generalize webcam detection with opt.source.isnumeric() (yolov3 #1462)

utils/datasets.py

@@ -235,7 +235,7 @@ class LoadStreams:  # multiple IP or RTSP cameras
         for i, s in enumerate(sources):
             # Start the thread to read frames from the video stream
             print('%g/%g: %s... ' % (i + 1, n, s), end='')
-            cap = cv2.VideoCapture(0 if s == '0' else s)
+            cap = cv2.VideoCapture(eval(s) if s.isnumeric() else s)
             assert cap.isOpened(), 'Failed to open %s' % s
             w = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
             h = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))