Error when using with ipython

[schlegelp]

Hi!

I recently updated my pygfx, wgpu and rendercanvas stack. After the update I started running into issues with a little 3D viewer built on top of your tools. Specifically, running it from ipython is now throwing errors I hadn't encountered before.

I managed to track the problem to the canvas - rendercanvas.qt.QRenderCanvas in my setup. Here is a minimal reproducible example:

>>> %gui qt
>>> from rendercanvas.auto import RenderCanvas
>>> canvas = RenderCanvas()  # <- hitting enter will make the canvas appear
>>>  print("foobar")  # <- entering any further command will lead to an error
foobar

Incompatible awaitable result <Future pending>. Maybe you used asyncio or trio (this does not run on either)?
Incompatible awaitable result <Future pending>. Maybe you used asyncio or trio (this does not run on either)?

Versions:

• rendercanvas == 2.6.3
• ipython == 9.13.0
• python == 3.11.8

Downgrading rendercanvas to 2.4.2 made the issue go away. I can see you had a major refactor just after that version which is the likely cause of the change in behavior.

I guess my question is: Is this a bug or expected? If the latter, is there anything I can do in my setup to make it work again?

Thanks in advance!

[Vipitis]

Hey,

One thing to try is to set present_method="screen" (might have to import from .qt instead of .auto) since the default is now bitmap which is also async.

Not sure what's going on with async in ipython, I will have to try it myself later.

[almarklein]

It looks like an ayncio Future somehow entered the event loop. Looking into it.

[Vipitis]

One thing to try is to set present_method="screen" (might have to import from .qt instead of .auto) since the default is now bitmap which is also async.

tried this and it didn't help.

[almarklein]

Did not find the cause yet. I'm on holiday now, will have another look when I'm back next week.

[almarklein]

My findings so far:

• Ipython, when using %gui qt, does not run the qt event loop, but rather keeps it alive together with (or on top of?) an asyncio event loop.
• This means that rendercanvas can pick either the qt loop or the asyncio loop, and it would run fine.
• However, rendercanvas picks the qt event loop, but when running, it tries to run an async sleep() in the asyncio event loop. And that's when things go wrong.
• The reason is that rendercanvas uses get_asyncgen_hooks() to determine what event loop is running. And when asyncio runs, it sets that hook. It should also set it back to what it was, but something goes sideways here ...

Selecting the right event-loop is a tricky problem. Maybe we should revisit it again ...

For now, one workaround could be:

>>> %gui qt
>>> from rendercanvas.auto import RenderCanvas
>>> from rendercanvas.asyncio import loop
>>> RenderCanvas.select_loop(loop)
>>> canvas = RenderCanvas() 
>>>

Could you try and see if this solves things? If it does, we could maybe detect IPython and do this automatically.

[schlegelp]

Thanks for the detective work!

Forcing asyncio prevents the error but it seems to also prevent the canvas from drawing anything. I've tried it in octarine and just get a blank canvas:

There is this line in the Viewer class which is supposed to kick off the render loop:

  # Start the animation loop
  self.canvas.request_draw(self._animate)

I added a print statement to self._animate and it never gets called.

[Vipitis]

When I tried it a while ago you could call loop.run() on the raw loop or asyncio loop... Which made the canvas animate - but it was blocking, so you didn't get the interactivity.

[schlegelp]

Yeah, I've been using loop.run() when running from a script but from ipython it always blocks which defeats the purpose.

Just to confirm: I tried it again and it does give the expected behaviour, i.e. correctly enters the rendering loop but blocks all other input.

[almarklein]

Forcing asyncio prevents the error but it seems to also prevent the canvas from drawing anything.

Thanks for trying. Ok, so maybe asyncio does not actually run, but ran briefly, or only when using await or something. In any case, IPython/asyncio did call sys.set_asyncgen_hooks(), causing rendercanvas to get confused about what loop is running.

How severe is this bug for you or your users? Can they just use glfw instead?

[schlegelp]

I have a set of qt/pyside6 GUI elements in my application. Haven't tried but I imagine it would be at least tricky if not impossible to use those in conjunction with glfw...

For now I could also pin the rendercanvas version but I guess that's not a long-term solution.