from PIL import Image
import piexif


fn = 'original.jpg'
im = Image.open(fn)
exif_dict = piexif.load(im.info['exif'])
tags = "tag1;tag2;tag3;"
ascii_tags = ()
for letter in tags:
    ascii_tags += (ord(letter), 0)
exif_dict['0th'][piexif.ImageIFD.ImageDescription] = b"Lorem ipsum dolor"
exif_dict['0th'][piexif.ImageIFD.XPKeywords] = ascii_tags
exif_bytes = piexif.dump(exif_dict)
im.save('new.jpeg', "jpeg", exif=exif_bytes)