[radarhere]

Alternative to #10061

When saving a GIF or PNG with multiple frames, you can specify the disposal or blend through a keyword argument, like

im.save("out.gif", save_all=True, disposal=2)
im.save("out.gif", save_all=True, disposal=(0, 1, 2))

However, if the user doesn't provide a keyword argument when saving, Pillow will fallback to the base image's info.

Pillow/src/PIL/GifImagePlugin.py

Line 664 in 2e758a2

disposal = im.encoderinfo.get("disposal", im.info.get("disposal"))

Pillow/src/PIL/PngImagePlugin.py

Lines 1207 to 1208 in 2e758a2

	disposal = im.encoderinfo.get("disposal", im.info.get("disposal", Disposal.OP_NONE))
	blend = im.encoderinfo.get("blend", im.info.get("blend", Blend.OP_SOURCE))

When a single integer is in the disposal, GIF ignores it. GIF doesn't write "disposal" to info when opening. im.info["disposal"] = (0, 1, 2) is the only scenario for which this would make a difference, but info["disposal"] isn't documented at https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#gif-saving, so I suggest removing it.

For PNG, a single integer in disposal and blend variables aren't used for that specific frame only, they are the default for all frames of all images. It isn't a documented feature at
and https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#apng-saving. I consider this unexpected.

If you would like a demonstration of the unexpected behaviour, #10061 notes that roundtripping

from PIL import Image
Image.open("Tests/images/iss634.apng").save("out.png", save_all=True)

produces

The test code that I'm removing here was added in #5611.