capturing video stream using piCam

Question

i'm trying to capture the video stream using piCam. but i get this warning and the execution stops

this the code:

this the code I used:

import numpy as np
import cv2
cap = cv2.VideoCapture(0)
cap.set(3,640) 
cap.set(4,480) 
while(True):
    ret, frame = cap.read()
    cv2.imshow('frame', frame)
    k = cv2.waitKey(30) & 0xff
    if k == 27: # press 'ESC' to quit
        break
cap.release()
cv2.destroyAllWindows()