Student Grades

admins = {'Pete':'123','John':'321'}
usernameInput = ''
passInput = ''
gradesDict = {'Pete':[1,2,3],'John':[9,8,7],'Johny':[]}

from statistics import *

#Check if the student name exists as key in the dictionary
def studentExists(name):
    flag = False

    if name in gradesDict:
        flag = True

    return flag


#Adding the student key and grade to the dictionary
def addStudent(name, grade):

    gradesDict[name] = [grade]
    print('\nUpdated list: ',gradesDict)


#Adding the already existing student grade to the dictionary
def addGrade(name, grade):

    gradesDict[name].append(grade)
    print('\nUpdated list: ',gradesDict)

#Passing the name of the student we want to remove from the main script
def removeStudent(name):

    del gradesDict[name]
    print('\nRemoving student: ',name)
    print('\nUpdated list: ',gradesDict)

def getAverage(name):
    print('\n'+name,'average grade is:',mean(gradesDict[name]))

def getAverageAll():
    for student in gradesDict:
        try:
            gradeList = gradesDict[student]
            print(student,'average grade is:',mean(gradesDict[student]))
        except StatisticsError as se:
            print(student,'average grades cannot be calculated!')

#MAIN PROGRAM
while usernameInput not in admins or admins[usernameInput]!=passInput:
    usernameInput = input('Username:')
    passInput = input('Password:')

    if usernameInput not in admins or admins[usernameInput]!=passInput:
        print('\nPlease provide valid username and password!\n')

print('\nWelcome to the Grading Centre')

while True:

    try:
        menuInput = input("""
[1] - Enter Grades
[2] - Remove Student
[3] - Calculate Average Grade of student
[4] - Get Average Grade for each student
[5] - Exit

What would you like to do today? [Enter a number from the menu]: """)
        menuInput = int(menuInput)
    except Exception as e:
        print(e)


    if menuInput == 1:
        name = ''
        grade = -1

        name = input('\nStudent Name: ')


        while grade <0 or grade >100:
            try:
                grade = input('Student Grade: ')
                grade = float(grade)

                if(grade<0 or grade >100):
                    print('\nEnter grade between 0 and 100!\n')

            except Exception as e:
                print('\nPlease enter valid grade!\n')
                grade = -1

        if studentExists(name):
            addGrade(name, grade)
        else:
            add = input('\nThe student you have entered does not exist, please confirm if you want to add a new Student? [Y/N]\n')
            add = add.upper()

            if add == 'Y':
                addStudent(name, grade)


    elif menuInput == 2:

        print('Student list: ',gradesDict,'\n')
        print('Please enter a student name from the list or type \'q\' to go back to the menu!')

        name = ''
        count = 0

        while not(studentExists(name)) and name.lower() != 'q':

            if count > 0:
                print('\nPlease enter valid student name!')

            name = input('\nStudent Name: ')

            count+=1

        if studentExists(name):
            removeStudent(name)


    elif menuInput == 3:
        print('Student list: ',gradesDict,'\n')
        print('Please enter a student name from the list or type \'q\' to go back to the menu!')

        name = ''
        count = 0

        while not(studentExists(name)) and name.lower() != 'q':

            name = input('\nStudent Name: ')

            if not(studentExists(name)):
                print('\nPlease enter valid student name or press \'q\'!')

            if studentExists(name):
                try:
                    getAverage(name)
                except StatisticsError as e:
                    name = ''
                    print('\nThe student you have entered has no grades!')
    elif menuInput == 4:
        getAverageAll()

    elif menuInput == 5:
        exit()

    else:

        print('\nPlease enter a valid option from the menu!\n')