GIF seek performance improvements

src/PIL/GifImagePlugin.py

@@ -97,7 +97,7 @@ def n_frames(self):
             current = self.tell()
             try:
                 while True:
-                    self.seek(self.tell() + 1)
+                    self._seek(self.tell() + 1, False)
             except EOFError:
                 self._n_frames = self.tell() + 1
             self.seek(current)
@@ -110,14 +110,16 @@ def is_animated(self):
                 self._is_animated = self._n_frames != 1
             else:
                 current = self.tell()
-
-                try:
-                    self.seek(1)
+                if current:
                     self._is_animated = True
-                except EOFError:
-                    self._is_animated = False
+                else:
+                    try:
+                        self._seek(1, False)
+                        self._is_animated = True
+                    except EOFError:
+                        self._is_animated = False
 
-                self.seek(current)
+                    self.seek(current)
         return self._is_animated
 
     def seek(self, frame):
@@ -135,7 +137,7 @@ def seek(self, frame):
                 self.seek(last_frame)
                 raise EOFError("no more images in GIF file") from e
 
-    def _seek(self, frame):
+    def _seek(self, frame, update_image=True):
 
         if frame == 0:
             # rewind
@@ -147,14 +149,11 @@ def _seek(self, frame):
             self.disposal_method = 0
         else:
             # ensure that the previous frame was loaded
-            if self.tile:
+            if self.tile and update_image:
                 self.load()
 
         if frame != self.__frame + 1:
             raise ValueError(f"cannot seek to frame {frame}")
-        self.__frame = frame
-
-        self.tile = []
 
         self.fp = self.__fp
         if self.__offset:
@@ -164,25 +163,34 @@ def _seek(self, frame):
                 pass
             self.__offset = 0
 
-        if self.__frame == 1:
-            self.pyaccess = None
-            if "transparency" in self.info:
-                if self.mode == "P":
-                    self.im.putpalettealpha(self.info["transparency"], 0)
-                    self.im = self.im.convert("RGBA", Image.Dither.FLOYDSTEINBERG)
-                    self.mode = "RGBA"
-                else:
-                    self.im = self.im.convert_transparent(
-                        "LA", self.info["transparency"]
-                    )
-                    self.mode = "LA"
+        s = self.fp.read(1)
+        if not s or s == b";":
+            raise EOFError
 
-                del self.info["transparency"]
-            else:
-                self.mode = "RGB"
-                self.im = self.im.convert("RGB", Image.Dither.FLOYDSTEINBERG)
-        if self.dispose:
-            self.im.paste(self.dispose, self.dispose_extent)
+        self.__frame = frame
+
+        self.tile = []
+
+        if update_image:
+            if self.__frame == 1:
+                self.pyaccess = None
+                if "transparency" in self.info:
+                    if self.mode == "P":
+                        self.im.putpalettealpha(self.info["transparency"], 0)
+                        self.im = self.im.convert("RGBA", Image.Dither.FLOYDSTEINBERG)
+                        self.mode = "RGBA"
+                    else:
+                        self.im = self.im.convert_transparent(
+                            "LA", self.info["transparency"]
+                        )
+                        self.mode = "LA"
+
+                    del self.info["transparency"]
+                else:
+                    self.mode = "RGB"
+                    self.im = self.im.convert("RGB", Image.Dither.FLOYDSTEINBERG)
+            if self.dispose:
+                self.im.paste(self.dispose, self.dispose_extent)
 
         palette = None
 
@@ -191,7 +199,8 @@ def _seek(self, frame):
         interlace = None
         while True:
 
-            s = self.fp.read(1)
+            if not s:
+                s = self.fp.read(1)
             if not s or s == b";":
                 break
 
@@ -229,6 +238,7 @@ def _seek(self, frame):
                         else:
                             info["comment"] = block
                         block = self.data()
+                    s = None
                     continue
                 elif s[0] == 255:
                     #
@@ -270,6 +280,13 @@ def _seek(self, frame):
             else:
                 pass
                 # raise OSError, "illegal GIF tag `%x`" % s[0]
+            s = None
+
+        if interlace is None:
+            # self.__fp = None
+            raise EOFError
+        if not update_image:
+            return
 
         frame_palette = palette or self.global_palette
 
@@ -329,9 +346,6 @@ def _rgb(color):
                     (bits, interlace),
                 )
             ]
-        else:
-            # self.__fp = None
-            raise EOFError
 
         for k in ["duration", "comment", "extension", "loop"]:
             if k in info: