Python GTIN number code - Enter a 7 digit number to find the check digit or enter an 8 digit number to check the validity.
while 2>1:
gtinnumber = input("Enter a 7 digit number to find the check digit or enter an 8 digit number to check the validity. ")
if gtinnumber.isdigit()==False:
continue
flip=3
total = [int(x) for x in gtinnumber]
for loop in range(len(total)):
total[loop]*=flip
flip=-1*flip+4
if len(str(gtinnumber))==8 and sum(total)%10==0:
print("Valid GTIN-8 number")
elif len(str(gtinnumber))==7:
if sum(total)%10!=0:
flip=((sum(total) + 9) // 10 * 10) - sum(total) + 1
print(flip-1)
else: print("Invalid number")