My GTIN-8 code doesn't work

Question

I am a beginner in coding and am trying to write this GTIN-8 code on python but it keep saying invalid syntax. Please help me. Thank you. BTW, it would be great if you could give me some advices on making the code more efficient and other constructive criticisms. Thank you.

DigitNumber = input ("Enter 7 different digit: ")
length=len(DigitNumber)
while length != 7:
       print("Please type 7 digit codes.")
       DN= input ("Enter 7 different digit: ")
if(length==7):
  GTIN1=int(DigitNumber[0])
  GTIN2=int(DigitNumber[1])
  GTIN3=int(DigitNumber[2])
  GTIN4=int(DigitNumber[3])
  GTIN5=int(DigitNumber[4])
  GTIN6=int(DigitNumber[5])
  GTIN7=int(DigitNumber[6])
TOTALGTIN=int(GTIN1*3+GTIN2+GTIN3*3+GTIN4+GTIN5*3+GTIN6+GTIN7*3)
roundingup=round(TOTALGTIN, -1)
GTIN8 = int(roundingup - TOTALGTIN) % 10
print("The full valid 8-Digit GTIN-8 code is: "+str(DigitNumber)+str(GTIN8)

yes = set(['yes','y', 'ye'])
no = set(['no','n'])

choice = input("Would you like to validate your 8-digit GTIN-8code?")
if choice in yes:
  Validate = input("Enter your 8 digit GTIN-8 number: ")
  GTIN1=int(DigitNumber[0])
  GTIN2=int(DigitNumber[1])
  GTIN3=int(DigitNumber[2])
  GTIN4=int(DigitNumber[3])
  GTIN5=int(DigitNumber[4])
  GTIN6=int(DigitNumber[5])
  GTIN7=int(DigitNumber[6])
  GTIN8=int(DigitNumber[7])
  Validifying=int(GTIN1*3+GTIN2+GTIN3*3+GTIN4+GTIN5*3+GTIN6+GTIN7*3+GTIN8)
  Check=Validifying%10
  if(Validifying/10.).is_integer():
     print("Your code is valid")    
  else:
     print("Your 8 digit GTIN code is invalid.")
elif choice in no:
  print ("Ok, thank you.")
  exit(1)
else:
  sys.stdout.write("Please respond with 'yes' or 'no'")

Answer 1

GS1 codes come in different lengths, ranging from GTIN-8 (8 digits) to SSCC (2 digit application ID + 18 digits). Here's a simple, general Python formula that works for any length GS1 identifier:

cd = lambda x: -sum(int(v) * [3,1][i%2] for i, v in enumerate(str(x)[::-1])) % 10

Explanation:

1. Convert input to string, so input can be numeric or string - just a convenience factor.
2. Reverse the string. This is a simpler way to align the 3x/1x weighting pattern with the input.
3. The weighting factor is selected based on odd and even input character position by calculating i mod 2. The last character in the input string (i=0 after the string has been reversed) gets 3x.
4. Calculate the negative weighted sum mod 10. Equivalent to the (10 - (sum mod 10)) mod 10 approach you'd get if you follow the GS1 manual calculation outline exactly, but that's ugly.

Test Cases

## GTIN-8
>>> cd(1234567)
0
>>> cd(9505000)
3

## GTIN-12
>>> cd(71941050001)
6
>>> cd('05042833241')
2

## GTIN-13
>>> cd(900223631103)
6
>>> cd(501234567890)
0

## GTIN-14
>>> cd(1038447886180)
4
>>> cd(1001234512345)
7

## SSCC (20 digits incl. application identifier)
>>> cd('0000718908562723189')
6
>>> cd('0037612345000001009')
1