Sorl-thumbnail: «OSError: cannot write mode RGBA as JPEG» when get_thumbnail for PNG in mode=P

What did I do?

Tried to get thumbnail (JPEG without transparency) for PNG image in mode=P (indexed colours?).
(It works good for PNG with mode=RGBA but not mode=P)

What did I expect to happen?

Saving thumbnail without any exceptions, flattened alpha-channel into solid background of some colour (eg THUMBNAIL_PADDING_COLOR, some specific colour from settings or 'white').

Test image

https://user-images.githubusercontent.com/244666/45594613-a5aba780-b9a6-11e8-9321-da6710f9fb4f.png

What actually happened?

>>> i = Image.open('forbes.png')
>>> i
<PIL.PngImagePlugin.PngImageFile image mode=P size=400x400 at 0x7FE650252CC0>

>>> get_thumbnail(i, '100x106', crop='center', quality=99, format='PNG')
<sorl.thumbnail.images.ImageFile at 0x7fe6501d6390>

>>> get_thumbnail(i, '100x100', crop='center', quality=99, format='JPEG')
OSError: cannot write mode RGBA as JPEG

Versions

Python == 3.6.5
Pillow == 5.2.0 (or any versions >= 4.2.0)
sorl-thumbnail == 12.4.1

Related issue

https://github.com/python-pillow/Pillow/issues/2609

How to fix it?

You need to make a background image with certain colour and merge source image into background:

    def _colorspace(self, image, colorspace, format):
        if colorspace == 'RGB':
            # Pillow JPEG doesn't allow RGBA anymore. It was converted to RGB before.
            if image.mode == 'RGBA' and format != 'JPEG':
                return image  # RGBA is just RGB + Alpha
            if image.mode == 'LA' or (image.mode == 'P' and 'transparency' in image.info):
                if format == 'JPEG':
                    newimage = Image.new('RGB', image.size, '#eebbaa')
                    mask = image.convert('RGBA').split()[-1]
                    newimage.paste(image.convert('RGBA'), (0, 0), mask)
                else:
                    newimage = image.convert('RGBA')
                    transparency = image.info.get('transparency')
                    if transparency is not None:
                        mask = image.convert('RGBA').split()[-1]
                        newimage.putalpha(mask)
                return newimage
            return image.convert('RGB')
        if colorspace == 'GRAY':
            return image.convert('L')
        return image

Correct result