[SOLVED] How do I get VideoCapture to return errors instead of warnings?

How do I get VideoCapture to return errors instead of warnings?

When trying to implement a basic try/except statement to get frames from a camera on Linux I noticed that OpenCV does not raise an error when the camera is missing or the device index is wrong; instead it prints a warning. How could I catch this?

Example:

import cv2

def foo(camera_index):
    try:
        cap = cv2.VideoCapture(camera_index)
    except Exception:
        print("Couldn't open camera at {}".format(camera_index))

Running foo(1) with a working camera at /dev/video/0 will print:

WARN:0] global /tmp/pip-req-build-kne9u3r2/opencv/modules/videoio/src/cap_v4l.cpp (893) open VIDEOIO(V4L2:/dev/video1): can't open camera by index

My except is then useless because cv2 will never raise a proper exception (yet the message text sounds like an error).

Solution

You can't catch those warnings, but you can check whether the VideoCapture object has been successfully created using isOpened() method.

As an example:

cap = cv2.VideoCapture(camera_index)
if not cap.isOpened():
  raise Exception("Couldn't open camera {}".format(camera_index))

Performing this check after creating a VideoCapture object is a common pattern; please see here for another example.