What exactly is the purpose of the test?

From a quick reading it is

1. a smoke test to exercise the anim code path for transparency
2. testing that a saved animation is exhausted.

Can you please describe more precisely what the test wants to ensure? Also it seems (2) is quite unrelated to transparency.

The goal of this test is to actually verify the non-covered section:

ifim.getextrema()[3][0]<255:# This frame has transparency, so we'll just add it as is.self._frames.append(im)

viasavefig_kwargs={"transparent": True}.

I tried just to mock up theclass PillowWriter(AbstractMovieWriter):, but it actually raise an error thatsavefig_kwargs={"transparent": True} is not included. Consequently, I extended a little bit the test. Considering, to leave this part out?

# Verify exhausted warningwithpytest.warns(UserWarning,match="exhausted"):anim._start()

@timhoffm what about this test? Only using the PillowWriter?

deftest_animation_with_transparency():"""Test animation exhaustion with transparency using PillowWriter directly"""fig,ax=plt.subplots()rect=plt.Rectangle((0,0),1,1,color='red',alpha=0.5)ax.add_patch(rect)ax.set_xlim(0,1)ax.set_ylim(0,1)writer=PillowWriter(fps=30)writer.setup(fig,'unused.gif',dpi=100)writer.grab_frame(transparent=True)frame=writer._frames[-1]# Check that the alpha channel is not 255, so frame has transparencyassertframe.getextrema()[3][0]<255plt.close(fig)

so the missing part is also captured? thx for feedback

If former test fits better, I will submit a revert commit