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Fix adding a new color when image palette has missing background or transparency index #9799

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Merged
radarhere merged 3 commits into from
Jul 19, 2026
+10 −2
Merged

Fix adding a new color when image palette has missing background or transparency index #9799

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48fa0c5
Store the color at the returned index in ImagePalette.getcolor
chuenchen309 Jul 19, 2026
d809d2b
Extend palette at the same time as increasing the index
radarhere Jul 19, 2026
71df484
Combine new test into existing test
radarhere Jul 19, 2026
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8 changes: 6 additions & 2 deletions Tests/test_imagepalette.py
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Original file line number Diff line number Diff line change
Expand Up @@ -87,13 +87,17 @@ def test_getcolor_not_special(index: int, palette: ImagePalette.ImagePalette) ->

# Do not use transparency index as a new color
im.info["transparency"] = index
index1 = palette.getcolor((0, 0, 0), im)
index1 = palette.getcolor((0, 0, 1), im)
assert index1 != index
roundtripped_palette = ImagePalette.ImagePalette(palette=palette.palette)
assert roundtripped_palette.colors[(0, 0, 1)] == index1

# Do not use background index as a new color
im.info["background"] = index1
index2 = palette.getcolor((0, 0, 1), im)
index2 = palette.getcolor((0, 0, 2), im)
assert index2 not in (index, index1)
roundtripped_palette = ImagePalette.ImagePalette(palette=palette.palette)
assert roundtripped_palette.colors[(0, 0, 2)] == index2


def test_file(tmp_path: Path) -> None:
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4 changes: 4 additions & 0 deletions src/PIL/ImagePalette.py
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Original file line number Diff line number Diff line change
Expand Up @@ -125,7 +125,11 @@ def _new_color_index(
image.info.get("background"),
image.info.get("transparency"),
)
assert isinstance(self._palette, bytearray)
while index in special_colors:
# Background or transparency index points past the end of the palette.
# Set it to black, so that the new color can be written afterwards.
self._palette += bytearray(len(self.mode))
index += 1
if index >= 256:
if image:
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