ImageOps.expand removes the alpha channel of palettes #9136

Calling ImageOps.expand on a image with an RGBA palette returns an image with an RGB palette.

Here's an example:

from PIL import Image, ImageOps

img = Image.new("P", (16, 16), 1)

img.putpalette((
    255, 0, 0, 0,  # TRANSPARENT RED
    0, 0, 0, 255,  # OPAQUE BLACK
), "RGBA")

framed_img = ImageOps.expand(img, 6, 0)

In this example I created an image of 16 × 16 pixels and with the second color of the palette (index 1), then I call ImageOps.expand to add a border with the first color of the palette (index 0).

I expected the second image to have a transparent border, but it is opaque:

(img)

(framed_img)

We can easily verify that both images have palettes and the palettes have different modes in a Python REPL:

>>> img.palette.mode
'RGBA'
>>> framed_img.palette.mode
'RGB'