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'

Ahmet · Answer 1 · 2020-09-04T10:47:51.537

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()

This doesn't work for remote RTSP urls by the way. E.g if you do ```cv2.VideoCapture('some_remote_rtsp_url')``` then it won't catch any thrown errors. — jaimish11, Jul 10 '21 at 11:33

score 1 · Answer 2 · answered Sep 04 '20 at 03:53

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

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

2 Answers2