TEST: Update animation test to use PillowWriter for transparency handling

Author: Anselmoo

lib/matplotlib/tests/test_animation.py

@@ -13,6 +13,7 @@
 import matplotlib as mpl
 from matplotlib import pyplot as plt
 from matplotlib import animation
+from matplotlib.animation import PillowWriter
 from matplotlib.testing.decorators import check_figures_equal
 
 
@@ -528,29 +529,18 @@ def test_movie_writer_invalid_path(anim):
                   writer=animation.FFMpegFileWriter())
 
 
-def test_exhausted_animation_with_transparency(tmp_path):
+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)
 
-    def update(frame):
-        # Modify transparency in each frame
-        rect.set_alpha(0.1 + frame * 0.1)
-        return [rect]
-
-    # Create and save animation
-    anim = animation.FuncAnimation(
-        fig, update, frames=iter(range(5)), repeat=False,
-        cache_frame_data=False,
-
-    )
-    tmp_file = tmp_path / "test_transparent.gif"
-    anim.save(tmp_file, writer='pillow', savefig_kwargs={"transparent": True})
-
-    # Verify exhausted warning
-    with pytest.warns(UserWarning, match="exhausted"):
-        anim._start()
-
+    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)