1 """

     2 Common checksum routines (used in multiple localflavor/ cases, for example).

     3 """

     4 

     5 __all__ = ['luhn',]

     6 

     7 LUHN_ODD_LOOKUP = (0, 2, 4, 6, 8, 1, 3, 5, 7, 9) # sum_of_digits(index * 2)

     8 

     9 def luhn(candidate):

    10     """

    11     Checks a candidate number for validity according to the Luhn

    12     algorithm (used in validation of, for example, credit cards).

    13     Both numeric and string candidates are accepted.

    14     """

    15     if not isinstance(candidate, basestring):

    16         candidate = str(candidate)

    17     try:

    18         evens = sum([int(c) for c in candidate[-1::-2]])

    19         odds = sum([LUHN_ODD_LOOKUP[int(c)] for c in candidate[-2::-2]])

    20         return ((evens + odds) % 10 == 0)

    21     except ValueError:  # Raised if an int conversion fails

    22         return False