singleuser: add some clarifying comments

Author: minrk

.prettierignore

@@ -1,2 +1,3 @@
 share/jupyterhub/templates/
 share/jupyterhub/static/js/admin-react.js
+jupyterhub/singleuser/templates/

jupyterhub/singleuser/__init__.py

@@ -7,6 +7,7 @@
 from .mixins import HubAuthenticatedHandler, make_singleuser_app
 
 if os.environ.get("JUPYTERHUB_SINGLEUSER_EXTENSION", "") not in ("", "0"):
+    _as_extension = True
     # check for conflict in singleuser entrypoint environment variables
     if os.environ.get("JUPYTERHUB_SINGLEUSER_APP", "") not in {
         "",
@@ -21,7 +22,6 @@
             f"Cannot use JUPYTERHUB_SINGLEUSER_EXTENSION={ext} with JUPYTERHUB_SINGLEUSER_APP={app}."
             " Please pick one or the other."
         )
-    _as_extension = True
     from .extension import main
 else:
     _as_extension = False

jupyterhub/singleuser/mixins.py

@@ -911,12 +911,14 @@ def make_singleuser_app(App):
     empty_parent_app = App()
     log = empty_parent_app.log
 
-    # detect base classes
+    # detect base handler classes
     if not getattr(empty_parent_app, "login_handler_class", None) and hasattr(
         empty_parent_app, "identity_provider_class"
     ):
+        # Jupyter Server 2 moves the login handler classes to the identity provider
         has_handlers = empty_parent_app.identity_provider_class(parent=empty_parent_app)
     else:
+        # prior to Jupyter Server 2, the app itself had handler class config
         has_handlers = empty_parent_app
     LoginHandler = has_handlers.login_handler_class
     LogoutHandler = has_handlers.logout_handler_class

jupyterhub/singleuser/templates/page.html

@@ -1,5 +1,17 @@
-{% extends "templates/page.html" %} {% block header_buttons %} {{super()}}
+{#
+this template customizes all pages (classic notebook, not jupyterlab) served by the single-user server
 
+It makes the following modifications:
+
+- add jupyterhub control panel link to the header
+- update logo url to jupyterhub
+- remove `?redirects` url param that may be added by jupyterhub
+#}
+
+{% extends "templates/page.html" %}
+
+{% block header_buttons %}
+{{ super() }}
 <span>
   <a
     href="{{hub_control_panel_url}}"
@@ -10,9 +22,14 @@
     Control Panel
   </a>
 </span>
-{% endblock %} {% block logo %}
+{% endblock %}
+
+{% block logo %}
 <img src="{{logo_url}}" alt="Jupyter Notebook" />
-{% endblock logo %} {% block script %} {{ super() }}
+{% endblock logo %}
+
+{% block script %}
+{{ super() }}
 <script type="text/javascript">
   function _remove_redirects_param() {
     // remove ?redirects= param from URL so that