If append_images is populated, default save_all to True

Tests/test_file_gif.py

@@ -1138,6 +1138,12 @@ def test_append_images(tmp_path: Path) -> None:
     ims = [Image.new("RGB", (100, 100), color) for color in ["#0f0", "#00f"]]
     im.copy().save(out, save_all=True, append_images=ims)
 
+    with Image.open(out) as reread:
+        assert reread.n_frames == 3
+
+    # Test append_images without save_all
+    im.copy().save(out, append_images=ims)
+
     with Image.open(out) as reread:
         assert reread.n_frames == 3
 

docs/handbook/image-file-formats.rst

@@ -235,8 +235,9 @@ following options are available::
     im.save(out, save_all=True, append_images=[im1, im2, ...])
 
 **save_all**
-    If present and true, all frames of the image will be saved. If
-    not, then only the first frame of a multiframe image will be saved.
+    If present and true, or if ``append_images`` is not empty, all frames of
+    the image will be saved. Otherwise, only the first frame of a multiframe
+    image will be saved.
 
 **append_images**
     A list of images to append as additional frames. Each of the
@@ -723,8 +724,8 @@ Saving
 
 When calling :py:meth:`~PIL.Image.Image.save` to write an MPO file, by default
 only the first frame of a multiframe image will be saved. If the ``save_all``
-argument is present and true, then all frames will be saved, and the following
-option will also be available.
+argument is present and true, or if ``append_images`` is not empty, all frames
+will be saved.
 
 **append_images**
     A list of images to append as additional pictures. Each of the
@@ -934,7 +935,8 @@ Saving
 
 When calling :py:meth:`~PIL.Image.Image.save`, by default only a single frame PNG file
 will be saved. To save an APNG file (including a single frame APNG), the ``save_all``
-parameter must be set to ``True``. The following parameters can also be set:
+parameter should be set to ``True`` or ``append_images`` should not be empty. The
+following parameters can also be set:
 
 **default_image**
     Boolean value, specifying whether or not the base image is a default image.
@@ -1163,7 +1165,8 @@ Saving
 The :py:meth:`~PIL.Image.Image.save` method can take the following keyword arguments:
 
 **save_all**
-    If true, Pillow will save all frames of the image to a multiframe tiff document.
+    If true, or if ``append_images`` is not empty, Pillow will save all frames of the
+    image to a multiframe tiff document.
 
     .. versionadded:: 3.4.0
 
@@ -1313,8 +1316,8 @@ Saving sequences
 
 When calling :py:meth:`~PIL.Image.Image.save` to write a WebP file, by default
 only the first frame of a multiframe image will be saved. If the ``save_all``
-argument is present and true, then all frames will be saved, and the following
-options will also be available.
+argument is present and true, or if ``append_images`` is not empty, all frames
+will be saved, and the following options will also be available.
 
 **append_images**
     A list of images to append as additional frames. Each of the
@@ -1616,15 +1619,14 @@ The :py:meth:`~PIL.Image.Image.save` method can take the following keyword argum
 **save_all**
     If a multiframe image is used, by default, only the first image will be saved.
     To save all frames, each frame to a separate page of the PDF, the ``save_all``
-    parameter must be present and set to ``True``.
+    parameter should be present and set to ``True`` or ``append_images`` should not be
+    empty.
 
     .. versionadded:: 3.0.0
 
 **append_images**
     A list of :py:class:`PIL.Image.Image` objects to append as additional pages. Each
-    of the images in the list can be single or multiframe images. The ``save_all``
-    parameter must be present and set to ``True`` in conjunction with
-    ``append_images``.
+    of the images in the list can be single or multiframe images.
 
     .. versionadded:: 4.2.0
 

docs/handbook/tutorial.rst

@@ -534,7 +534,6 @@ You can create animated GIFs with Pillow, e.g.
     # Save the images as an animated GIF
     images[0].save(
         "animated_hopper.gif",
-        save_all=True,
         append_images=images[1:],
         duration=500,  # duration of each frame in milliseconds
         loop=0,  # loop forever

src/PIL/Image.py

@@ -2527,13 +2527,17 @@ def save(
         # may mutate self!
         self._ensure_mutable()
 
-        save_all = params.pop("save_all", False)
+        save_all = params.pop("save_all", None)
         self.encoderinfo = {**getattr(self, "encoderinfo", {}), **params}
         self.encoderconfig: tuple[Any, ...] = ()
 
         if format.upper() not in SAVE:
             init()
-        if save_all:
+        if save_all or (
+            save_all is None
+            and params.get("append_images")
+            and format.upper() in SAVE_ALL
+        ):
             save_handler = SAVE_ALL[format.upper()]
         else:
             save_handler = SAVE[format.upper()]