Error when reading videos with OPenCV

[Theodore Papadopoulo]

On Fedora 29 (x86-64), I have trouble reading videos with OPenCV and
python3.

The script is as simple as:

import cv2
import sys

video = cv2.VideoCapture(sys.argv[1])

And the error message is:

-> python3 test.py 170621_ch1_ROI2_20h38pt.mp4
Failed to query video capabilities: Inappropriate ioctl for device
libv4l2: error getting capabilities: Inappropriate ioctl for device
VIDEOIO ERROR: V4L: device 170621_ch1_ROI2_20h38pt.mp4: Unable to query
number of channels
warning: GStreamer: unable to query duration of stream
(/builddir/build/BUILD/opencv-3.4.1/modules/videoio/src/cap_gstreamer.cpp:904)
warning: Cannot query video position: status=1 value=0 duration=-1
 (/builddir/build/BUILD/opencv-3.4.1/modules/videoio/src/cap_gstreamer.cpp:952)

I have tried with other videos formats (includng nn-patent encumbered
ones such as webp or mpeg) and got the same error.
This seems remnant of some old bugs that are marked as solved in the
fedora 25/26 timeframe. Did they reappear ? Certainly, the problem was
not here two ot three years ago...

From the opencv website, it seems that the same problem is said to be
solved when building with FFMPEG which seems to be uncompatible with
free software... But since it was working a while ago, would it be
possible to restore this functionality ?? Or at the very least to get
some understandable error message ?

    Thank's a lot,

	Theo