im not sure what ive done wrong code doesnt run

[dgizzly]

def main():
    again='y'
    while again=='y' or again=='Y':
        try:
            key=('A', 'C', 'A', 'B', 'B', 'D', 'D', 'A', 'C', 'A',
'B', 'C', 'D', 'C', 'B')
            while again=='y':
                print('_-_-_-_-_-'*5)
                name=input('Enter student name:')

                filename=input('\tEnter student file name: ')
                file=open(filename, 'r')
                i=0
                choice=file.readline()
                while choice!='':
                    choice=choice.rstrip('\n')
                    ans[i]=choice
                    choice=file.readline()
                    i+=1
                    correct=0
main()