Merge pull request matplotlib#29520 from Anselmoo/Anselmoo/issue29519

FIX: Correct variable name from _frame to _frames in PillowWriter class

Author: timhoffm

lib/matplotlib/animation.py

@@ -496,7 +496,7 @@ def grab_frame(self, **savefig_kwargs):
             "RGBA", self.frame_size, buf.getbuffer(), "raw", "RGBA", 0, 1)
         if im.getextrema()[3][0] < 255:
             # This frame has transparency, so we'll just add it as is.
-            self._frame.append(im)
+            self._frames.append(im)
         else:
             # Without transparency, we switch to RGB mode, which converts to P mode a
             # little better if needed (specifically, this helps with GIF output.)

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
 
 
@@ -526,3 +527,20 @@ def test_movie_writer_invalid_path(anim):
     with pytest.raises(FileNotFoundError, match=match_str):
         anim.save("/foo/bar/aardvark/thiscannotreallyexist.mp4",
                   writer=animation.FFMpegFileWriter())
+
+
+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)