Pillow fade edges

from PIL import Image, ImageFilter, ImageDraw
import random

img = Image.open("1.jpg")

width = img.size[0]
height = img.size[1]

mask = Image.new('L', img.size, 255)
maskd = ImageDraw.Draw(mask)

border_thick = 50
sigma = 15

for i in range(0, border_thick):
    alpha = random.gauss(i * 255 / border_thick, sigma)
    maskd.line([i, i, width - i, i], alpha ) #up
    maskd.line([i, height - i, width - i, height - i], alpha) #down
    maskd.line([i, i, i, height - i], alpha) #left
    maskd.line([width - i, i, width - i, height - i], alpha) #right

mask = mask.filter(ImageFilter.GaussianBlur(3))

img.putalpha(mask)
img.save("1.png")