Only use an RGBA palette for images with an alpha channel

Tests/test_file_apng.py

@@ -249,8 +249,8 @@ def test_apng_mode():
         assert im.mode == "P"
         im.seek(im.n_frames - 1)
         im = im.convert("RGBA")
-        assert im.getpixel((0, 0)) == (0, 255, 0, 255)
-        assert im.getpixel((64, 32)) == (0, 255, 0, 255)
+        assert im.getpixel((0, 0)) == (255, 0, 0, 0)
+        assert im.getpixel((64, 32)) == (255, 0, 0, 0)
 
     with Image.open("Tests/images/apng/mode_palette_1bit_alpha.png") as im:
         assert im.mode == "P"

Tests/test_image_convert.py

@@ -167,7 +167,7 @@ def test_gif_with_rgba_palette_to_p():
     with Image.open("Tests/images/hopper.gif") as im:
         im.info["transparency"] = 255
         im.load()
-        assert im.palette.mode == "RGBA"
+        assert im.palette.mode == "RGB"
         im_p = im.convert("P")
 
     # Should not raise ValueError: unrecognized raw mode

src/PIL/Image.py

@@ -833,7 +833,7 @@ class Image:
             palette_length = self.im.putpalette(mode, arr)
             self.palette.dirty = 0
             self.palette.rawmode = None
-            if "transparency" in self.info:
+            if "transparency" in self.info and mode in ("RGBA", "LA", "PA"):
                 if isinstance(self.info["transparency"], int):
                     self.im.putpalettealpha(self.info["transparency"], 0)
                 else:

src/PIL/WebPImagePlugin.py

@@ -320,7 +320,7 @@ def _save(im, fp, filename):
         alpha = (
             "A" in im.mode
             or "a" in im.mode
-            or (im.mode == "P" and "A" in im.im.getpalettemode())
+            or (im.mode == "P" and "transparency" in im.info)
         )
         im = im.convert("RGBA" if alpha else "RGB")