Reverted converting L with transparency to LA after first frame

Tests/test_file_gif.py

@@ -59,23 +59,15 @@ def test_invalid_file():
         GifImagePlugin.GifImageFile(invalid_file)
 
 
-def test_l_mode_subsequent_frames():
-    with Image.open("Tests/images/no_palette.gif") as im:
-        assert im.mode == "L"
-        assert im.load()[0, 0] == 0
-
-        im.seek(1)
-        assert im.mode == "L"
-        assert im.load()[0, 0] == 0
-
+def test_l_mode_transparency():
     with Image.open("Tests/images/no_palette_with_transparency.gif") as im:
         assert im.mode == "L"
-        assert im.load()[0, 0] == 0
+        assert im.load()[0, 0] == 128
         assert im.info["transparency"] == 255
 
         im.seek(1)
-        assert im.mode == "LA"
-        assert im.load()[0, 0] == (0, 255)
+        assert im.mode == "L"
+        assert im.load()[0, 0] == 128
 
 
 def test_strategy():
@@ -440,31 +432,34 @@ def test_dispose_background_transparency():
     "mode_strategy, expected_colors",
     (
         (
-            GifImagePlugin.ModeStrategy.DIFFERENT_PALETTE_ONLY,
+            GifImagePlugin.ModeStrategy.AFTER_FIRST,
             (
                 (2, 1, 2),
-                (0, 1, 0),
-                (2, 1, 2),
+                ((0, 255, 24, 255), (0, 0, 255, 255), (0, 255, 24, 255)),
+                ((0, 0, 0, 0), (0, 0, 255, 255), (0, 0, 0, 0)),
             ),
         ),
         (
-            GifImagePlugin.ModeStrategy.AFTER_FIRST,
+            GifImagePlugin.ModeStrategy.DIFFERENT_PALETTE_ONLY,
             (
                 (2, 1, 2),
-                ((0, 255, 24, 255), (0, 0, 255, 255), (0, 255, 24, 255)),
-                ((0, 0, 0, 0), (0, 0, 255, 255), (0, 0, 0, 0)),
+                (0, 1, 0),
+                (2, 1, 2),
             ),
         ),
     ),
 )
 def test_transparent_dispose(mode_strategy, expected_colors):
     GifImagePlugin.PALETTE_TO_RGB = mode_strategy
-    with Image.open("Tests/images/transparent_dispose.gif") as img:
-        for frame in range(3):
-            img.seek(frame)
-            for x in range(3):
-                color = img.getpixel((x, 0))
-                assert color == expected_colors[frame][x]
+    try:
+        with Image.open("Tests/images/transparent_dispose.gif") as img:
+            for frame in range(3):
+                img.seek(frame)
+                for x in range(3):
+                    color = img.getpixel((x, 0))
+                    assert color == expected_colors[frame][x]
+    finally:
+        GifImagePlugin.PALETTE_TO_RGB = GifImagePlugin.ModeStrategy.AFTER_FIRST
 
 
 def test_dispose_previous():

docs/handbook/image-file-formats.rst

@@ -107,8 +107,7 @@ are used or GIF89a is already in use.
 
 GIF files are initially read as grayscale (``L``) or palette mode (``P``)
 images. Seeking to later frames in a ``P`` image will change the image to
-``RGB`` (or ``RGBA`` if the first frame had transparency). ``L`` images will
-stay in ``L`` mode (or change to ``LA`` if the first frame had transparency).
+``RGB`` (or ``RGBA`` if the first frame had transparency).
 
 ``P`` mode images are changed to ``RGB`` because each frame of a GIF may
 introduce up to 256 colors. Because ``P`` can only have up to 256 colors, the

src/PIL/GifImagePlugin.py

@@ -71,6 +71,12 @@ def data(self):
             return self.fp.read(s[0])
         return None
 
+    def _is_palette_needed(self, p):
+        for i in range(0, len(p), 3):
+            if not (i // 3 == p[i] == p[i + 1] == p[i + 2]):
+                return True
+        return False
+
     def _open(self):
 
         # Screen
@@ -89,11 +95,9 @@ def _open(self):
             self.info["background"] = s[11]
             # check if palette contains colour indices
             p = self.fp.read(3 << bits)
-            for i in range(0, len(p), 3):
-                if not (i // 3 == p[i] == p[i + 1] == p[i + 2]):
-                    p = ImagePalette.raw("RGB", p)
-                    self.global_palette = self.palette = p
-                    break
+            if self._is_palette_needed(p):
+                p = ImagePalette.raw("RGB", p)
+                self.global_palette = self.palette = p
 
         self.__fp = self.fp  # FIXME: hack
         self.__rewind = self.fp.tell()
@@ -259,7 +263,9 @@ def _seek(self, frame, update_image=True):
 
                 if flags & 128:
                     bits = (flags & 7) + 1
-                    palette = ImagePalette.raw("RGB", self.fp.read(3 << bits))
+                    p = self.fp.read(3 << bits)
+                    if self._is_palette_needed(p):
+                        palette = ImagePalette.raw("RGB", p)
 
                 # image data
                 bits = self.fp.read(1)[0]
@@ -305,11 +311,6 @@ def _seek(self, frame, update_image=True):
                     else:
                         self.mode = "RGB"
                         self.im = self.im.convert("RGB", Image.Dither.FLOYDSTEINBERG)
-            elif self.mode == "L" and "transparency" in self.info:
-                self.pyaccess = None
-                self.im = self.im.convert_transparent("LA", self.info["transparency"])
-                self.mode = "LA"
-                del self.info["transparency"]
 
         def _rgb(color):
             if self._frame_palette:
@@ -369,7 +370,7 @@ def _rgb(color):
             if frame_transparency is not None:
                 if frame == 0:
                     self.info["transparency"] = frame_transparency
-                elif self.mode not in ("RGB", "RGBA", "LA"):
+                elif self.mode not in ("RGB", "RGBA"):
                     transparency = frame_transparency
             self.tile = [
                 (
@@ -394,7 +395,7 @@ def load_prepare(self):
                 self.im = Image.core.fill(
                     temp_mode, self.size, self.info["transparency"]
                 )
-        elif self.mode in ("RGB", "RGBA", "LA"):
+        elif self.mode in ("RGB", "RGBA"):
             self._prev_im = self.im
             if self._frame_palette:
                 self.im = Image.core.fill("P", self.size, self._frame_transparency or 0)
@@ -418,8 +419,6 @@ def load_end(self):
                 frame_im = self.im.convert("RGBA")
             else:
                 frame_im = self.im.convert("RGB")
-        elif self.mode == "L" and self._frame_transparency is not None:
-            frame_im = self.im.convert_transparent("LA", self._frame_transparency)
         else:
             if not self._prev_im:
                 return
@@ -428,7 +427,7 @@ def load_end(self):
 
         self.im = self._prev_im
         self.mode = self.im.mode
-        if frame_im.mode in ("LA", "RGBA"):
+        if frame_im.mode == "RGBA":
             self.im.paste(frame_im, self.dispose_extent, frame_im)
         else:
             self.im.paste(frame_im, self.dispose_extent)