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Only update Python palette when loading an image if rawmode was different #9309

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Only update Python palette when loading an image if rawmode was different #9309

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6a9960e
Only update Python palette if rawmode was different to the mode
radarhere Nov 25, 2025
d06c8b3
Test drawing a new color onto a dirty palette
radarhere Nov 27, 2025
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12 changes: 11 additions & 1 deletion Tests/test_imagedraw.py
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Original file line number Diff line number Diff line change
Expand Up @@ -68,10 +68,20 @@ def test_sanity() -> None:
draw.rectangle(list(range(4)))


def test_valueerror() -> None:
def test_new_color() -> None:
with Image.open("Tests/images/chi.gif") as im:
draw = ImageDraw.Draw(im)
assert len(im.palette.colors) == 249

# Test drawing a new color onto the palette
draw.line((0, 0), fill=(0, 0, 0))
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@markmentovai markmentovai Nov 26, 2025

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I don’t think that this is the best possible test for the fix, because it doesn’t actually draw anything. You’re only giving a single coordinate pair to ImageDraw.line.

I don’t know that it’s great that ImageDraw.line even accepts this since it’s (almost) a functional no-op, but I’m assuming that you wouldn’t want to change it now because existing code might be reliant on this behavior. (But that existing code might be broken?) Regardless, it’s not in scope for this change.

But really, as long as ImageDraw.line might accept a single coordinate pair, or even zero coordinate pairs (which it also accepts), it’d still be well within its rights to not alter the palette given that there’s nothing to draw in these cases. That it does now is maybe even a little unexpected, and it’s why I referred to the operation as “almost” a no-op above. At best, it’s probably just a side effect of the implementation, and nothing that anyone (including this test) should rely on.

I’m not sure what the original form of this test, test_valueerror, was intended to achieve, but if it’s important to get test coverage of the single coordinate pair ImageDraw.line case, I don’t think it’s prudent to replace that test with something that tests the fix for the behavior originally reported in #9308.

It would be best if the test for this palette did actual drawing that altered the image, such as by providing (at least) 2 unequal coordinate pairs to ImageDraw.draw, or by using ImageDraw.point as the test I originally proposed in #9308 did.

Even better if the test further asserted that the draw operation had an effect on the image, not just the palette, because if it’s possible for the image to not change, it’s also reasonable that the palette might not.

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@radarhere radarhere Nov 27, 2025

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I've updated the test to use point() and to check what is drawn afterwards.

I'm somewhat inclined to leave existing tests as they are, so I haven't adjusted the original code, but in practical terms, no, I don't think the use of line() is important here, and could easily be replaced with point().

assert len(im.palette.colors) == 250
assert im.palette.dirty

# Test drawing another new color, now that the palette is dirty
draw.point((0, 0), fill=(1, 0, 0))
assert len(im.palette.colors) == 251
assert im.convert("RGB").getpixel((0, 0)) == (1, 0, 0)


def test_mode_mismatch() -> None:
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4 changes: 3 additions & 1 deletion src/PIL/Image.py
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Expand Up @@ -897,7 +897,9 @@ def load(self) -> core.PixelAccess | None:
else:
self.im.putpalettealphas(self.info["transparency"])
self.palette.mode = "RGBA"
else:
elif self.palette.mode != mode:
# If the palette rawmode is different to the mode,
# then update the Python palette data
self.palette.palette = self.im.getpalette(
self.palette.mode, self.palette.mode
)
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