init() if one of the formats is unrecognised

[radarhere]

Resolves #5036

from PIL import Image
im = Image.open('Tests/images/hopper.webp', formats=('WEBP',))

Image.open calls preinit() first and then expects to loop through them. However, formats overrides this, and so WEBP is checked, which throws a KeyError because init() has not been called yet, as Image.open() only calls it if the preinit() formats do not match.

This PR resolves that.

[hugovk]

Can this be tested too?

[radarhere]

The key part of the change here is causing a module to be imported, so that Image.register_open is run.

Pillow/src/PIL/WebPImagePlugin.py

Line 345 in 2d6e51e

Image.register_open(WebPImageFile.format, WebPImageFile, _accept)

The problem is that conftest

Pillow/Tests/conftest.py

Lines 4 to 9 in 2d6e51e

	def pytest_report_header(config):
	try:
	from PIL import features
	with io.StringIO() as out:
	features.pilinfo(out=out, supported_formats=False)

calls Image.init() at the start of the tests

Pillow/src/PIL/features.py

Lines 216 to 230 in 2d6e51e

	def pilinfo(out=None, supported_formats=True):
	"""
	Prints information about this installation of Pillow.
	This function can be called with ``python -m PIL``.
	:param out:
	The output stream to print to. Defaults to ``sys.stdout`` if ``None``.
	:param supported_formats:
	If ``True``, a list of all supported image file formats will be printed.
	"""
	if out is None:
	out = sys.stdout
	Image.init()

So the plugins are already imported. I'm not seeing a way to re-import a Python module.

I think it would be possible to mock a plugin, but I feel like that would be overly complex.

[hugovk]

Okay, thanks!