关于非典型的无法解释的错误

Question

我试图访问函数中的全局变量，但它似乎认为它是非类型的。有什么帮助吗？ 这个错误似乎认为字典“students”是非类型的，即使我告诉它，它也没有把它作为一个全局变量来读取。这是我的代码的错误还是这个版本的python的问题，我不能在任何其他程序上复制这个错误（使用2.7-3.3.3的版本！）你知道吗

#Imports
from math import floor
#Global Declarations (for later)
#Functions
def loadData():
    """Loads the students.txt file if it exists."""
    studentsFile = open("students.txt", "r")
    data = studentsFile.read()
    students = exec(data)
    studentsFile.close()
    return students
def fileExists():
    """Checks if the students.txt file exists."""
    try:
        studentsFile = open("students.txt", "r")
        studentsFile.close()
        students = loadData()
    except FileNotFoundError:
        studentsFile = open("students.txt", "w")
        studentsFile.close()
        students = {}
    print(students)
    return students
def saveToFile():
    global students
    studentsFile = open("students.txt", "w")
    studentsFile.write(str(students))
    studentsFile.close()
def gradeCalc(score):
    possibleGrades = {"A*" : [x for x in range(90, 101)], "A" : [x for x in range(80, 90)], "B" : [x for x in range(70, 80)], "C" : [x for x in range(60, 70)], "D" : [x for x in range(50, 60)], "E" : [x for x in range(40, 50)], "F" : [x for x in range(30, 40)], "G" : [x for x in range(20, 30)], "U" : [x for x in range(0, 20)]}
    for key in possibleGrades:
            if int(floor(score)) in possibleGrades[key]:
                grade = key
def studentExists(studentName):
    global students
    print(students)
    print(studentName)
    try:
        testVar = students[studentName]
        return True
    except KeyError:
        return False
def doneTest(studentName, testName):
    try:
        testVar = students[studentName][testName]
        return True
    except KeyError:
        return False
def integerInput(msg):
    while True:
        try:
            integer = int(input(msg))
            return integer
        except ValueError:
            print("Not a whole number.")
def scoreInput():
    while True:
        studentScore = integerInput("Enter the students score: ")
        if studentScore <= 100 and studentScore >= 0:
            break
        else:
            print("Not between (and including) 0 and 100.")
    return studentScore
def getData():
    global students
    quantity = integerInput("How many students do you wish to add? ")
    test = str(input("Please enter a test name: ").lower())
    for i in range(0, quantity):
        name = str(input("Student name: ").title())
        score = scoreInput()
        if studentExists(name) == False:
            students[name] = {}
        students[name][test] = score
        students[name][test+"Grade"]=gradeCalc(score)
def displayData():
    global students
    studentsToDisplay = []
    listOfGrades = []
    print("You must choose a mode. How many students got each grade or the grades for an individual student.")
    mode = input("Please select 'grade' or 'student'").lower()
    if mode == 'grade':
        test = str(input("For which test do you want to access the grades for? ").lower())
        for key in students:
            if doneTest(key, test)==True:
                studentsToDisplay.append(key)
        for item in studentsToDisplay:
            listOfGrades.append(students[item][test+"Grade"])
        print("Grades:\nA*:", ListOfGrades.count("A*"), "\nA:", ListOfGrades.count("A"), "\nB:", ListOfGrades.count("B"), "\nC:", ListOfGrades.count("C"), "\nD:", ListOfGrades.count("D"), "\nE:", ListOfGrades.count("E"), "\nF:", ListOfGrades.count("F"), "\nG:", ListOfGrades.count("G"), "\nU:", ListOfGrades.count("U"))
    elif mode == "student":
        studentName = str(input("Student name: ").title())
        test = str(input("Test name: ").lower())
        if doneTest(studentName, test) == True:
            print(studentName, "got", students[studentName][test], "and this is grade:", students[studentName][test+"Grade"])
def main():
    global students
    students = fileExists()
    while True:
        choice = str(input("Do you want to add students/tests or do you want to review previous data? (accepts 'add' and 'review') ").lower())
        if choice == "add":
            getData()
            saveToFile()
        elif choice == "review":
            displayData()
        else:
            exitChoice = str(input("Do you want to exit? ").lower())
            if exitChoice == "yes" or exitChoice == "y":
                break
main()

我得到这个错误：

Traceback (most recent call last):
  File "V:\GCSE 2013\sj.sully\Programme Library\Assessed Tasks and CAT Practice\CAT Practice Scenario 1 Code.py", line 108, in <module>
    main()
  File "V:\GCSE 2013\sj.sully\Programme Library\Assessed Tasks and CAT Practice\CAT Practice Scenario 1 Code.py", line 100, in main
    getData()
  File "V:\GCSE 2013\sj.sully\Programme Library\Assessed Tasks and CAT Practice\CAT Practice Scenario 1 Code.py", line 71, in getData
    if studentExists(name) == False:
  File "V:\GCSE 2013\sj.sully\Programme Library\Assessed Tasks and CAT Practice\CAT Practice Scenario 1 Code.py", line 39, in studentExists
    testVar = students[studentName]
TypeError: 'NoneType' object is not subscriptable

我使用的是python3.3.0。你知道吗

Answer 1

如果students是None，那是因为在使用它之前没有定义students。你知道吗

但是用globals来解决这样的问题是愚蠢的。实际上，使用globals是邪恶的，不应该这样做。你知道吗

Python是一种OOP语言，因此您应该为正确的任务使用正确的工具。 对于特定的用例，可以使用类，例如：

class StudentsData:
    def __init__(self):
        self.students = []

    def loadData(self):
        pass

    def fileExists(self):
        pass

    def save(self):
        pass

    def scoreInput(self):
        pass

    def display(self):
        pass

但有些函数并不打算在类中使用，例如：

def gradeCalc(score):
    pass

def main():
    pass

我不会给你实现，因为你需要尝试和失败来理解如何做出正确的面向对象设计。我敢肯定你也在上计算机编程课，所以也要读一下你的课程！；—）

编辑：要阅读有关该主题的更多信息，请参阅下面的参考资料列表，以便回答：

• https://stackoverflow.com/a/19158418/1290438

编辑#2：我能想到的唯一例外是“global is evil”规则，它用于声明常量（即您不会修改的变量），这样您就可以在程序的一个地方定义所有常量。我读过，人们倾向于使用state/env对象作为全局对象，这可能被认为是单例的，很容易被认为是糟糕的编程。在做这样的事情之前，你需要三思而后行，因为经常使用类可能是更好的解决方案

另外，如果您阅读python的源代码，您会发现globals的使用示例。但这并不是python中唯一一个粗心实现的糟糕东西