Save Coordinates Edges OpenCV tuples to txt

Question

Im trying to save data obtained from edge detection. I would like to save the coordenates (X,Y) from the edges in a txt file.

Im trying to created the file but i only obtain this Data -> [255 255 255 ... 255 255 255].

Edges

import cv2
import numpy as np
# read image
img = cv2.imread("SPIF.png")

# convert to gray
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# threshold
#thresh = cv2.threshold(gray, 180, 255, cv2.THRESH_BINARY)[1]
thresh = cv2.threshold(gray, 210, 255, cv2.THRESH_BINARY)[1]

# morphology edgeout = dilated_mask - mask
# morphology dilate
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (5,5))
dilate = cv2.morphologyEx(thresh, cv2.MORPH_DILATE, kernel)

# get absolute difference between dilate and thresh
diff = cv2.absdiff(dilate, thresh)

# invert
edges = 255 - diff

indices = np.where(diff != [0])
coordinates = zip(indices[0], indices[1])
print(coordinates)

# write result to disk
cv2.imwrite("cartoon_diff.jpg", diff)
cv2.imwrite("cartoon_edges.jpg", edges)

# display it
cv2.imshow("diff", diff)
cv2.imshow("edges", edges)
cv2.waitKey(0)