Check return value in python tutorial

doc/py_tutorials/py_gui/py_video_display/py_video_display.markdown

@@ -25,23 +25,27 @@ import numpy as np
 import cv2 as cv
 
 cap = cv.VideoCapture(0)
-
-while(True):
+if not cap.isOpened():
+    print("Cannot open camera")
+    exit()
+while True:
     # Capture frame-by-frame
     ret, frame = cap.read()
 
+    # if frame is read correctly ret is True
+    if not ret:
+        print("Can't receive frame (stream end?). Exiting ...")
+        break
     # Our operations on the frame come here
     gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
-
     # Display the resulting frame
-    cv.imshow('frame',gray)
-    if cv.waitKey(1) & 0xFF == ord('q'):
+    cv.imshow('frame', gray)
+    if cv.waitKey(1) == ord('q'):
         break
 
 # When everything done, release the capture
 cap.release()
-cv.destroyAllWindows()
-@endcode
+cv.destroyAllWindows()@endcode
 `cap.read()` returns a bool (`True`/`False`). If frame is read correctly, it will be `True`. So you can
 check end of the video by checking this return value.
 
@@ -75,13 +79,17 @@ import cv2 as cv
 
 cap = cv.VideoCapture('vtest.avi')
 
-while(cap.isOpened()):
+while cap.isOpened():
     ret, frame = cap.read()
 
+    # if frame is read correctly ret is True
+    if not ret:
+        print("Can't receive frame (stream end?). Exiting ...")
+        break
     gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
 
-    cv.imshow('frame',gray)
-    if cv.waitKey(1) & 0xFF == ord('q'):
+    cv.imshow('frame', gray)
+    if cv.waitKey(1) == ord('q'):
         break
 
 cap.release()
@@ -123,20 +131,20 @@ cap = cv.VideoCapture(0)
 
 # Define the codec and create VideoWriter object
 fourcc = cv.VideoWriter_fourcc(*'XVID')
-out = cv.VideoWriter('output.avi',fourcc, 20.0, (640,480))
+out = cv.VideoWriter('output.avi', fourcc, 20.0, (640,  480))
 
-while(cap.isOpened()):
+while cap.isOpened():
     ret, frame = cap.read()
-    if ret==True:
-        frame = cv.flip(frame,0)
+    if not ret:
+        print("Can't receive frame (stream end?). Exiting ...")
+        break
+    frame = cv.flip(frame, 0)
 
-        # write the flipped frame
-        out.write(frame)
+    # write the flipped frame
+    out.write(frame)
 
-        cv.imshow('frame',frame)
-        if cv.waitKey(1) & 0xFF == ord('q'):
-            break
-    else:
+    cv.imshow('frame', frame)
+    if cv.waitKey(1) == ord('q'):
         break
 
 # Release everything if job is finished