importing PIL.PngImagePlugin breaks Image.registered_extensions()

[aliencaocao]

What are your OS, Python and Pillow versions?

• OS: Windows 10
• Python: 3.9.13
• Pillow: 9.3.0

from PIL import Image
print(Image.registered_extensions())

The code above outputs, all what I expected.
{'.blp': 'BLP', '.bmp': 'BMP', '.dib': 'DIB', '.bufr': 'BUFR', '.cur': 'CUR', '.pcx': 'PCX', '.dcx': 'DCX', '.dds': 'DDS', '.ps': 'EPS', '.eps': 'EPS', '.fit': 'FITS', '.fits': 'FITS', '.fli': 'FLI', '.flc': 'FLI', '.ftc': 'FTEX', '.ftu': 'FTEX', '.gbr': 'GBR', '.gif': 'GIF', '.grib': 'GRIB', '.h5': 'HDF5', '.hdf': 'HDF5', '.png': 'PNG', '.apng': 'PNG', '.jp2': 'JPEG2000', '.j2k': 'JPEG2000', '.jpc': 'JPEG2000', '.jpf': 'JPEG2000', '.jpx': 'JPEG2000', '.j2c': 'JPEG2000', '.icns': 'ICNS', '.ico': 'ICO', '.im': 'IM', '.iim': 'IPTC', '.tif': 'TIFF', '.tiff': 'TIFF', '.jfif': 'JPEG', '.jpe': 'JPEG', '.jpg': 'JPEG', '.jpeg': 'JPEG', '.mpg': 'MPEG', '.mpeg': 'MPEG', '.mpo': 'MPO', '.msp': 'MSP', '.palm': 'PALM', '.pcd': 'PCD', '.pdf': 'PDF', '.pxr': 'PIXAR', '.pbm': 'PPM', '.pgm': 'PPM', '.ppm': 'PPM', '.pnm': 'PPM', '.psd': 'PSD', '.bw': 'SGI', '.rgb': 'SGI', '.rgba': 'SGI', '.sgi': 'SGI', '.ras': 'SUN', '.tga': 'TGA', '.icb': 'TGA', '.vda': 'TGA', '.vst': 'TGA', '.webp': 'WEBP', '.wmf': 'WMF', '.emf': 'WMF', '.xbm': 'XBM', '.xpm': 'XPM'}

However, if I try to import PIL.PngImagePlugin, it will override the entire list and leave with ONLY PNG. I do not think this should be the behaviour because no where in the doc says just importing it will remove existing registered extensions.

from PIL import Image
import PIL.PngImagePlugin
print(Image.registered_extensions())

The code above outputs:
{'.png': 'PNG', '.apng': 'PNG'}

What's even more interesting is, if I call Image.registered_extensions() before I import PIL.PngImagePlugin, it works as expected, the existing list is untouched. This should be the behaviour even if I do not call Image.registered_extensions() before importing.

from PIL import Image
print(Image.registered_extensions())
import PIL.PngImagePlugin
print(Image.registered_extensions())

