def render(self, name, value, attrs=None, choices=()):
        if value is None: value = []
        has_id = attrs and 'id' in attrs
        final_attrs = self.build_attrs(attrs, name=name)
        str_values = set([force_unicode(v) for v in value]) # Normalize to strings.
        for i, (option_value, option_label) in enumerate(chain(self.choices, choices)):
            # If an ID attribute was given, add a numeric index as a suffix,
            # so that the checkboxes don't all have the same ID attribute.
            if has_id:
                final_attrs = dict(final_attrs, id='%s_%s' % (attrs['id'], i))
            cb = forms.CheckboxInput(final_attrs, check_test=lambda value: value in str_values)
            option_value = force_unicode(option_value)
            yield mark_safe(u'<label>%s %s</label>' % (cb.render(name, option_value), force_unicode(option_label)))