I have had a comparable issue on Ubuntu, using the pip install opencv-python process. Solution used : I recompiled both ffmpeg and opencv 3.2 from sources on my station (following the install processes indicated on offical portals). Everything worked well, but I still was not able to open video from cv2.videocapture on python.

My solution has been to replace the cv2.so library provided by pypi with the one compiled from sources : sudo cp opencv-3.2.0/build/lib/cv2.so /usr/local/lib/python2.7/dist-packages/cv2/

And I now can play videos with cv2, from my python scripts. Hope this will help those experiencing the same issue (I spent some time on trying to find a solution and never found this suggestion so …).

This is still looks like an usage question. OpenCV requires 3rdparty libraries to work with cameras/videos. You need to install them properly before building OpenCV Python bindings.

opencv-python doesn’t support them (almost all 3rdparty dependencies are turned off, because they are system dependent - system with minimal configuration is used during build).

For more information please refer to this question and others:

Q: Why I can’t open video files on GNU/Linux distribution X or on macOS?

(https://pypi.python.org/pypi/opencv-python)

opencv-python binary package is not officially supported by OpenCV dev team, so we can’t help here.

Usage questions should go to Users OpenCV Q/A forum: http://answers.opencv.org

For future reference:

If you have installed either one of the precompiled distributions opencv-python or opencv-contrib-python from pip there are a few things to note if you are using Linux or macOS (Windows builds have had video support always):

• versions < 3.3.1.11 are not compiled with FFmpeg (macOS, Linux) and V4L (webcam support, Linux)
• versions >= 3.3.1.11 have been compiled with FFmpeg and V4L

Changelogs are available for every release at the opencv-python package repository https://github.com/skvark/opencv-python/releases.

In my case i simply un-installed and installed back opencv-python Before re-installation i had opencv-python-3.3.0.10. after re-install i got a newer version opencv-python-3.4.0.12

• pip3 uninstall opencv_python
• pip3 install opencv_python --user

I did a similar thing to @iiAnthony where I compiled OpenCV 3.2.0 from source, double checking with CMake that it can find both python 2 and python 3

then I replaced the compiled .so dynamic library with the one that comes with opencv-python python3 package. The difference between my comment and @iiAnthony’s is that his caters for Python2 users?

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

Or better yet, just the ln command to symlink the built .so instead of copying, so that if you decide to compile OpenCV3 with different compilation flags you don’t have to copy the library file over again.

Note: Your .so library file may have a different name to mine, but the idea is the same, compile your own opencv3 .so library file and replace the one that opencv-python comes with.

Can’t open videos or webcam:

System information (version) opencv-python Version: 3.2.0.7 python Version: 3.6.1 Operating System / Platform: 4.10.11-1-ARCH Linux

Detailed description VideoCapture doesn’t work with files or with /dev/video

Steps to reproduce

# Fails with files, direct instantiation
import cv2
camera = cv2.VideoCapture("path/to/a/valid/video.mp4")
print(camera.isOpened()) # False
print(camera.read()) # (False, None)

# Fails with files, using .open() method
import cv2
camera = cv2.VideoCapture()
camera.open("path/to/a/valid/video.mp4") # False
print(camera.read()) # (False, None)

# Fails with webcam
import cv2
camera = cv2.VideoCapture(0)
print(camera.isOpened()) # False
print(camera.read()) # (False, None)

I had this problem and couldn’t find a solution, but on the web, it was working. My solution was almost funny - you can’t have two streams. I turned off the web stream and then it was fine!

I know people don’t appreciate you @FilipBielickiPL, but that is certainly my problem, so thx! Basically, I have a configurable program (video vs webcam) and despite the WebcamVideoStream is not used during video mode, it’ll disturb the way VideoCapture working.

