v2.0.0: 'NotebookWebApplication' object has no attribute 'append'

[robnagler]

Downgrading to jupyterlab-server==1.2.0 works. Here's the stack of the error:

jupyter lab --no-browser --port=8888 --ip=0.0.0.0
[I 23:36:06.549 LabApp] Writing notebook server cookie secret to /home/vagrant/.local/share/jupyter/runtime/notebook_cookie_secret
[I 23:36:07.049 LabApp] JupyterLab extension loaded from /home/vagrant/.pyenv/versions/3.7.2/envs/py3/lib/python3.7/site-packages/jupyterlab
[I 23:36:07.049 LabApp] JupyterLab application directory is /home/vagrant/.pyenv/versions/3.7.2/envs/py3/share/jupyter/lab
zzzz[W 23:36:07.051 LabApp] Error loading server extension jupyterlab
    Traceback (most recent call last):
      File "/home/vagrant/.pyenv/versions/3.7.2/envs/py3/lib/python3.7/site-packages/notebook/notebookapp.py", line 1991, in init_server_extensions
        func(self)
      File "/home/vagrant/.pyenv/versions/3.7.2/envs/py3/lib/python3.7/site-packages/jupyterlab/extension.py", line 222, in load_jupyter_server_extension
        add_handlers(web_app, config)
      File "/home/vagrant/.pyenv/versions/3.7.2/envs/py3/lib/python3.7/site-packages/jupyterlab_server/handlers.py", line 165, in add_handlers
        handlers.append((url_pattern, LabHandler))
    AttributeError: 'NotebookWebApplication' object has no attribute 'append'

[blink1073]

Hi @robnagler, it looks like you have jupyterlab < 3.0 installed, which pins its jupyterlab_server compatibility: https://github.com/jupyterlab/jupyterlab/blob/9a8dadf59424ed2a6ff2a3c84001d46e37104d75/setup.py#L154. You can try either upgrading jupyterlab or downgrading the jupyterlab_server.

[robnagler]

Thanks @blink1073! I fixed it by pinning the version in our Docker build. It was pulling in the latest independently of jupyterlab, and pip wasn't complaining (don't ask me why).

[krassowski]

Linking back to SO post with the same error message, but slightly different underlying issue in case if Google leads ussers here.