Facebook event selector field

from django.utils.translation import ugettext as _
from django.forms import MultiValueField
from django.forms.widgets import Widget, Select
from django.utils.safestring import mark_safe
from django.utils import simplejson

"""    
var categorySelect = function(el, target, tree){
  var list = tree[el.getValue()];
  target.setTextValue('');
  for (var key = 0; key < list.length; key++){
    var option = document.createElement('option');
    option.setValue(list[key][0]);
    option.setTextValue(list[key][1]);
    target.appendChild(option);
  }
};

"""

class CategorySelect(Widget):
    
    category_name = '%s_category'
    
    def __init__(self, choice_tree, attrs=None):
        self.attrs = attrs or {}
        category_choices = []
        choices_map = {}
        for cat_id, cat_label, subcats in choice_tree:
            category_choices += ((cat_id, cat_label),)
            choices_map[cat_id] = subcats
        self.category_choices = category_choices
        self.choices_map = choices_map
    
    def value_from_datadict(self, data, files, name):
        cat_value = data.get(self.category_name % name, '')
        subcat_value = data.get(name, '')
        return cat_value, subcat_value
    
    def render(self, name, value, attrs={}):
        # Value is id of the bottom choice.
        category_value = None
        choices = ()
        if value:
            for cat_id, subcats in self.choices_map.items():
                for subcat_id, subcat_label in subcats:
                    if str(subcat_id) == str(value):
                        category_value = str(cat_id)
                        choices = subcats
                        break
        else:
            category_value = self.category_choices[0][1]
            choices = self.choices_map.values()[0]
        output = []
        category_id = 'id_%s_category' % name
        category_attrs = {
            'id': category_id,
            'onchange': 'categorySelect(this, document.getElementById(\'id_%s\'), %s);' % \
                    (name, simplejson.dumps(self.choices_map))
        }
        output.append(Select().render(self.category_name % name, category_value, category_attrs, self.category_choices))
        output.append(Select().render(name, value, attrs, choices))
        
        return mark_safe(u'\n'.join(output))


class CategoryField(MultiValueField):
    
    FACEBOOK_EVENT_SUBCATEGORIES = [
        _('Birthday Party'), 
        _('Cocktail Party'), 
        _('Club Party'), 
        _('Potluck'), 
        _('Fraternity/Sorority Party'), 
        _('Business Meeting'), 
        _('Barbecue'), 
        _('Card Night'), 
        _('Dinner Party'), 
        _('Holiday Party'), 
        _('Night of Mayhem'), 
        _('Movie/TV Night'), 
        _('Drinking Games'), 
        _('Bar Night'), 
        _('LAN Party'), 
        _('Brunch'), 
        _('Mixer'), 
        _('Slumber Party'), 
        _('Erotic Party'), 
        _('Benefit'), 
        _('Goodbye Party'), 
        _('House Party'), 
        _('Reunion'), 
        _('Fundraiser'), 
        _('Protest'), 
        _('Rally'), 
        _('Class'), 
        _('Lecture'), 
        _('Office Hours'), 
        _('Workshop'), 
        _('Club/Group Meeting'), 
        _('Convention'), 
        _('Dorm/House Meeting'), 
        _('Informational Meeting'), 
        _('Audition'), 
        _('Exhibit'), 
        _('Jam Session'), 
        _('Listening Party'), 
        _('Opening'), 
        _('Performance'), 
        _('Preview'), 
        _('Recital'), 
        _('Rehearsal'), 
        _('Pep Rally'), 
        _('Pick-Up'), 
        _('Sporting Event'), 
        _('Sports Practice'), 
        _('Tournament'), 
        _('Camping Trip'), 
        _('Daytrip'), 
        _('Group Trip'), 
        _('Roadtrip'), 
        _('Carnival'), 
        _('Ceremony'), 
        _('Festival'), 
        _('Flea Market'), 
        _('Retail')]

    FACEBOOK_EVENTS = (
        (_('Party'),       (0,23)),
        (_('Causes'),     (23,27)),
        (_('Education'),  (27,31)),
        (_('Meetings'),   (31,36)),
        (_('Music/Arts'), (36,45)),
        (_('Sports'),     (45,48)),
        (_('Trips'),      (48,53)),
        (_('Other'),      (53,-1))
        )
    
    def __init__(self, *args, **kwargs):
        super(CategoryField, self).__init__(widget=CategorySelect(self.get_facebook_event_tree()), *args, **kwargs)
    
    def get_facebook_event_tree(self):
        # Output a tree structure, more easily handled by client side code
        results = []
        for i, category in enumerate(self.FACEBOOK_EVENTS):
            category_label, slices = category
            s1, s2 = slices
            subcats = zip(range(*slices), self.FACEBOOK_EVENT_SUBCATEGORIES[s1:s2])
            results += ((i, category_label, subcats),)
        return results
    
    def clean(self, value):
        try:
            if [int(b) for b in value if b]:
                return value
        except ValueError:
            raise forms.ValidationError(_('You must select a category and subcategory for your event.'))
        return [None, None]