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:

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

via savefig_kwargs={"transparent": True}.

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

# Verify exhausted warning
    with pytest.warns(UserWarning, match="exhausted"):
        anim._start()

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

def test_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 transparency
    assert frame.getextrema()[3][0] < 255
    plt.close(fig)

so the missing part is also captured? thx for feedback

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