Python function to calculate the check digit of a UPC-12 barcode.

calc_check_digit.py

def calc_check_digit(digits):
  digits = [int(i) for i in str(digits)][:-1]
  
  odd_pos_list = digits[0::2]  # 1st, 3rd, 5th, etc
  even_pos_list = digits[1::2] # 2nd, 4th, 6th, etc
  
  n = (sum(odd_pos_list) * 3) + sum(even_pos_list)
  rmndr = n % 10
  return (10 - rmndr) if rmndr > 0 else 0

# Full 12 digits of a UPC-12 barcode
digits = '123601057072'
check_digit = calc_check_digit(digits)

# Return value should match the 12th digit on the barcode
print(check_digit)