default to auth extensions with Jupyter Server 2

jupyterhub/singleuser/__init__.py

@@ -1,40 +1,75 @@
 """JupyterHub single-user server entrypoints
 
 Contains default notebook-app subclass and mixins
+
+Defaults to:
+
+- Jupyter server extension with Jupyter Server >=2
+- Subclass with Jupyter Server <2 or clasic notebook
+
+Application subclass can be controlled with environment variables:
+
+- JUPYTERHUB_SINGLEUSER_EXTENSION=1 to opt-in to the extension (requires Jupyter Server 2)
+- JUPYTERHUB_SINGLEUSER_APP=notebook (or jupyter-server) to opt-in
 """
 import os
 
 from .mixins import HubAuthenticatedHandler, make_singleuser_app
 
-if os.environ.get("JUPYTERHUB_SINGLEUSER_EXTENSION", "") not in ("", "0"):
+_as_extension = False
+_extension_env = os.environ.get("JUPYTERHUB_SINGLEUSER_EXTENSION", "")
+_app_env = os.environ.get("JUPYTERHUB_SINGLEUSER_APP", "")
+
+if not _extension_env:
+    # extension env not set, check app env
+    if not _app_env or 'jupyter_server' in _app_env.replace("-", "_"):
+        # no app env set or using jupyter-server, this is the default branch
+        # default behavior:
+        # - extension, if jupyter server 2
+        # - older subclass app, otherwise
+        try:
+            import jupyter_server
+
+            _server_major = int(jupyter_server.__version__.split(".", 1)[0])
+        except Exception:
+            # don't have jupyter-server, assume classic notebook
+            _as_extension = False
+        else:
+            # default to extension if jupyter-server >=2
+            _as_extension = _server_major >= 2
+
+    elif _app_env == "extension":
+        _as_extension = True
+    else:
+        # app env set and not to jupyter-server, that opts out of extension
+        _as_extension = False
+elif _extension_env == "0":
+    _as_extension = False
+else:
+    # extension env set to anything non-empty other than '0' enables the extension
     _as_extension = True
+
+if _as_extension:
     # check for conflict in singleuser entrypoint environment variables
-    if os.environ.get("JUPYTERHUB_SINGLEUSER_APP", "") not in {
+    if _app_env not in {
         "",
         "jupyter_server",
         "jupyter-server",
         "extension",
         "jupyter_server.serverapp.ServerApp",
     }:
-        ext = os.environ["JUPYTERHUB_SINGLEUSER_EXTENSION"]
-        app = os.environ["JUPYTERHUB_SINGLEUSER_APP"]
         raise ValueError(
-            f"Cannot use JUPYTERHUB_SINGLEUSER_EXTENSION={ext} with JUPYTERHUB_SINGLEUSER_APP={app}."
+            f"Cannot use JUPYTERHUB_SINGLEUSER_EXTENSION={_extension_env} with JUPYTERHUB_SINGLEUSER_APP={_app_env}."
             " Please pick one or the other."
         )
     from .extension import main
 else:
-    _as_extension = False
-    try:
-        from .app import SingleUserNotebookApp, main
-    except ImportError:
-        # check for Jupyter Server 2.0 ?
-        from .extension import main
-    else:
-        # backward-compatibility
-        JupyterHubLoginHandler = SingleUserNotebookApp.login_handler_class
-        JupyterHubLogoutHandler = SingleUserNotebookApp.logout_handler_class
-        OAuthCallbackHandler = SingleUserNotebookApp.oauth_callback_handler_class
+    from .app import SingleUserNotebookApp, main
+
+    # backward-compatibility
+    JupyterHubLoginHandler = SingleUserNotebookApp.login_handler_class
+    JupyterHubLogoutHandler = SingleUserNotebookApp.logout_handler_class
+    OAuthCallbackHandler = SingleUserNotebookApp.oauth_callback_handler_class
 
 
 __all__ = [