[Python] OpenCV ret=False, is it reading the video, or not?

I want to detect edges of a video file (640x640 9sn.) and save the result. I followed OpenCV documents and some other examples. Most of the examples that I found was reading from camera.

Here is my code. I checked cap.isOpened(), it returns True but ret does False and frame is NoneType object. What is confusing is that I'm having the gray array which depends the condition if ret == True. How can I get gray matrix if ret = False?

(I installed ffmpeg pip install ffmpeg-python)

(andy.avi was saved in folder but it's broke, empty)

import cv2
import numpy as np


cap = cv2.VideoCapture("...\\video.mp4")

while(cap.isOpened()):
ret, frame = cap.read()

frame_width = int(cap.get(3))
frame_height = int(cap.get(4))

size = (frame_width, frame_height)

result = cv2.VideoWriter('andy.avi',
cv2.VideoWriter_fourcc(*'DIVX'),
30, size)

if ret == True:
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
edges = cv2.Canny(gray, 45, 90)
result.write(edges)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
else:
break

cap.release()
result.release()
cv2.destroyAllWindows()

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