title: django python cannot concatenate 'str' and '__proxy__' objects
body: <p>I have <code>forms.py</code> in a Django app.</p>
  
  <pre><code>from django import forms
  from django.core.urlresolvers import reverse_lazy
  
  class ComposeMessageForm(forms.Form):
  ccc = reverse_lazy('user_api')
  st = 'customSelectize:u_r,modelUrl:"'+ reverse_lazy('user_api') +'", choiceField:"username"'
  user_receivers = forms.ModelMultipleChoiceField(
      queryset=User.objects.all(),
      required=False,
      widget=forms.SelectMultiple(
          attrs={
              'data-bind': st,
              'placeholder': 'Select USERS'}))
  </code></pre>
  
  <p>When I run it I get</p>
  
  <pre><code>TypeError at /message/compose/
  
  cannot concatenate 'str' and '__proxy__' objects
  </code></pre>
  
  <p>How can this be done? I have no idea what <code>__proxy__</code> object is.</p>
  
  <p><code>urls.py</code> is:</p>
  
  <pre><code>from django.conf.urls import patterns, include, url
  from rest_framework.urlpatterns import format_suffix_patterns
  from django.contrib import admin
  from msgin import views
  
  urlpatterns = patterns('',
                     url(r'^$', 'ms.views.home', name='home'),
                     url(r'^message/', include('msgin.urls')),
                     url(r'^admin/', include(admin.site.urls)),
                     url(r'^msgs/$', views.MessageList.as_view()),
                     url(r'^msgs/(?P&lt;pk&gt;[0-9]+)/$',
                         views.MessageDetail.as_view()),
                     url(r'^users/$', views.UserList.as_view(),
                         name='user_api'),
                     url(r'^users/(?P&lt;pk&gt;[0-9]+)/$',
                         views.UserDetail.as_view()),
                     url(r'^groups/$', views.GroupList.as_view(),
                         name='groups_api'),
                     url(r'^groups/(?P&lt;pk&gt;[0-9]+)/$',
                         views.GroupDetail.as_view()),
                     )
  </code></pre>