updated django-mptt enabled FilteredSelectMultiple m2m widget, Django 1.11 compatible

from itertools import chain
from django import forms
from django.contrib.admin import widgets
from django.utils.encoding import smart_unicode, force_unicode, force_text
from django.utils.html import escape, conditional_escape


class MPTTModelChoiceIterator(forms.models.ModelChoiceIterator):
    """MPTT version of ModelChoiceIterator"""
    def choice(self, obj):
        """Overriding choice method"""
        tree_id = getattr(obj, self.queryset.model._mptt_meta.tree_id_attr, 0)
        left = getattr(obj, self.queryset.model._mptt_meta.left_attr, 0)
        return super(MPTTModelChoiceIterator,
                     self).choice(obj) + ((tree_id, left),)


class MPTTModelMultipleChoiceField(forms.ModelMultipleChoiceField):
    """MPTT version of ModelMultipleChoiceField"""
    def __init__(self, *args, **kwargs):
        self.level_indicator = kwargs.pop('level_indicator', '+--')
        super(MPTTModelMultipleChoiceField, self).__init__(*args, **kwargs)

    def label_from_instance(self, obj):
        """Creates labels which represent the tree level of each node
        when generating option labels."""
        return u'%s %s' % (self.level_indicator * getattr(
            obj, obj._mptt_meta.level_attr), smart_unicode(obj))

    def _get_choices(self):
        """Overriding _get_choices"""
        if hasattr(self, '_choices'):
            return self._choices
        return MPTTModelChoiceIterator(self)

    choices = property(_get_choices, forms.ChoiceField._set_choices)


class MPTTFilteredSelectMultiple(widgets.FilteredSelectMultiple):
    """MPTT version of FilteredSelectMultiple"""
    def render_options(self, choices, selected_choices):
        """
        This is copy'n'pasted from django.forms.widgets Select(Widget)
        change to the for loop and render_option so they will unpack
        and use our extra tuple of mptt sort fields (if you pass in
        some default choices for this field, make sure they have the
        extra tuple too!)
        """
        def render_option(option_value, option_label, sort_fields):
            """Inner scope render_option"""
            option_value = force_unicode(option_value)
            selected_html = (option_value in selected_choices) \
                and u' selected="selected"' or ''
            return u'<option value="%s" data-tree-id="%s" ' \
                   'data-left-value="%s"%s>%s</option>' % (
                       escape(option_value),
                       sort_fields[0],
                       sort_fields[1],
                       selected_html,
                       conditional_escape(force_unicode(option_label)),
                   )
        # Normalize to strings.
        selected_choices = set([force_unicode(v) for v in selected_choices])
        output = []
        for option_value, option_label, sort_fields in chain(
                self.choices, choices):
            if isinstance(option_label, (list, tuple)):
                output.append(u'<optgroup label="%s">' % escape(
                    force_unicode(option_value)))
                for option in option_label:
                    output.append(render_option(*option))
                output.append(u'</optgroup>')
            else:
                output.append(render_option(option_value, option_label,
                                            sort_fields))
        return u'\n'.join(output)

    def optgroups(self, name, value, attrs=None):
        """Return a list of optgroups for this widget.

        Adapted from django.forms.widgets.ChoiceWidget.optgroups method, the only change was
        adding `(t, p)` in the for loop"""
        groups = []
        has_selected = False

        for index, (option_value, option_label, (t, l)) in enumerate(chain(self.choices)):
            if option_value is None:
                option_value = ''

            subgroup = []
            if isinstance(option_label, (list, tuple)):
                group_name = option_value
                subindex = 0
                choices = option_label
            else:
                group_name = None
                subindex = None
                choices = [(option_value, option_label)]
            groups.append((group_name, subgroup, index))

            for subvalue, sublabel in choices:
                selected = (
                    force_text(subvalue) in value and
                    (has_selected is False or self.allow_multiple_selected)
                )
                if selected is True and has_selected is False:
                    has_selected = True
                subgroup.append(self.create_option(
                    name, subvalue, sublabel, selected, index,
                    subindex=subindex, attrs=attrs,
                ))
                if subindex is not None:
                    subindex += 1
        return groups

    class Media:
        # extend = False
        js = ("admin/js/core.js",
              "js/mptt_m2m_selectbox.js",
              "admin/js/SelectFilter2.js",)