Jupyter server extension in Python namespace package

I’ve been running into traceback whilst trying to develop a Jupyter Server extension within a Python namespace package.

Here’s a minimal reproducible example based on the example in the docs:

And here is some example traceback from attempting to run it:

$ t_ns_jps_ext
[I 2021-05-19 12:40:39.457 ServerApp] t_namespace | extension was successfully linked.
Traceback (most recent call last):
  File ".../t_ns_jps_ext", line 33, in <module>
    sys.exit(load_entry_point('jps-namespace-extension', 'console_scripts', 't_ns_jps_ext')())
  File ".../python3.9/site-packages/jupyter_server/extension/application.py", line 518, in launch_instance
    serverapp = cls.initialize_server(argv=args)
  File ".../python3.9/site-packages/jupyter_server/extension/application.py", line 488, in initialize_server
  File ".../python3.9/site-packages/traitlets/config/application.py", line 87, in inner
    return method(app, *args, **kwargs)
  File ".../python3.9/site-packages/jupyter_server/serverapp.py", line 1855, in initialize
    point = self.extension_manager.extension_points[starter_extension]
KeyError: 'test_namespace_extension'

Note the log line t_namespace | extension was successfully linked. which shows the extension being loaded with the name of the namespace package rather than the extension name.

If I patch this code to use the namespace name for the extension name I hit more traceback as Jupyter Server goes looking for load_jupyter_server_extension in the namespace package rather than the namespace.

$ t_ns_jps_ext
[I 2021-05-19 12:48:25.936 ServerApp] t_namespace | extension was successfully linked.
> .../python3.9/site-packages/jupyter_server/serverapp.py(1856)initialize()
-> point = self.extension_manager.extension_points[starter_extension]
(Pdb) starter_extension
(Pdb) starter_extension = 't_namespace'
(Pdb) c
[W 2021-05-19 12:48:37.635 ServerApp] t_namespace | extension failed loading with message: module 't_namespace' has no attribute 'load_jupyter_server_extension'

I’m not sure if this is a mistake on my part, a bug or a design limitation?

Any ideas?

Raise a PR - extensions: allow extensions in namespace packages by oliver-sanders · Pull Request #523 · jupyter-server/jupyter_server · GitHub