpickle_database('student_data.pkl'):
        property_list = [student.name, student.grade, student.average]
        print(''.join([v.ljust(20,' ') for v in property_list]))

def delete_student(student_to_delete, filename):
    students = list(unpickle_database(filename))
    del students[student_to_delete-1]
    clear_database()
    for student in students:
        save_object(student, filename)

def store_grades_in_list():
    students = list(unpickle_database('student_data.pkl'))
    students_grade = []
    for student in students:
        students_grade.append(int(student.grade))
    return students_grade

def store_averages_in_list():
    students = list(unpickle_database('student_data.pkl'))
    students_average = []
    for student in students:
        students_average.append(int(student.average))
    return students_average

def merge(st1, st2, st3):
    i1 = i2 = i3 = 0
    n1, n2 = len(st1), len(st2)

    while i1 < n1 and i2 < n2:
        if st1[i1] < st2[i2]:
            st3[i3] = st1[i1]
            i1 = i1 + 1
        else:
            st3[i3] = st2[i2]
            i2 = i2 + 1
        i3 = i3 + 1

    while i1 < len(st1):
        st3[i3] = st1[i1]
        i1 = i1 + 1
        i3 = i3 + 1

    while i2 < len(st2):
        st3[i3] = st2[i2]
        i2 = i2 + 1
        i3 = i3 + 1

def mergeSort(st):
    n = len(st)
    if n > 1:
        m = n//2
        st1, st2 = st[:m], st[m:]
        mergeSort(st1)
        mergeSort(st2)
        merge(st1, st2, st)
    return st

def bin_search(x):
    my_list = store_averages_in_list()
    bottom = 0
    top = len(my_list)-1
    found = False
    location = -1
    while (bottom <= top) and not(found):
        middle = int((bottom + top)/2)

        if (my_list[middle] == x):
            location = middle
            found = True
        elif (my_list[middle] < x):
            bottom = middle + 1
        else:
            top = middle -1
    return location

while True:
    user_input = input("\nType \"Clear\" to clear the database. Type \"Add\" to add a student. Type \"Display\" to display the database contents. Type \"Quit\" to quit the program. Type \"Remove\" to remove a student. Type \"Search\" to serach for an average. Type \"Sort av\" to display sorted list of averages.\n")
    if user_input == "Quit":
        break
    elif user_input == "Clear":
        clear_database()
        print("\nThe database has been cleared.")
    elif user_input == "Add":
        Student.create_item()
        print("\nThe student has been added. The updated database is: ")
        display_database()
    elif user_input == "Display":
        display_database()
    elif user_input == "Remove":
        student_to_delete = int(input("Type the student number that you would like to delete: "))
        delete_student(student_to_delete,'student_data.pkl')
        print("\nThe student has been deleted. The updated database is: ")
        display_database()
    elif user_input == "Display Characteristics":
        store_chars_in_list()
    elif user_input == "Sort av":
        print(mergeSort(store_averages_in_list()))
    elif user_input == "Sort gr":
        print(mergeSort(store_grades_in_list()))
    elif user_input == "Search":
        print(bin_search(int(input("Enter average to search for: "))))

Answer 1

所以你需要做两件事来完成这个。

不是将学生平均值的列表传递给合并排序函数，而是需要传递Student对象本身的列表。
mergesort 函数不会改变，但 merge 函数会。由于列表中的元素不再是数字，而是 Student 对象，因此您需要比较 st1[i1].average < st2[i2].average 之类的东西，而不是比较 st1[i1] < st2[i2] 或类似的东西。这样，您排序的不是 平均值 的列表，而是 Student 的列表。然后，一旦你有了一个排序的学生列表，你就可以随心所欲地显示它。

这个答案有意尽可能少地包含明确的代码，因为这个问题感觉就像是学校布置的作业，你应该接受我提供的信息并尝试一下:)

旁注：如果您只是想对 Student 的列表进行排序并且不需要或不想自己编写合并排序，则以下代码将起作用：

students = ... # list of Student
sorted_students = list(sorted(students, key=attrgetter('average'))

使用 operator 库中的 attrgetter 函数。

如何使用 MergeSort 根据属性对 pickle 对象进行排序？

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