The failure definitely looks real so that will need to be addressed.

I do not understand why this change suppresses the warning. The issue seems to be that in Figure.show() we are warning if a) the manager.show() raises NonGuiException and b) we are not on a headless linux system. This change is to the list of backends we ship as part of Matplotlib on the rcparams file which is only used to validate that a backend name is valid.

I think the correct fix for this is to change what manager.show() does in the inline backend (which lives at https://github.com/ipython/ipykernel/blob/master/ipykernel/pylab/backend_inline.py ). Probably can not update it to use the new helper machinery (as they will need to support old versions of Matplotlib).

Also, thank you for your work on this! Sorry my previous comment is mostly negative.

You're right! I got confused by using the warn=False param in the show method while experimenting. Because ipynb backends are defined in the ipykernel project, do you think putting a condition on ipynb backends before the warning is raised would be a viable solution?

At figure.py#L440, it would look like:

if manager is not None: 
    try:
        manager.show()
        return
    except NonGuiException:
        pass
    if (backends._get_running_interactive_framework() != "headless"
        and get_backend() != "module://ipykernel.pylab.backend_inline"
        and warn):
        cbook._warn_external('Matplotlib is currently using %s, which is '
                                 'a non-GUI backend, so cannot show the '
                                 'figure.' % get_backend())

As suggested by @tacaswell above, I think the correct fix lives on ipykernel's side, as matplotlib can't possibly keep track of each and every backend that exists out there.

Yes, makes sense!

I'm going to close this @nitrofortin please let us know if you need any help over on the iptyhon/jupyter side!

Hope we hear from you again.

Thank you, I will investigate on the ipython side how this could be fixed :)

Was an issue / PR ever opened on the jupyter side?

No!