Remove unused ImagingConvertInPlace by radarhere · Pull Request #9878 · python-pillow/Pillow · GitHub

im.putalpha() is the only place where C's setmode() is called.

Lines 2050 to 2052 in 3220aca

    
           mode = getmodebase(self.mode) + "A" 
        
           try: 
        
               self.im.setmode(mode)

This is only trying to convert the image to LA, PA or RGBA.

However, the only operation setmode() can actually succeed at is converting RGB or RGBX to RGBA.

Pillow/src/_imaging.c

Lines 2079 to 2090 in 3220aca

    
           if (im->mode == mode) { 
        
               ; /* same mode; always succeeds */ 
        
           } else if (isRGB(im->mode) && isRGB(mode)) { 
        
               /* color to color */ 
        
               im->mode = mode; 
        
               im->bands = modelen; 
        
               if (mode == IMAGING_MODE_RGBA) { 
        
                   (void)ImagingFillBand(im, 3, 255); 
        
               } 
        
           } else { 
        
               /* trying doing an in-place conversion */ 
        
               if (!ImagingConvertInPlace(im, mode)) {

Pillow/src/libImaging/Convert.c

Lines 1739 to 1748 in 3220aca

    
           ImagingConvertInPlace(Imaging imIn, const ModeID mode) { 
        
               ImagingSectionCookie cookie; 
        
               ImagingShuffler convert; 
        
               int y; 
        
               /* limited support for inplace conversion */ 
        
               if (imIn->mode == IMAGING_MODE_L && mode == IMAGING_MODE_1) { 
        
                   convert = l2bit; 
        
               } else if (imIn->mode == IMAGING_MODE_1 && mode == IMAGING_MODE_L) { 
        
                   convert = bit2l;

ImagingConvertInPlace() isn't able to convert anything to LA, PA or RGBA.

So this PR

removes ImagingConvertInPlace(). setmode() is the only place where ImagingConvertInPlace() is called.
simplifies setmode() to more clearly only convert RGB or RGBX images to RGBA. I've also renamed it to setalpha().

I had been looking at removing ImagingConvertInPlace (in a bid to see if the shuffler functions could be implemented in a faster way) but didn't dare go through with it.

Some thoughts within though :)

This comment is a little stale now?

Is it? The method is still modifying the mode of an image in place. It's more specific now, is all.

I suppose...

Should this function know how to do P-to-PA and L-to-LA too?

The thought occurred to me. It is a bit different to RGB, since RGB already has the same pixelsize as RGBA.

Do you mind if that is a follow-up PR? I would rather tidy things up first.

I'll click the rebase button to test automerge (#9126) on a non-docs PR:

akx reviewed Aug 20, 2026

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radarhere mentioned this pull request Aug 20, 2026

C interface is internal #9879

Merged

radarhere added 3 commits August 28, 2026 11:02

Remove unused ImagingConvertInPlace

d2825d5

Rename setmode to setalpha

28a9180

Add error for incorrect mode

cc41fe3

hugovk force-pushed the convert branch from 8abe0b5 to cc41fe3 Compare August 28, 2026 08:02

hugovk enabled auto-merge August 28, 2026 08:02

hugovk merged commit 20510d4 into python-pillow:main Aug 28, 2026
72 of 79 checks passed

radarhere deleted the convert branch August 28, 2026 08:44

hugovk mentioned this pull request Aug 28, 2026

Use GitHub's automerge instead of Mergify #9126

Merged

               static PyObject *
-              im_setmode(ImagingObject *self, PyObject *args) {
+              im_setalpha(ImagingObject *self, PyObject *args) {
                   /* attempt to modify the mode of an image in place */

Remove unused ImagingConvertInPlace by radarhere · Pull Request #9878 · python-pillow/Pillow · GitHub

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Remove unused ImagingConvertInPlace - #9878

Remove unused ImagingConvertInPlace#9878
hugovk merged 3 commits into
python-pillow:mainfrom
radarhere:convert

radarhere commented Aug 19, 2026 •
edited
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	mode = getmodebase(self.mode) + "A"
	try:
	self.im.setmode(mode)

	if (im->mode == mode) {
	; /* same mode; always succeeds */
	} else if (isRGB(im->mode) && isRGB(mode)) {
	/* color to color */
	im->mode = mode;
	im->bands = modelen;
	if (mode == IMAGING_MODE_RGBA) {
	(void)ImagingFillBand(im, 3, 255);
	}
	} else {
	/* trying doing an in-place conversion */
	if (!ImagingConvertInPlace(im, mode)) {

	ImagingConvertInPlace(Imaging imIn, const ModeID mode) {
	ImagingSectionCookie cookie;
	ImagingShuffler convert;
	int y;

	/* limited support for inplace conversion */
	if (imIn->mode == IMAGING_MODE_L && mode == IMAGING_MODE_1) {
	convert = l2bit;
	} else if (imIn->mode == IMAGING_MODE_1 && mode == IMAGING_MODE_L) {
	convert = bit2l;

Remove unused ImagingConvertInPlace by radarhere · Pull Request #9878 · python-pillow/Pillow · GitHub

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Remove unused ImagingConvertInPlace - #9878

Remove unused ImagingConvertInPlace#9878hugovk merged 3 commits intopython-pillow:mainpython-pillow/Pillow:mainfrom radarhere:convertradarhere/Pillow:convertCopy head branch name to clipboard

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Remove unused ImagingConvertInPlace#9878
hugovk merged 3 commits into
python-pillow:mainfrom
radarhere:convert

radarhere commented Aug 19, 2026 •
edited
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