base_url not set

[guimou]

Hi!

Bug description

The base_url variable used to set --www-root-path (L93 of _init.py) is not replaced by any value.
If I test it in a standalone mode of JupyterLab, I end up with the command .... --www-root-path={base_url}rstudio/ ... exactly as written in the script, no variable substitution. Same thing behing with JupyterHub behind a Reverse Proxy (although here I have other issues with the auth-sign-in rewrite, but this must be related).
Trying to debug it, base_url is indeed an unknown variable from Python's point of view.
As per jupyter-server-proxy documentation, this variable should be replaced.

Expected behaviour

Variable set correctly.

Your personal set up

• OS: containerized image of JupyterLab installation, based on Fedora 34.
• Packages:
  ipython<8.0.0
  notebook==6.4.6
  jupyterhub==1.3.0
  jupyterlab==3.1.18
  jupyterlab-requirements==0.15.0
  jupyter_kernel_gateway==2.4.0
  jupyter-nbrequirements==0.7.3
  supervisor==4.1.0
  jupyterlab-git==0.30
  jupyterlab-spellchecker==0.7.2
  jupyter-server-proxy==3.2.1
  jupyter-rsession-proxy==2.0.1

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[guimou]

Wrong diagnosis on my part, I was not debugging at the right place.
Problem solved with the solution provided by #124 (and comments in #121).