ENH: support for PySide6 in %gui

[shaperilio]

Addresses #13859

Changes here parallel the changes in ipykernel, i.e. prefer PyQt over PySide and allow requesting explicit versions (e.g. 'qt5') with one difference (see below).

I believe that, eventually, the Qt importing logic should be all here, and ipykernel should defer toipython. I chose not to do that at this time since it would mean the latest ipykernel would require the latest IPython; I'm happy to discuss further.

The only difference between IPython and ipykernel, after these two pull requests, is that, in IPython, it's not possible to explicitly request an event loop for Qt4. This is because an alias exists here which effectively makes "qt4" be "the latest Qt available". I did not remove the alias because I don't know the history behind it.

[shaperilio]

I'm not sure what to do about the docs failing to build - it doesn't seem to be an issue in main?

[shaperilio]

@Carreau what's this force push about?

[Carreau]

@Carreau what's this force push about?

Some failures were due to a problem on the main branch, I fixed the test on the main branch and rebase this one top of main, to get the test to pass.

[shaperilio]

This is ready for review. The equivalent changes in ipykernel have already been merged.

[shaperilio]

There's a problem here with matplotlib, related to commit 5c57ffe.

matplotlib.get_backend() will actually set a backend if none has been set... and then returns it. I think the relevant code is here.

I have an application where I require the TkAgg backend, so I do something like this:

if matplotlib.get_backend() != 'TkAgg':
  _l.debug('IPython shell detected. Attempting to change '
           'matplotlib backend to `TkAgg`...')
  try:
    matplotlib.use('TkAgg')
  except ImportError as e:
    raise RuntimeError('Cannot run. Matplotlib backend cannot be changed to `TkAgg`:\n' 
                       f'   {str(e)}')

Sadly, the call to get_backend actually sets the backend to QtAgg on my system, which hooks in the Qt eventloop. The error I added in 5c57ffe then prevents me from switching to TkAgg, but evidently the exception is getting absorbed somewhere in the call to matplotlib.use.

So I'm changing the error to a warning. My rationale was that I'd want to prevent people from unknowingly changing which event loop was tied in - you could stop some other application from functioning if you do so - but I suppose if you really wanted to prevent that, %gui would have to be changed to support multiple event loops being stacked in the hook.

[ccordoba12]

Thanks @shaperilio for your work on this! I left some comments for you, which are basically the things I changed in ipykernel to provide a better user experience, in my opinion.

[shaperilio]

Thanks @shaperilio for your work on this! I left some comments for you, which are basically the things I changed in ipykernel to provide a better user experience, in my opinion.

Thanks for these. I should have learned my lesson with the matplotlib issue. :)

[shaperilio]

Any thoughts on when this may be merged, or if there's more feedback? I believe it's been released in ipykernel.

[Carreau]

Sorry, this fell under the radar. Let's try it.