The code above outputs:
{'.blp': 'BLP', '.bmp': 'BMP', '.dib': 'DIB', '.bufr': 'BUFR', '.cur': 'CUR', '.pcx': 'PCX', '.dcx': 'DCX', '.dds': 'DDS', '.ps': 'EPS', '.eps': 'EPS', '.fit': 'FITS', '.fits': 'FITS', '.fli': 'FLI', '.flc': 'FLI', '.ftc': 'FTEX', '.ftu': 'FTEX', '.gbr': 'GBR', '.gif': 'GIF', '.grib': 'GRIB', '.h5': 'HDF5', '.hdf': 'HDF5', '.png': 'PNG', '.apng': 'PNG', '.jp2': 'JPEG2000', '.j2k': 'JPEG2000', '.jpc': 'JPEG2000', '.jpf': 'JPEG2000', '.jpx': 'JPEG2000', '.j2c': 'JPEG2000', '.icns': 'ICNS', '.ico': 'ICO', '.im': 'IM', '.iim': 'IPTC', '.tif': 'TIFF', '.tiff': 'TIFF', '.jfif': 'JPEG', '.jpe': 'JPEG', '.jpg': 'JPEG', '.jpeg': 'JPEG', '.mpg': 'MPEG', '.mpeg': 'MPEG', '.mpo': 'MPO', '.msp': 'MSP', '.palm': 'PALM', '.pcd': 'PCD', '.pdf': 'PDF', '.pxr': 'PIXAR', '.pbm': 'PPM', '.pgm': 'PPM', '.ppm': 'PPM', '.pnm': 'PPM', '.psd': 'PSD', '.bw': 'SGI', '.rgb': 'SGI', '.rgba': 'SGI', '.sgi': 'SGI', '.ras': 'SUN', '.tga': 'TGA', '.icb': 'TGA', '.vda': 'TGA', '.vst': 'TGA', '.webp': 'WEBP', '.wmf': 'WMF', '.emf': 'WMF', '.xbm': 'XBM', '.xpm': 'XPM'} {'.blp': 'BLP', '.bmp': 'BMP', '.dib': 'DIB', '.bufr': 'BUFR', '.cur': 'CUR', '.pcx': 'PCX', '.dcx': 'DCX', '.dds': 'DDS', '.ps': 'EPS', '.eps': 'EPS', '.fit': 'FITS', '.fits': 'FITS', '.fli': 'FLI', '.flc': 'FLI', '.ftc': 'FTEX', '.ftu': 'FTEX', '.gbr': 'GBR', '.gif': 'GIF', '.grib': 'GRIB', '.h5': 'HDF5', '.hdf': 'HDF5', '.png': 'PNG', '.apng': 'PNG', '.jp2': 'JPEG2000', '.j2k': 'JPEG2000', '.jpc': 'JPEG2000', '.jpf': 'JPEG2000', '.jpx': 'JPEG2000', '.j2c': 'JPEG2000', '.icns': 'ICNS', '.ico': 'ICO', '.im': 'IM', '.iim': 'IPTC', '.tif': 'TIFF', '.tiff': 'TIFF', '.jfif': 'JPEG', '.jpe': 'JPEG', '.jpg': 'JPEG', '.jpeg': 'JPEG', '.mpg': 'MPEG', '.mpeg': 'MPEG', '.mpo': 'MPO', '.msp': 'MSP', '.palm': 'PALM', '.pcd': 'PCD', '.pdf': 'PDF', '.pxr': 'PIXAR', '.pbm': 'PPM', '.pgm': 'PPM', '.ppm': 'PPM', '.pnm': 'PPM', '.psd': 'PSD', '.bw': 'SGI', '.rgb': 'SGI', '.rgba': 'SGI', '.sgi': 'SGI', '.ras': 'SUN', '.tga': 'TGA', '.icb': 'TGA', '.vda': 'TGA', '.vst': 'TGA', '.webp': 'WEBP', '.wmf': 'WMF', '.emf': 'WMF', '.xbm': 'XBM', '.xpm': 'XPM'}

I've done some simple troubleshooting and found that

Pillow/src/PIL/PngImagePlugin.py

Line 1453 in d594f4c

Image.register_extensions(PngImageFile.format, [".png", ".apng"])

causes it. If I print(Image.registered_extensions()) before that line, everything is good, but after that line, it overrides.

This directly causes gradio-app/gradio#2843 AUTOMATIC1111/stable-diffusion-webui#5603 AUTOMATIC1111/stable-diffusion-webui#5664 and I believe many other downstream uses.

[jokkebk]

Can verify that this happens also on my Win10 machine on Python 3.10 as well as Ubuntu on Python 3.8 (running in WSL2).

[Yay295]

So when you get the registered extensions, if nothing has been registered, it first initializes everything. But if something has been registered (like when you import the PngImagePlugin), then it just returns that instead of initializing everything.

Pillow/src/PIL/Image.py

Lines 3416 to 3423 in 7f6fe3c

	def registered_extensions():
	"""
	Returns a dictionary containing all file extensions belonging
	to registered plugins
	"""
	if not EXTENSION:
	init()
	return EXTENSION

[Yay295]

So

from PIL import Image
Image.init()

will load everything.

[radarhere]

I've created PR #6811 to resolve this.

[aliencaocao]

Thanks for the explanation and the PR.