'Return the integer value to be stored from the hex string'

def get_prep_value(self, value):

    if ((value is None) or (value == '')):
        return None
    if isinstance(value, six.string_types):
        value = _hex_string_to_unsigned_integer(value)
    if _using_signed_storage():
        value = _unsigned_to_signed_integer(value)
    return value
