total = 0
count = 0
while True :
    hey = input('Enter your mark: ')
    if hey == 'done' : break
    value = float(hey)
total = total + value 
count = count + 1

average = total / count
print('Average: ', average)

prints, (gets the mark and gets the average of the last num until type done)

Enter your mark: 34 Enter your mark: 67 Enter your mark: 80 Enter your mark: done Average: 80.0

,,,,,The same using lists

num = list()
while True :
    inp = input('Enter the mark: ')
    if inp == 'done' : break
    value = float(inp)
    num.append(value)
average = sum(num) / len(num)
print('Average: ', average)

Enter the mark: 67 Enter the mark: 78 Enter the mark: 90 Enter the mark: 56 Enter the mark: done Average: 72.75