add integration with django-autocomplete-light with UserSuDalForm

adds UserSuDalForm form class which leverages the autocomplete
facilities provided by django-autocomplete-light.
It expects:
  * 'django-light-autocomplete' to be installed
  * 'dal' to be present into INSTALLED_APPS
  * 'SU_DAL_VIEW_NAME' setting to contain the name of the autocomplete view

If 'SU_DAL_VIEW_NAME' setting key is present and form_class is default su_login view tries to
automatically use UserSuDalForm, avoiding the end user to customize
django_su's urls.

Required configuration is added into README

README.rst

@@ -72,13 +72,51 @@ The following apps are optional but will enhance the user experience:
 
 * The 'login su' form will render using `django-form-admin`_
 * The user selection widget will render using `django-ajax-selects`_
+* The user selection widget will render using `django-autocomplete-light`_
+
+`django-autocomplete-light`_ and `django-ajax-selects`_ are mutually exclusive.
 
 Note that `django-ajax-selects`_ requires the following settings:
 
 .. code-block:: python
 
     AJAX_LOOKUP_CHANNELS = {'django_su':  dict(model='auth.user', search_field='username')}
 
+`django-autocomplete-light`_ requires the following configuration:
+
+.. code-block:: python
+
+    # settings.py
+    # this setting will be used to find the named url for the autocomplete view
+    SU_DAL_VIEW_NAME = 'user-autocomplete'
+
+    # views.py
+    # we need an autocomplete view
+    from dal import autocomplete
+    from django.contrib.auth.models import User
+
+    class UserAutoComplete(autocomplete.Select2QuerySetView):
+      def get_queryset(self):
+        user = self.request.user
+        qs = User.objects.none()
+        if user.is_authenticated and user.is_staff:
+          qs = User.objects.filter(username__icontains=self.q)
+
+        return qs.order_by('pk')
+
+    # urls.py
+    from .views import UserAutoComplete
+    from django.conf.urls import url
+    from django.conf import settings
+
+    urlpatterns = [
+      #...
+      url(r'^user-autocomplete/$',
+          UserAutoComplete.as_view(),
+          name=settings.SU_DAL_VIEW_NAME
+      )
+      #....
+    ]
 
 Configuration (optional)
 ------------------------
@@ -174,4 +212,5 @@ django-su is packaged using seed_.
 
 .. _django-form-admin: http://pypi.python.org/pypi/django-form-admin
 .. _django-ajax-selects: http://pypi.python.org/pypi/django-ajax-selects
+.. _django-autocomplete-light: https://pypi.org/project/django-autocomplete-light/
 .. _seed: https://github.com/adamcharnock/seed/

django_su/forms.py

@@ -53,3 +53,24 @@ def __str__(self):
             except ImportError:
                 pass
         return super(UserSuForm, self).__str__()
+
+
+class UserSuDalForm(UserSuForm):
+    def __init__(self, *args, **kwargs):
+        super(UserSuForm, self).__init__(*args, **kwargs)
+
+        if "dal" in settings.INSTALLED_APPS and getattr(
+            settings, "SU_DAL_VIEW_NAME", None
+        ):
+            try:
+                from dal import autocomplete
+            except ImportError:
+                return
+            old_field = self.fields["user"]
+
+            self.fields["user"] = forms.ModelChoiceField(
+                required=old_field.required,
+                label=old_field.label,
+                queryset=old_field.queryset,
+                widget=autocomplete.ModelSelect2(url=settings.SU_DAL_VIEW_NAME),
+            )

django_su/views.py

@@ -16,7 +16,7 @@
 from django.views.decorators.csrf import csrf_protect
 from django.views.decorators.http import require_http_methods
 
-from .forms import UserSuForm
+from .forms import UserSuDalForm, UserSuForm
 from .utils import custom_login_action, su_login_callback
 
 
@@ -61,7 +61,13 @@ def login_as_user(request, user_id):
 @csrf_protect
 @require_http_methods(["POST", "GET"])
 @user_passes_test(su_login_callback)
-def su_login(request, form_class=UserSuForm, template_name="su/login.html"):
+def su_login(request, form_class=None, template_name="su/login.html"):
+    if form_class is None:
+        if getattr(settings, "SU_DAL_VIEW_NAME", None):
+            form_class = UserSuDalForm
+        else:
+            form_class = UserSuForm
+
     form = form_class(request.POST or None)
     if form.is_valid():
         return login_as_user(request, form.get_user().pk)