Allow RGBA palettes to work with ImageOps.expand() (#9138)

Author: hugovk

Tests/test_imageops.py

@@ -186,6 +186,21 @@ def test_palette(mode: str) -> None:
     )
 
 
+def test_rgba_palette() -> None:
+    im = Image.new("P", (1, 1))
+
+    red = (255, 0, 0, 255)
+    translucent_black = (0, 0, 0, 127)
+    im.putpalette(red + translucent_black, "RGBA")
+
+    expanded_im = ImageOps.expand(im, 1, 1)
+
+    palette = expanded_im.palette
+    assert palette is not None
+    assert palette.mode == "RGBA"
+    assert expanded_im.convert("RGBA").getpixel((0, 0)) == translucent_black
+
+
 def test_pil163() -> None:
     # Division by zero in equalize if < 255 pixels in image (@PIL163)
 

src/PIL/ImageOps.py

@@ -499,14 +499,15 @@ def expand(
     height = top + image.size[1] + bottom
     color = _color(fill, image.mode)
     if image.palette:
-        palette = ImagePalette.ImagePalette(palette=image.getpalette())
+        mode = image.palette.mode
+        palette = ImagePalette.ImagePalette(mode, image.getpalette(mode))
         if isinstance(color, tuple) and (len(color) == 3 or len(color) == 4):
             color = palette.getcolor(color)
     else:
         palette = None
     out = Image.new(image.mode, (width, height), color)
     if palette:
-        out.putpalette(palette.palette)
+        out.putpalette(palette.palette, mode)
     out.paste(image, (left, top))
     return out