flag = False
count = 0
while not flag:
    num = float(input('Enter a number: '))
    if num > 0:
        count += 1
        if count == 1:
            largest_num = num
        else:
            if largest_num < num:
                largest_num = num

    else:
        flag = True
if count > 0:
    print('The largest number entered was', '{:.2f}'.format(largest_num))
else:
    print('No positive number was entered')

Name 'largest_num' can be undefined 10