from PIL import Image
from pathlib import Path
import json
r=[]
for dpi in (72,300):
 im=Image.open(Path(__file__).parent/f'density-{dpi}.png');v=im.info['dpi'][0];r.append({'dpi_read':v,'pixels':im.size,'width_mm':im.width/v*25.4})
assert r[0]['pixels']==r[1]['pixels']==(600,300)
print(json.dumps(r,indent=2))