import cv2

cv2.namedWindow("preview")
vc=cv2.VideoCapture(0)

if vc.isOpened(): # try to get the firstframe
	rval, frame=vc.read()
else:
	rval=False

while rval:
	cv2.imshow("preview", frame)
	rval,frame=vc.read()
	key=cv2.waitKey(20)
	if key==27: # exit on ESC
		break
cv2.destroyWindow("preview")