callable arguments fail with TraitError(" .. instance expected a list, not the function")

[jhgoebbert]

The pull request #507 has introduced an issue if command is passing a callable function.

[W 2025-02-24 19:28:56.126 ServerApp] jupyter_server_proxy | extension failed loading with message: TraitError("The 'command' trait of a ServerProcess instance expected a list, not the function '_get_cmd'.")

This breaks multiple server proxies as it is not possible anymore to pass a function using command.

Looking at the documentation of jupyter-server-proxy it should be possible to pass a callable function

• https://jupyter-server-proxy.readthedocs.io/en/latest/server-process.html#callable-arguments
  • https://jupyter-server-proxy.readthedocs.io/en/latest/server-process.html#command

Here are some examples on how and why for example command is used in server-proxies:

• https://github.com/jhgoebbert/jupyter-rsession-proxy/blob/main/jupyter_rsession_proxy/__init__.py#L206
• https://github.com/FZJ-JSC/jupyter-xprahtml5-proxy/blob/main/jupyter_xprahtml5_proxy/__init__.py#L91
• https://github.com/jhgoebbert/jupyter-nestdesktop-proxy/blob/main/jupyter_nestdesktop_proxy/__init__.py#L130

[jhgoebbert]

Thanks @jwindgassen for providing a fix with this PR #519