python cv2 video resolution

import cv2
cap = cv2.VideoCapture(0)
while(cap.isOpened()):
    cv2.waitKey(10)

    ret, frame = cap.read()
    cap.set(3, 800)
    cap.set(4, 600)

    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2BGRA) 
    print cap.get(3) # return default 1280       

    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

This your code works with webcame, not with file

for a video file, you can resize the window

cv2.resizeWindow(winname, width, height) 

for that first define window with name and resize it

example

  cv2.namedWindow("frame", 0);
  cv2.resizeWindow("frame", 800,600);

for Detail resize window

I think there are a few things in your code that might need attention.

1. As described in the OpenCV documentation for VideoCapture, if you want to access your default WebCam, you'd need to initialise the class as follows:

   cap = cv2.VideoCapture('file')
   

   If you are trying to then change the resolution of the camera, I'd suggest to move the two set lines right below the initialisation of cap and only perform it once - not each time you read in the frame. You can also use constants to access the right attributes:

   cap = cv2.VideoCapture('file')
   cap.set(cv2.CAP_PROP_FRAME_WIDTH, width)
   cap.set(cv2.CAP_PROP_FRAME_HEIGHT, height)
   
   # Your while loop and the rest of the code...
   

2. If you are trying to read the frame from a file and want to change it's resolution, you'd probably want to use the resize method as described here. This would need to be done inside the loop, right after you read in the frame. It could be something like:

   resize(ret, ret, Size(800, 600), 0, 0, INTER_CUBIC); 
   

I hope this helps.