GifImagePlugin seek-related optimizations

Tests/test_file_gif.py

@@ -277,6 +277,7 @@ def test_seek():
                 img.seek(img.tell() + 1)
         except EOFError:
             assert frame_count == 5
+            assert img._n_frames == frame_count
 
 
 def test_seek_info():

docs/releasenotes/9.1.0.rst

@@ -121,6 +121,34 @@ can be used to return data in the current mode of the palette.
 Other Changes
 =============
 
+GifImagePlugin seek-related optimizations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Optimized :py:meth:`~PIL.GifImagePlugin.seek()`,
+:py:property:`~PIL.GifImagePlugin.n_frames` and
+:py:property:`~PIL.GifImagePlugin.is_animated` when seeking back to the *current* frame
+before invoking the operation, after an ``EOFError`` is raised.
+
+These operations no longer have to seek back to **frame 0** before seeking forward,
+instead they seek back to the frame just before the *current* frame and then seek
+just one frame forward.
+
+Eliminated seek checks while computing :py:property:`~PIL.GifImagePlugin.n_frames`
+by using :py:meth:`PIL.GifImagePlugin._seek()` in place of
+:py:meth:`~PIL.GifImagePlugin.seek()` since the frame numbers cannot be invalid until
+the final one, which raises ``EOFError``.
+
+When an ``EOFError`` is raised within :py:meth:`~PIL.GifImagePlugin.seek()` i.e just
+seeked past the last frame, ``self._n_frames`` is updated if it hadn't been updated
+earlier, to prevent re-computation when :py:property:`~PIL.GifImagePlugin.n_frames` is
+later invoked.
+
+Added ``self.__prev_offset`` to always hold the offset of the frame just before the
+current frame.
+
+Replaced all internal calls to :py:meth:`~PIL.GifImagePlugin.tell()` with direct
+references to ``self.__frame``.
+
 ImageShow temporary files on Unix
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/PIL/GifImagePlugin.py

@@ -94,13 +94,17 @@ def _open(self):
     @property
     def n_frames(self):
         if self._n_frames is None:
-            current = self.tell()
+            prev_offset = self.__prev_offset
+            current = self.__frame
             try:
                 while True:
-                    self.seek(self.tell() + 1)
+                    self._seek(self.__frame + 1)
             except EOFError:
-                self._n_frames = self.tell() + 1
-            self.seek(current)
+                self._n_frames = self.__frame
+
+            self.__offset = prev_offset
+            self.__frame = current - 1
+            self._seek(current)
         return self._n_frames
 
     @property
@@ -109,15 +113,18 @@ def is_animated(self):
             if self._n_frames is not None:
                 self._is_animated = self._n_frames != 1
             else:
-                current = self.tell()
+                prev_offset = self.__prev_offset
+                current = self.__frame
 
                 try:
                     self.seek(1)
                     self._is_animated = True
                 except EOFError:
                     self._is_animated = False
 
-                self.seek(current)
+                self.__offset = prev_offset
+                self.__frame = current - 1
+                self._seek(current)
         return self._is_animated
 
     def seek(self, frame):
@@ -127,12 +134,17 @@ def seek(self, frame):
             self.im = None
             self._seek(0)
 
+        prev_offset = self.__prev_offset
         last_frame = self.__frame
         for f in range(self.__frame + 1, frame + 1):
             try:
                 self._seek(f)
             except EOFError as e:
-                self.seek(last_frame)
+                if not self._n_frames:
+                    self._n_frames = f
+                self.__offset = prev_offset
+                self.__frame = last_frame - 1
+                self._seek(last_frame)
                 raise EOFError("no more images in GIF file") from e
 
     def _seek(self, frame):
@@ -157,6 +169,7 @@ def _seek(self, frame):
         self.tile = []
 
         self.fp = self.__fp
+        self.__prev_offset = self.__offset
         if self.__offset:
             # backup to last frame
             self.fp.seek(self.__offset)