How to correctly check if a camera is available?

Another solution, which is available in Linux is to use the /dev/videoX device in the VideoCapture() call. The devices are there when the cam is plugged in. Together with glob(), it is trivial to get all the cameras:

import cv2, glob

for camera in glob.glob("/dev/video?"):
    c = cv2.VideoCapture(camera)

Of course a check is needed on c using isOpened(), but you are sure you only scan the available cameras.

Using cv2.VideoCapture( invalid device number ) does not throw exceptions. It constructs a <VideoCapture object> containing an invalid device - if you use it you get exceptions.

Test the constructed object for None and not isOpened() to weed out invalid ones.

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For me this works (1 laptop camera device):

import cv2 as cv 

def testDevice(source):
   cap = cv.VideoCapture(source) 
   if cap is None or not cap.isOpened():
       print('Warning: unable to open video source: ', source)

testDevice(0) # no printout
testDevice(1) # prints message

Output with 1:

Warning: unable to open video source:  1

Example from: https://github.com/opencv/opencv_contrib/blob/master/samples/python2/video.py lines 159ff

cap = cv.VideoCapture(source)
    if 'size' in params:
        w, h = map(int, params['size'].split('x'))
        cap.set(cv.CAP_PROP_FRAME_WIDTH, w)
        cap.set(cv.CAP_PROP_FRAME_HEIGHT, h)
if cap is None or not cap.isOpened():
    print 'Warning: unable to open video source: ', source