Eigen to Numpy with Swig

Introduction

This package exists as a demonstration of some of the tools I've been playing with to wrap C++ libraries which use Eigen Matrix types in Python (using numpy arrays). Specifically, it demonstrates:

• Wrapping a C++ class in a Python class
• Passing numpy arrays to and from C++ methods which accept and return Eigen Matrix types
• Adding additional Python-defined methods to a C++ class
• Calling templated C++ functions from Python.
• Calling C++ methods which take Eigen::Ref arguments from Python
• Support for Python 2.7 and Python 3

To show this off, I've written a C++ function which uses Eigen's LU module to invert matrices. This is a trivially simple example, but hopefully it will make someone's life a little easier down the road.

Requirements

You'll need, at the very least:

• cmake
• swig
• a c/c++ compiler for your system

On OSX, you can follow the instructions from Homebrew to install a compiler and use Homebrew to install the cmake swig packages. On Ubuntu, you can sudo apt-get install cmake swig.

If you don't have a copy of eigen3 installed, one will be automatically downloaded and built for you. If you don't want that, just install the eigen3 package yourself. In Homebrew it's called eigen and in apt-get it's called libeigen3-dev.

Building

This project is entirely configured using CMake, so building should be pretty easy. Just:

mkdir build
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=../install
make
make install

Nothing should get installed into your system; instead, the inverter library will get put in install/lib/. If you want to install globally, then you can just remove the -DCMAKE_INSTALL_PREFIX=../install argument.

Using the python bindings

You'll need to make sure that the inverter shared library is accessible to your system at runtime. If you installed globally (without the -DCMAKE_INSTALL_PREFIX=../install argument), then you're all set. Otherweise, on OSX, you can do:

export DYLD_LIBRARY_PATH="$DYLD_LIBRARY_PATH:/path/to/wherever/you/put/swig-eigen-numpy/install/lib"

Or on Linux:

export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/path/to/wherever/you/put/swig-eigen-numpy/install/lib"

That will only affect the current instance of your terminal. To make that change permanent, put that line inside your ~/.bashrc file.

The python bindings are demonstrated in python/test_inverter.py. To run it, just do:

cd python
python test_inverter.py

Structure of this repo

src

contains our c++ library, header, and a test executable

python

contains the main swig interface file and our python source code

swigmake/swig

contains general-purpose swig interface files for eigen and numpy

swigmake/cmake

contains some helper scripts for cmake to find the eigen and numpy libraries

Python 3 support

This project should build and run correctly on Python 2 or Python 3. To change python versions, you can add another argument to cmake:

mkdir build
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=../install -DPYTHON_EXECUTABLE=`which python3`
make
make install
cd ../python
python3 test_inverter.py

In the above code I've told cmake to build with python3 instead of my default python2.