remap_palette loses transparency information

[JensRestemeier]

What did you do?

I have palettized images that use "0xFF 0x00 0xFF" to mark transparency. After converting this into an alpha channel I want to reorder colours so that transparency is at the start of the palette.
I am using "image.remap_palette" to reorder colours.

What did you expect to happen?

I expect the transparency information in a palette to be preserved when reordering colours.

What actually happened?

Transparency information in a palette is lost when reordering colours

What are your OS, Python and Pillow versions?

• OS: Windows 10
• Python: 3.9.5 (64 bit)
• Pillow: 9.1.1

from PIL import Image

img = Image.new("P", (256, 256))
pixel = []
for y in range(256):
    for x in range(256):
        v = ((y // 16) & 1) ^ ((x // 16) & 1)
        pixel.append(v)
img.putdata(pixel)

pal = [255,0,0,0 ,0,255,0,255] + [255] * 248 * 4
img.putpalette(pal, "RGBA")
img.save("test_A.png")

img = img.remap_palette([1,0])
img.save("test_B.png")

This produces two images: test_A is the input, test_B is the output
Both images should look identical when opened in a program like GIMP. The colours (red and green) are correctly swapped, though test_B loses the transparency on red.

[radarhere]

I've created PR #6350 to resolve this.