Well spotted, @fredrik-1. I do however fear that people might not understand the motivation and implication.

So the following code works fine on 2.2.2

import matplotlib.pyplot as plt
import matplotlib.animation

fig, ax = plt.subplots()
line, = ax.plot([0,10])

def animate(i):
    line.set_ydata([i, 10-i])
    
ani = matplotlib.animation.FuncAnimation(fig, animate, 10)
plt.show()

Due to a change in #11370 the same code now breaks in master with

TypeError: 'NoneType' object is not iterable

This is due to the animating function not returning an iterable of artists.

According to the documentation such animating function should indeed return an iterable of artists, however there is in principle no such need if blit=False (the default) is used. This leads to the construct from above being widely spread.

If a version with #11370 in is now being released, it will break a lot of people's codes.

Options:

• Make it explicit that from now on you really need to return an iterable of artists. This would at least get a what's new entry. It would also need a proper Error message to tell people what to do.
• Be forgiving about not returning any artists (just as it's always been). In this case the fix proposed in this PR may be useful.

I would consider this release critical for 3.0.

For 3.0 this fix is the pragmatic change to prevent back-compatibility issues.

For 3.1 we may want to start warning if no artists are returned, but that should happen in a different PR.

For 3.1 we may want to start warning if no artists are returned

although it is a bit questionable why one would force users to add some useless line like

def animate(i):
    line.set_ydata([i, 10-i])
    return fig.patch,

just to prevent some warning.

Until I finally finish refactoring the blitting code so that it's more reliable, I think it's fine to only care about having the callback return artists for the blitting path.

As say this as someone who just spent the last month writing a library that wraps funcanimation. It is a really bad idea to require that the callback has returns artists (if blitting is false).

My library is built on the concept "blocks" while most of my blocks return artists some don't. (which means I might return a list of artists with some NoneTypes though I can issue a warning if blitting is true and certain blocks are used)

Animating things in matplotlib can be very difficult. It is very common for people to do:

def animate(i):
    ax.clear()
    #re-plot everything

This is a common tactic for pcolormesh despite there being a proper way to update the data, and for some types of plots (like pie charts), it's the only reasonably easy way. Requiring it to return artists just make's it a lot harder for people.

I'm seriously against requiring the callback to return artists in the future (if blitting is false). Totally for an error if blit is true.

So that seems to be pretty conclusive that we do not want to start warning on no returns for 3.1 !