[Bug]: Swallowed error in Jupyter+Matplotlib FuncAnimation.

[Carreau]

Bug summary

In some case it seem that FuncAnimation in Jupyter + matplotlib widgets + funcanimation just swallow error, leading to hard to debug error as to why animations don't show.

Code for reproduction

Here is a dumb animation where the animate part, raise an error (in the original code this was a typo so a NameError, but for readability, here an assert).

%matplotlib widget

import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation 

Nx = 101

x = list(range(10))
def u_func(t):         
    return [x+t//10 for x in range(10)]

fig, ax = plt.subplots()
dt_ms =  50

u0 = u_func(0)
line = ax.plot(x, u0)[0] 



def animate(n):
    """ Fonction principale qui met à jour l'image d'un pas de temps à un autre
    Entrée : 
        n : pas de temps courant
    """
    line.set_ydata(u_func(n))
    assert False, "anything that triggers and error like a typo"
    return line
 
anim = FuncAnimation(fig, animate, interval=dt_ms, frames=50, repeat=True)

Actual outcome

Animation not happening

Expected outcome

Show at least a warnings that the animate function raised

Additional information

I tried to print a trace as to when the animate function get called see

https://gist.github.com/Carreau/e6510855ecab88630f1ea5985a98358c

This is likely multiple problems, matpltolib may want have an explicit warning that animate crash, but ipympl, comm, and ipykernel itself should likely have error messages as well.

Operating system

osx

Matplotlib Version

3.8.3

Matplotlib Backend

module://ipympl.backend_nbagg

Python version

3.11

Jupyter version

4.0.9

Installation

pip

[dopplershift]

I think you need to trigger the animation to render. You can do this directly with anim.to_jshtml() or anim.to_html5_video(). Should also work by having anim at the end of the cell...BUT:

On my system, mpl.rcParams['animation.html'] is defaulting to 'none', which is causing the _repr_html_() method to return None.

[tacaswell]

I suspect this is related to the issue with event callbacks where any print calls are not forwarded to the browser.

[Carreau]

think you need to trigger the animation to render. You can do this directly with anim.to_jshtml() or anim.to_html5_video(). Should also work by having anim at the end of the cell...BUT:

On my system, mpl.rcParams['animation.html'] is defaulting to 'none', which is causing the _repr_html_() method to return None.

Yeah, but with the default settings, if you just remove the assert False, the IPython display hook is triggered anyway and the animation works, the animation rendering is triggered, and I don't expect end user to understand those subtleties.

I also confirm that by adding a write to a file and this is triggered.

I suspect this is related to the issue with event callbacks where any print calls are not forwarded to the browser.

Hum that seem more likely, I didn't tracked if the is an try/except in this stack and if it would print anything. I'll see if I can track that down.

[dopplershift]

@Carreau I can reproduce your behavior that it auto-displays properly with the assert removed but we get just a normal repr if the assertion is present. Weird.

None of this makes sense, because the auto-display AFAICT is going through _repr_html_(), which is in its entirety:

    def _repr_html_(self):
        """IPython display hook for rendering."""
        fmt = mpl.rcParams['animation.html']
        if fmt == 'html5':
            return self.to_html5_video()
        elif fmt == 'jshtml':
            return self.to_jshtml()

and I can get the assertion output from both methods called in there.