From 11df01145dea9a7afbcf9cec3f6bd0587f970f3b Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Tue, 28 Jan 2025 00:29:21 +0100
Subject: [PATCH] Backport PR #29520: FIX: Correct variable name from _frame to
 _frames in PillowWriter class

---
 lib/matplotlib/animation.py            |  2 +-
 lib/matplotlib/tests/test_animation.py | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
index 2be61284073a..a87f00201124 100644
--- a/lib/matplotlib/animation.py
+++ b/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.)
diff --git a/lib/matplotlib/tests/test_animation.py b/lib/matplotlib/tests/test_animation.py
index d026dae59533..e3382f662c73 100644
--- a/lib/matplotlib/tests/test_animation.py
+++ b/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
 
 
@@ -551,3 +552,20 @@ 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)