How to build OpenCV 3.x with Python 3.x bindings on Ubuntu 20.04 LTS

build-opencv3-with-python-on-ubuntu20lts.sh

# Download and unpack your chosen OpenCV version., eg. as `/usr/local/src/opencv-3.4.1`
# You can use https://github.com/pisecurity/opencv-manager repository as well.

# This is important. On Ubuntu 20.04 LTS, it is not enough to install just python-dev.
apt-get install python-dev python-numpy python-is-python2 python2.7 python2.7-dev python2.7-minimal

# All unrelated build settings were cut off.
cd /usr/local/src/opencv-3.4.1/build
cmake \
    -DENABLE_PRECOMPILED_HEADERS=OFF \
    -DBUILD_opencv_python2=ON \
    -DBUILD_opencv_python3=ON \
    -DCMAKE_INSTALL_PREFIX=/usr/local/opencv-$version \
    ..
make -k -j2

# Now wait. Everything should build properly, except for `opencv_python3.dir/__/src2/cv2.cpp.o` file.

mcedit modules/python3/CMakeFiles/opencv_python3.dir/flags.make

# In this file, you should see a veeeery long `CXX_INCLUDES` variable. Near the end, you should see:

`-isystem /usr/include/python3.8`

# change it to:

`-isystem /usr/include/python2.7`

# and run "make" again. Now Python 3.x module should be built properly.