I recently solved issue for my use case. So my old (BAD configs) were:

• OpenCV 2.4.5 (after yum install opencv)
• opencv_python 3.3.0.10 (after pip3 install opencv_python)

And in fact I have Assertion error, as it’s described here: https://github.com/ageitgey/face_recognition/issues/191#issuecomment-350223266

Then, I compiled GOOD config - OpenCV from sources (git cloned and checked out 3.3.1 branch) and I had 3.3.1 version. But even with opencv 3.3.0.10 IT WAS NOT working, because

/usr/lib64/python3.4/site-packages/cv2/cv2.cpython-34m.so is 3.3.0

yes, opencv_python installs into ... lib64 ...

So, then there are 2 ways of resolving issue:

1. replace of so file

sudo cp ~/opencv/build/lib/python3/cv2.cpython-34m.so /usr/lib64/python3.4/site-packages/cv2 
# meaning 3.3.1 version replaces 3.3.0 version of so file

1. Remove opencv_python and don’t use it later

sudo pip3 uninstall opencv_python

And later on import cv2 will import from /usr/lib/python3.4/site-packages/ (which is 3.3.1)

I did both, and my application was working in both cases. But still, I am not confident what is the best way.

PS.

So far I have 2 web cameras: built in and external via USB, and my python code can’t access external. It always open built in. If someone know how to do it, please let me know. Here is piece of code:

print(cv2.__file__)
print(cv2.__version__)
# pprint(vars(cv2))

# https://docs.opencv.org/3.3.0/d8/dfe/classcv_1_1VideoCapture.html#a5d5f5dacb77bbebdcbfb341e3d4355c1
# to open default camera using default backend just pass 0. 
self.video = cv2.VideoCapture(0) # connects to built in camera in 640x480
# self.video = cv2.VideoCapture(1) # connects to built in camera, but in 1280x720

# self.video = cv2.VideoCapture(2) # should connect to 3nd installed camera - BUT NOT
# https://docs.opencv.org/3.3.0/d4/d15/group__videoio__flags__base.html#gga023786be1ee68a9105bf2e48c700294dacf10e9692c4166f74de62b7d00c377d0
# self.video = cv2.VideoCapture(cv2.CAP_FFMPEG) # connects to built in camera in 640x480

# TODO - HOW TO connect/switch to another web cam?

That could be a number of things:

1. CMake failed to find your python version, double check it is listed in the cmake .. stage (like in the screenshot I took in the previous post).
2. You don’t have the python development library (on debian based systems this corresponds to python-dev or python3-dev packages)
3. Try and checkout a specific version of OpenCV instead of building master (i.e. git checkout 3.3.1)
4. Maybe do a find . -name "cv2.*.so" to find if it is hidden somewhere? Though I am not convinced that building on CentOS would mean the .so file is at a different location during build.

I just downloaded and built the zipped OpenCV 3.3.1 in Ubuntu 16.04 and have found the cv2.so in the build/lib/python3 folder so you’ll have to double check your environment.

Same issue with Python 3.6.0 under MacOS 10.12 Sierra.

I did a similar thing to @iiAnthony where I compiled OpenCV 3.2.0 from source, double checking with CMake that it can find both python 2 and python 3

then I replaced the compiled .so dynamic library with the one that comes with opencv-python python3 package. The difference between my comment and @iiAnthony’s is that his caters for Python2 users?

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

Or better yet, just the ln command to symlink the built .so instead of copying, so that if you decide to compile OpenCV3 with different compilation flags you don’t have to copy the library file over again.

Note: Your .so library file may have a different name to mine, but the idea is the same, compile your own opencv3 .so library file and replace the one that opencv-python comes with.

Great answer! It works for me

@chutsu Why the thumbs down? I solved my error exactly like what @FilipBielickiPL did. Is that not clear enough?

it would help if you can share a minimal reproducible example so that people can try out what the error is. Without example code we can’t help you.

BTW,

which python3 => /usr/local/bin/python3

You have custom python setup (without any information how you get it).

ls -la /usr/local/lib/python3.6/site-packages/cv2/

This file set is not generated by original OpenCV. You have installed something else from other sources.