A Class handling choices and it's schema type.

choice.py

import colandar


class InvalidChoice(Exception):
    pass


class Choice(str):
    """Class for choice value of some schema
    """
    choices = ()
    IsInvalidChoice = InvalidChoice

    def __init__(self, string=''):
        self.validate(string)
        self.choice = string

    @property
    def choice_keys(self):
        return [choice[0] for choice in self.choices]

    def validate(self, value):
        if not value in self.choice_keys:
            raise self.IsInvalidChoice


class LikingChoice(Choice):
    choices = (('like', 'Like'),
               ('soso', 'Soso'),
               ('unlike', 'Unlike'))

    int_mapping = {
        'like': 1,
        'soso': 0,
        'unlike': -1,
    }

    def __int__(self):
        return self.int_mapping[self]


class ChoiceSchemaType(colander.String):
    def __init__(self, choice_class, encoding=None):
        self.choice_class = choice_class
        self.encoding = encoding

    def deserialize(self, node, cstruct):
        value = super(Choice, self).deserialize(node, cstruct)
        try:
            choice = self.choice_class(value)
        except self.choice_class.IsInvalidChoice:
            choices = ', '.join(['%s' % x for x in self.choice_class.choices])
            err = colander._('"${val}" is not one of ${choices}',
                    mapping={'val':value, 'choices':choices})
            raise colander.Invalid(node, err)
        return choice