Unpopular `formats` argument of `PIL.Image.open` cause exceptions

[tsangwpx]

In open(), preinit() only initialize popular formats,
_open_core() will raise if the given formats have not been initialized. The init() failback become unreachable.

Pillow/src/PIL/Image.py

Lines 2901 to 2934 in 2d6e51e

	preinit()
	accept_warnings = []
	def _open_core(fp, filename, prefix, formats):
	for i in formats:
	try:
	factory, accept = OPEN[i]
	result = not accept or accept(prefix)
	if type(result) in [str, bytes]:
	accept_warnings.append(result)
	elif result:
	fp.seek(0)
	im = factory(fp, filename)
	_decompression_bomb_check(im.size)
	return im
	except (SyntaxError, IndexError, TypeError, struct.error):
	# Leave disabled by default, spams the logs with image
	# opening failures that are entirely expected.
	# logger.debug("", exc_info=True)
	continue
	except BaseException:
	if exclusive_fp:
	fp.close()
	raise
	return None
	im = _open_core(fp, filename, prefix, formats)
	if im is None:
	if init():
	im = _open_core(fp, filename, prefix, formats)

What did you do?

from __future__ import annotations

from PIL import Image


def main():
    with open('any.webp', 'rb') as f:
        try:
            im = Image.open(f, formats=('WEBP',))
        except Exception:
            print(Image.ID)  # No WEBP in the list
            raise
        else:
            print(im.size)


if __name__ == '__main__':
    main()

What did you expect to happen?

Output the size

(640, 480)

What actually happened?

['BMP', 'DIB', 'GIF', 'TIFF', 'JPEG', 'PPM', 'PNG']
Traceback (most recent call last):
  File "pil_debug.py", line 18, in <module>
    main()
  File "pil_debug.py", line 9, in main
    im = Image.open(f, formats=('WEBP',))
  File "lib\site-packages\PIL\Image.py", line 2929, in open
    im = _open_core(fp, filename, prefix, formats)
  File "lib\site-packages\PIL\Image.py", line 2909, in _open_core
    factory, accept = OPEN[i]
KeyError: 'WEBP'

What are your OS, Python and Pillow versions?

• OS: Windows 10
• Python: 3.8
• Pillow: 8.0.1

[nulano]

You can import the WebP plugin explicitly to make sure it is loaded:

from PIL import Image, WebPImagePlugin
im = Image.open("Downloads/1.webp", formats=("WEBP",))
# im = <PIL.WebPImagePlugin.WebPImageFile image mode=RGB size=550x368 at 0x233A9546508>

[radarhere]

I've created PR #5037 to resolve this.