Validate a VIN using Python

vin_validation.py

class VINLookupForm(Form):
    """
    Lookup a VIN form
    """
    vin = TextField(u'Enter VIN',
        validators=[validators.Required(u'VIN is required for lookup.')])

    def validate_vin(self, field):
        """
        Validate a VIN against the 9th position checksum

        See: http://en.wikipedia.org/wiki/Vehicle_Identification_Number#Check_digit_calculation

        Test VINs:
            1M8GDM9AXKP042788
            11111111111111111
        """
        POSITIONAL_WEIGHTS = [8,7,6,5,4,3,2,10,0,9,8,7,6,5,4,3,2]
        ILLEGAL_ALL = ['I', 'O', 'Q']
        ILLEGAL_TENTH = ['U','Z','0']
        LETTER_KEY = dict(
            A=1,B=2,C=3,D=4,E=5,F=6,G=7,H=8,
            J=1,K=2,L=3,M=4,N=5,    P=7,    R=9,
                S=2,T=3,U=4,V=5,W=6,X=7,Y=8,Z=9,
        )

        if len(field.data) == 17:
            vin = field.data.upper()

            for char in ILLEGAL_ALL:
                if char in vin:
                    raise ValidationError('Field cannot contain "I", "O", or "Q".')

            if vin[10] in ILLEGAL_TENTH:
                raise ValidationError('Field cannot contain "U", "Z", or "0" in position 10.')

            check_digit = vin[8]

            pos=sum=0
            for char in vin:
                value = int(LETTER_KEY[char]) if char in LETTER_KEY else int(char)
                weight = POSITIONAL_WEIGHTS[pos]
                sum += (value * weight)
                pos += 1

            calc_check_digit = int(sum) % 11

            if calc_check_digit == 10:
                calc_check_digit = 'X'

            if str(check_digit) != str(calc_check_digit):
                raise ValidationError('Invalid VIN.')
        else:
            raise ValidationError('Field must be 17 characters.')