Q: - I can't open the webcam or video file.
A: Try re-install opencv-python with version >= 3.3.1.11 by
pip3 uninstall opencv_python
pip3 install opencv_python --user
Many people meet this problem at opencv/opencv#8471. The solution I use is
sudo cp <path to opencv source repo>/build/lib/python3/cv2.cpython-35m-x86_64-linux-gnu.so /usr/local/lib/python3.5/dist-packages/cv2/cv2.cpython-35m-x86_64-linux-gnu.so
The idea is to replace the cv2.so library provided by pypi with the one compiled from sources. You can check for more info at opencv/opencv#8471.
Q: - I can't open the webcam with the latest opencv-python
A: Check if your device is valid by
ls /dev/video*
Usually you can find video0, but if you have a device with other index like video3, you can run the program by
python3 webcam_demo.py --webcam 3 --outdir examples/res --vis