# Read a TIFF whose 4th channel is untagged alpha (ExtraSamples = 0) as RGBA.
from PIL import Image
def open_rgba(path):
    im = Image.open(path)
    if im.tag_v2.get(277) == 4 and im.mode == 'RGB':
        im.tile = [(t[0], t[1], t[2], ('RGBA',) + tuple(t[3][1:]) if isinstance(t[3], tuple) else 'RGBA') for t in im.tile]
        im._mode = 'RGBA'
    return im.convert('RGBA')
if __name__ == '__main__':
    import sys
    im = open_rgba(sys.argv[1]); print(im.mode, im.getextrema()); im.save(sys.argv[2])