Render a noise pattern onto an image in Blender

noise.py

import bpy
import numpy as np
from mathutils import noise

img = bpy.data.images['some-img']
w, h = img.size

# 4 channel image (RGBA), f64
a = np.array(img.pixels).reshape((h, w, 4))

for j in range(h):
    for i in range(w):
        v = noise.fractal([float(i)/w, float(j)/h, 0.5], 1.0, 2.0, 8)
        a[j, i, :3] = v

a[:,:,3] = 1 # alpha

img.pixels = a.reshape((-1,))