The source of the problem is that we by default try to pick the "best" backend and use that. If you have an active X session and a GUI framework installed we will select that. In order for the GUI window to be responsive someone needs to take care of running the GUI event loop while waiting for the human to type into the terminal. What enable_gui is doing is telling IPython to set up its inputhooks [1]. However because you are launching IPython from the base class to hook to install the hooks is not implemented, and hence the failure.

The immediate work around is to either force the backend to be non-interactive [2] via

MPLBACKEND=agg python -c 'import IPython.core.interactiveshell as ipsh; ipsh.InteractiveShell.instance(); import matplotlib.pyplot as plt; plt.figure()'

(svg and pdf would also work, pick the one that matches the type you expect to save most often). If you want to have GUI integration then you need to launch one of the sub-classes of InteractiveShell that has the GUI hooks implemented.

Medium term, we should catch errors in the call to enable_gui and continue down the fallback chain.

I am going to label this as good first issue and medium difficulty. There is no API design involved here (we have a reproduction case that should not fail), but the backend selection code / fallback logic in pyplot is a bit complicated (it is a complicated thing and while we are working on simplifying it we are currently in the awkward state where both the old and new are around). The exact steps here are:

• adjust pyplot so in this case we will (eventually) fallback to a non-interactive backend (probably convert the NotImplementedError into an ImportError and undo a bunch of work we just did)
• write a test that will reliably exercise this path (probably will require a subprocess? We have some tools to make that easy)

The relevant code:

[1] : more details than anyone wants about input hooks: https://matplotlib.org/stable/users/explain/interactive_guide.html
[2]: https://matplotlib.org/stable/users/explain/backends.html

Thanks! This all makes sense.

For posterity / future googling: I discovered this issue because bokeh.io.output_notebook() actually (and unexpectedly to me at least) winds up invoking IPython.core.interactiveshell.InteractiveShell.instance() if no ipython is running already. So, for example, a stray output_notebook() in a script will summon this above exception.

I can try to take this issue! if you guys would guide me

@nithinivi Do you have specific questions about #23770 (comment) ?

@tacaswell I'm working from a windows laptop, and I was able to reproduce the error, Is there something is should specifically consider while developing windows? so sorry for the late reply ! :)

No, I do not think that this issue is platform specific.

Hi! It seems that this issue is still around (tried and got it on windows, at least).
Is it free to work on? I'd like to give it a try.

Hi, @tacaswell! I had a couple questions regarding this issue.

So far I changed the code so that it looks like this in the "switch_backend()" method:

  # make sure the repl display hook is installed in case we become
  # interactive
    try:
        install_repl_displayhook()
    except ImportError as err:
        _log.error(str(err))
        raise ImportError

and like this in the "install_repl_displayhook()" method:

 try:
        ipython_gui_name = backend2gui.get(get_backend())
        if ipython_gui_name:
            ip.enable_gui(ipython_gui_name)
 except NotImplementedError as err:
        raise ImportError('Fallback to a different backend.') from err

I tried running the original prompt in the terminal -- nothing happened, but there were no error messages except for the logger output.
I altered it like so:

python -c "import IPython.core.interactiveshell as ipsh; ipsh.InteractiveShell.instance(); import matplotlib.pyplot as plt; import numpy as np; f = plt.figure(); ny = int(f.get_figheight() * f.dpi); nx = int(f.get_figwidth() * f.dpi); data = np.random.random((ny,nx)); f.figimage(data); plt.show()"

...and got this as an output:

It seems to be working with my changes (but unable to load the icons, even though the menus themselves are functional). However -- the backend this defaulted to is GTK4Agg. In your comment you said that we should be defaulting to a non-interactive backend. Plus the code mentions that it's not ideal since it's cairo-based:

  # Don't try to fallback on the cairo-based backends as they each have
  # an additional dependency (pycairo) over the agg-based backend, and
  # are of worse quality.

but after qtagg "fails", the next two backends are cairo-based.

candidates += [
            "macosx", "qtagg", "gtk4agg", "gtk3agg", "tkagg", "wxagg"]

The issue with icons only appeared for gtk4 and gtk3.

I also got an additional failed test after making these changes:

 FAILED lib/matplotlib/tests/test_pickle.py::test_pickle_load_from_subprocess[png] - UserWarning: This figure was saved with matplotlib version 3.8.0.dev952+g3133213d8b.d19700101 and is unlikely to function correctly.

This is my first issue so it's very likely that I misunderstood something, but I still wanted to ask.

1. Is this what you had in mind for these changes?
2. Should I change it so that gtk4agg and gtk3agg go after tkagg and wxagg?
3. Is the failed test something to worry about?

Thanks in advance :)

Hi @turnipseason, as there's no active open PR, you're welcome to open one. For 3, you may want to do a build clean and then rebuild matplotlib.

but after qtagg "fails", the next two backends are cairo-based.

we also have *cairo variant backends which are what depend on pycairo. Please do not re-arrange the backend order.

If you commit your changes it will work (we write the version Matpltolib used to generate the pickle into the file and then check if it matches the current version on the way out, the reported version of Matpltolib depends on the current state of git).

we also have *cairo variant backends which are what depend on pycairo. Please do not re-arrange the backend order.

If you commit your changes it will work (we write the version Matpltolib used to generate the pickle into the file and then check if it matches the current version on the way out, the reported version of Matpltolib depends on the current state of git).

Thanks for replying and the clarification! I'll check everything once again and try submitting a PR in the morning then :)