gistfile1.py

class ChoiceField(Field):
    ...
    def to_representation(self, value):
        if data == '' and self.allow_blank:
            return ''

        try:
            return self.choice_strings_to_values[six.text_type(value)]
        except KeyError:
            return value


class MultipleChoiceField(ChoiceField):
    ...
    def to_representation(self, value):
        return set([
            self.choice_strings_to_values[six.text_type(item)]
            if six.text_type(value) in self.choice_strings_to_values
            else item
            for item in value
        ])