How to detect non-transparent shape corner of a png image with Python?

I would like to determine the pixels of corner of non-transparent shape of a png image with Python?
Here is the image . For better understanding the transparent area marked red color as:
Here is my code:

from PIL import Image

img = Image.open('image.png')
columnsize,rowsize=img.size
img = img.convert("RGBA")


for i in range(rowsize):
    for j in range(columnsize):
        X=img.getpixel((j,i))
        print X