Support python 3

[quicklizard99]

We are tied to Python 2.7.x because one of our dependencies - OpenCV - provides only Python 2.x bindings. Although Python 2.7 will be supported until 2020, there will be only bugfix releases - no new development.

Python 3 is the future. OpenCV 3 (currently in alpha) will provide Python 3 bindings.

[quicklizard99]

More information. Suggest we aim to support both python 2.7.x and 3.x, at least in the short term, once OpenCV 3 has been released. This work requires that we have thorough tests with excellent code coverage (#84).

[quicklizard99]

Python 3 has a different API for C extension modules. Support for Python 3 requires that we (or someone else) rewrites the pydmtx module, which provides an interface to the libdmtx Data Matrix barcode reading library. Also the zbar module, if we decide to support the zbar barcode reading library.

[stefanv]

Both of these projects are quite old, so I guess we shouldn't hold out too much hope that they'll port to 3. Since `libdtmx` is written in C, we should be able to construct a Cython wrapper for the functionality used in inselect fairly easily. If you think that would be useful, I'd be happy to put some time into it.

[quicklizard99]

Thanks @stefanv - that would be great. Probably best to wait until I have tried Inselect with Python 3 - I think it likely that there will be other sticking points - cx_freeze, PyInstaller, PySide - to sort out before we get to barcodes. Ideally, barcode reading would be part of OpenCV because neither libdmtx nor zbar haven't been developed for years.

[stefanv]

Alright, let me know when you are done with that phase, then I will dive into this in more detail!

[quicklizard99]

A first pass at python 3 support on iss95.

[quicklizard99]

How I built OpenCV 3.0.0 RC1 on Ubuntu 14.04 (with help from 1 and 2). Download opencv-3.0.0-rc1.zip to ~/build/. The following will make and install to ~/local/opencv-3.0.0-rc1. The cv2 extension python will be installed to the virtual env's site-packages: ~/Envs/dig343/lib/python3.4/site-packages for me.

cd ~/build
unzip opencv-3.0.0-rc1.zip
cd opencv-3.0.0-rc1
mkdir build
cd build
workon dig343    # the name of my virtual env
rm -rf * && cmake -D CMAKE_BUILD_TYPE=RELEASE \
        -D PYTHON3_EXECUTABLE=$(which python3) \
        -D PYTHON3_PACKAGES_PATH=`python3 -c 'from distutils.sysconfig import get_python_lib; print(get_python_lib())'` \
        -D PYTHON3_INCLUDE_DIR=`python3 -c 'from distutils.sysconfig import get_python_inc; print(get_python_inc())'` \
        -D PYTHON3_LIBRARY=`python3 -c 'from distutils import sysconfig; print("{LIBDIR}/{LDLIBRARY}".format(**sysconfig.get_config_vars()))'` \
        -D CMAKE_INSTALL_PREFIX=~/local/opencv-3.0.0-rc1 \
        -D BUILD_EXAMPLES=ON \
        -D INSTALL_PYTHON_EXAMPLES=ON \
        -D WITH_QT=ON \
        ~/build/opencv-3.0.0-rc1/

    make -j4 # 4 cores - use the appropriate number here
    make install

Test

python -c "import cv2; print(cv2.__version__)"

[stefanv]

Another few potential options for wrapping:

https://github.com/dlbeer/quirc
https://github.com/glassechidna/zxing-cpp

What kind of barcodes do we need to support?

[quicklizard99]

#372 and #374 add Python 3 compatible wrappers for libdmtx and zbar respectively.

[quicklizard99]

Things blocking Python 3

• PySide - not updated for years, stuck on Python 2 (and Qt 4) - fixed by PyQt4 #389
• zbar - not updated for years, stuck on Python 2 - fixed by Support pyzbar #374
• libdmtx - not updated for years, stuck on Python 2 - fixed by Support pydmtxlib #372
• OpenCV 2.4 - stuck on Python 2 - fixed by OpenCV3 #391

PyInstaller and cx_Freeze (used to freeze Inselect for Mac OS X and Windows respectively) both claim to support Python 3.

[quicklizard99]

Pull #403: all tests pass on Mac OS X, Windows and on Ubuntu 12.04, 14.04 and 16.04.

Neither the Mac OS X nor the Windows installers build properly - the next job.