OpenCV GTK+2.x error

First check whether libgtk2.0-dev is installed properly. If you have installed aptitude package manager, run the following:

sudo aptitude search libgtk2.0-dev

It should return like this:

i  libgtk2.0-dev              - development files for the GTK+ library 
p  libgtk2.0-dev:i386         - development files for the GTK+ library

You need to build the files once again.Locate your OpenCV folder. Create a new folder and name it as release. Enter into this folder. For example

cd /home/user_name/OpenCv
mkdir Release
cd Release

Now build using cmake with following command:

cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -D WITH_TBB=ON -D BUILD_NEW_PYTHON_SUPPORT=ON -D WITH_V4L=ON -D INSTALL_C_EXAMPLES=ON -D INSTALL_PYTHON_EXAMPLES=ON -D BUILD_EXAMPLES=ON -D WITH_QT=ON -D WITH_GTK=ON -D WITH_OPENGL=ON ..

Remember to put WITH_GTK=ON during cmake. After this step enter the command,

make
sudo make install

This should resolve your problem.If you have broken dependencies for libgtk2.0-dev, then install a fresh copy of libgtk2.0-dev using aptitude.

sudo aptitude install libgtk2.0-dev

If you installed OpenCV using the opencv-python pip package, be aware of the following note, taken from https://pypi.python.org/pypi/opencv-python

IMPORTANT NOTE MacOS and Linux wheels have currently some limitations:

• video related functionality is not supported (not compiled with FFmpeg)
• for example cv2.imshow() will not work (not compiled with GTK+ 2.x or Carbon support)

Also note that to install from another source, first you must remove the opencv-python package

To install OpenCV in Ubuntu I followed this guide, and it worked perfectly fine: http://www.pyimagesearch.com/2016/10/24/ubuntu-16-04-how-to-install-opencv/

In order to improve @Nic Szer's answer I want to explain how to fix this error on Mac OS in three simple steps.

1. Remove installed OpenCV version to avoid mess up later

   pip3 uninstall opencv-python 
   

2. Lower your python version to 3.5 (current version 3.6 has problems with conda which we will use to install OpenCV)

   conda install python=3.5
   

3. Finally, use conda to install working version of OpenCV

   conda install -c menpo opencv3 
   

And then voila: OpenCV will start working on your Mac OS(Siera 10.12.4).