Inspect arg as Python object, instead of using PyErr_Clear()

[radarhere]

In _convert_transparent(), instead of failing a call to PyArg_ParseTuple(), calling PyErr_Clear() afterwards and then trying again,

Pillow/src/_imaging.c

Lines 1084 to 1089 in 095bbc3

	if (PyArg_ParseTuple(args, "s(iii)", &mode_name, &r, &g, &b)) {
	const ModeID mode = findModeID(mode_name);
	return PyImagingNew(ImagingConvertTransparent(self->image, mode, r, g, b));
	}
	PyErr_Clear();
	if (PyArg_ParseTuple(args, "si", &mode_name, &r)) {

an alternative strategy would be to parse the second argument as a PyObject ("O") and then check if it is a tuple or not.

A similar change could be made in _convert_matrix().

[hugovk]

This fails with lists:

from PIL import Image

rgb = Image.new("RGB", (8, 8), (255, 0, 0))


def transparent(t):
    rgb.info["transparency"] = t
    return rgb.convert("RGBA")


cases = {
    "matrix tuple(4)": lambda: rgb.convert("L", matrix=(0.3, 0.59, 0.11, 0.0)),
    "matrix list(4)": lambda: rgb.convert("L", matrix=[0.3, 0.59, 0.11, 0.0]),
    "matrix tuple(12)": lambda: rgb.convert("RGB", matrix=tuple(range(12))),
    "matrix list(12)": lambda: rgb.convert("RGB", matrix=list(range(12))),
    "transparency tuple": lambda: transparent((255, 0, 0)),
    "transparency list": lambda: transparent([255, 0, 0]),
}

for label, fn in cases.items():
    try:
        fn()
        print(f"OK    {label}")
    except Exception as e:
        print(f"RAISE {label}: {type(e).__name__}: {e}")

Before:

OK    matrix tuple(4)
OK    matrix list(4)
OK    matrix tuple(12)
OK    matrix list(12)
OK    transparency tuple
OK    transparency list

After:

OK    matrix tuple(4)
RAISE matrix list(4): SystemError: <method 'convert_matrix' of 'ImagingCore' objects> returned a result with an exception set
OK    matrix tuple(12)
RAISE matrix list(12): SystemError: /Users/nad/build_macos_installer/installer/variant/binaries/build_source/Objects/tupleobject.c:96: bad argument to internal function
OK    transparency tuple
RAISE transparency list: TypeError: 'list' object cannot be interpreted as an integer

Maybe worth adding a test?

Suggested change

	if (PyTuple_Size(matrix) == 12) {
	if (PySequence_Size(matrix) == 12) {

[radarhere]

matrix should be a tuple?

Pillow/src/PIL/Image.py

Lines 1017 to 1020 in 9c9fece

	def convert(
	self,
	mode: str | None = None,
	matrix: tuple[float, ...] | None = None,

Pillow/src/PIL/Image.py

Lines 1054 to 1055 in 9c9fece

	:param matrix: An optional conversion matrix. If given, this
	should be 4- or 12-tuple containing floating point values.

[hugovk]

Should according to type hint and docstring, but not enforced.

There is existing code using lists:

https://github.com/search?q=%2F%5C.convert%5C%28.*%2C+matrix%3D%5C%5B%2F&type=code

Do we want to change the behaviour in this PR?

[codspeed-hq]

Merging this PR will not alter performance

✅ 331 untouched benchmarks

---

_{Comparing radarhere:clear (9197939) with main (6590b1b)}