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def serialize(data):
    return simplejson.dumps(data, sort_keys=True, indent=4)

def deserialize(stream):
    return simplejson.loads(stream)


class DictFormWidget(forms.Textarea):
    """ form widget for preferences dict """
    def render(self, name, value, attrs=None):
        """
        FIXME: Can we get the original non-serialized db value here?
        """
        value = serialize(value)
        return super(DictFormWidget, self).render(name, value, attrs)


class DictFormField(forms.CharField):
    """ form field for preferences dict """
    widget = DictFormWidget
    
    def clean(self, value):
        """
        validate the form data
        FIXME: How can we get the pref form class for validating???
        """
        value = super(DictFormField, self).clean(value)
        try:
            return deserialize(value)
        except Exception, err:
            raise forms.ValidationError("Can't deserialize: %s" % err)


class DictField(models.TextField):
    """
    A dict field.
    Stores a python dict into a text field. 
    """
    __metaclass__ = models.SubfieldBase

    def to_python(self, value):
        """ decode the data dict using simplejson.loads() """
        if isinstance(value, dict):
            return value
        return deserialize(value)

    def get_db_prep_save(self, value):
        "Returns field's value prepared for saving into a database."
        assert isinstance(value, dict)
        return serialize(value)

    def formfield(self, **kwargs):
        # Always use own form field and widget:
        kwargs['form_class'] = DictFormField
        kwargs['widget'] = DictFormWidget
        return super(DictField, self).formfield(**kwargs)