iso6346.py

#!/usr/bin/env python
# Calculate an ISO 6346 check digit (probably).
# The official algorithm costs about $100 to know.
# By Charlie Loyd, 2008-04-11.

from re import compile as regex
from string import uppercase

def iso6346checksum(code, ten='error'):
  '''Call with a string like 'TEXU 328422' and optionally ten=
  what you want a check digit of 10 to return instead of an error
  (probably 0 or 10)'''
  valid = regex('[A-Z]{3}[UJZ] [0-9]{6}')
  if not valid.match(code):
    raise ValueError, code + ' should look like "ZZZZ 111111".'
  
  prefix, suffix = code.split()
  prefix = list(prefix)
  # Letter values are A=10, etc., skipping those divisible by 11, so Z=38
  lettervals = filter(lambda e: e % 11 != 0, range(10, 39))
  lettertable = dict(zip(uppercase, lettervals))
  prefix = [lettertable[l] for l in prefix]
  
  code = prefix + map(int, list(suffix))
  s = sum([code[ind] * 2**ind for ind in range(0, 10)])
  
  dummy = int(s/11) * 11 # futureproof!
  cksum = s - dummy
  if cksum == 10:
    if ten == 'error':
      raise ValueError, 'ISO 6346 checksum of 10.'
    else: return ten
  else: return cksum