Try-Catch Exception won't work in OpenCV-python

Question

So i purposely put a file in the cv2.VideoCapture() that doesn't exist in my computer to test the try-catch system i've put into my code but for some reason it still gives off the same error it gives when i didn't put a try-catch into my code. how do i fix this??

code:

import numpy as np
import cv2



try:
    cap = cv2.VideoCapture('trial.mkv')
    while (cap.isOpened()):
        ret, frame = cap.read()

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

    cap.release()
    cv2.destroyAllWindows()
except:
    print("no file found")

error:

[ERROR:0] global C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-cff9bdsm\opencv\modules\videoio\src\cap.cpp (142) cv::VideoCapture::open VIDEOIO(CV_IMAGES): raised OpenCV exception:

OpenCV(4.4.0) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-cff9bdsm\opencv\modules\videoio\src\cap_images.cpp:253: error: (-5:Bad argument) CAP_IMAGES: can't find starting number (in the name of file): trial.mkv in function 'cv::icvExtractPattern'

Answer 1

When an error is caught by the try/except statement and stderr msg is still printed it just means that the library is simply sending all the error messages to the stderr .

OpenCV library was known to have this problems and there are ways around it. You should read the detailed answers here - OpenCv error and How to stop OpenCV error message from printing in Python

Answer 2

I definitely agree @AzyCrw4282 but I also point-out two issues on your code, therefore I'm writing as an answer.

• Issue #1: Use except cv2.error as error to catch the error:

---

•  except cv2.error as error: print("[Error]: {}".format(error))

  Result:

•  [Error]: OpenCV(4.0.0) /Users/opencv/modules/highgui/src/window.cpp:350: error: (-215:Assertion failed) size.width>0 && size.height>0 in function 'imshow' Process finished with exit code 0

• Even if I give an invalid video file:

   OpenCV: Couldn't read video stream from file "trial.mkv" Process finished with exit code 0

• Actually try-catch block is working if you use except cv2.error as error .

• Issue #2: Please always check frame

---

For instance: If you want to stop the application use:

•  if frame is None break

  or

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

  if frame returns then display

Full code:

import cv2

try:
    cap = cv2.VideoCapture('dread.mp4')
    while cap.isOpened():
        ret, frame = cap.read()
        if ret:
            cv2.imshow('frame', frame)
            if cv2.waitKey(1) & 0xFF == ord('q'):
                break
except cv2.error as error:
    print("[Error]: {}".format(error))

cap.release()
cv2.destroyAllWindows()