Compile lpsolve module for Python 3

Introduction

lpsolve is a library for solving Mixed Integer Linear Programming (MILP) implemented in C. It does have a Python extension but haven't been updated for a long time and only support Python 2. As the Python C API changed a lot from 2 to 3, support for Python 3 is not as simple as just recompile it for Python 3. This guide will walk through the process for compiling and installing lpsolve module for Python 3.

Steps

1. Download and extract the latest version of lpsolve and Python module source code. The current version is 5.5.2.5 and these two directories can be merged into lp_solve_5.5 (extra directory inside lp_solve_5.5).

2. Compile the library following the readme. The script for building libraries are in lp_solve_5.5/lpsolve55. You should follow the readme.txt in this directory. This should produce liblpsolve55.a and liblpsolve55.so(liblpsolve55.dll on Windows) in lp_solve_5.5/lpsolve55/bin/<your os>.

3. Copy (do not delete) these files to where installed libraries should go. On Linux/MacOS (it should be /usr/local/lib)

4. In lp_solve_5.5/extra/Python, you will need to modify the setup.py to build it for Python3. The script is a bit messy and you need to perform the following changes:

• Change print ... to print(...)
• Delete all the if ... else, everything after all import ... and above setup(...). Don't delete
• print(...) you just changed.
• Add the followings between import ... and print(...)
  • Add LPSOLVE55 = '../../lpsolve55/bin/<your os>'
  • Add WIN32 = 'NOWIN32' unless you are compiling for win32 (not tested)
  • If you want to build with Numpy, add NUMPY = 'NUMPY' otherwise NUMPY = 'NONUMPY'
  • If you want to build with Numpy, you also need to find your Numpy include directory. It should be located in site-packages directory of you Python installation directory, and the overall path looks something like this '<??>/Python/<3.x>/lib/python/site-packages/numpy/core/include'. If you go to this directory, inside numpy directory, there should be a lot of .h files. Add NUMPYPATH='<??>/Python/<3.x>/lib/python/site-packages/numpy/core/include'
• In setup(...) set include_dirs=['../..', NUMPYPATH]

5. If you are on MacOS, change #include <malloc.h> in hash.c to #include <stdlib.h>.

6. Follow the changed to pythonmod.c mentioned here except for the followings: You should change all PyString_<...> to PyBytes_<...>. It is advised to use search and replace functionality of the text editor. DO NOT change it to PyUnicode_<...> The code will compile, but you will run into issues when you use it in Python 3. This is because of the change of Python C API, if you want to learn more about the change, see this. The short answer is that String in C (lpsolve source code) is equivalent to bytes type but not str type in Python3.

7. Now you should be able to compile the library. Use python3 setup.py install to compile and install the package. (Note: if your system default python is Python 3, you can just run python3 setup.py install, it is important not to run setup.py from Python 2).

8. If you see no errors, the package should be installed. To test whether it works, opens a Python 3 shell in your terminal, type from lpsolve55 import *. It should not print anything. If it prints errors, you probably need to change some code again :(.

9. Follow the documentation to try a few examples. You need to change all string literals '...' to bytes literals b'...' (just add a b in the front of the literal) in order to make it work. All other things should be the same as the module in Python 2.

Common Issues

1. If your module could not found library liblpsolve55.so, it means your library is not in the path. Try to set the environmental variable LD_LIBRARY_PATH to the location of your library.

2. If your module could not initalize because of unknown symbols xxx. Double check the changes in pythonmod.c, especially for the replacements.

3. On Linux, if gcc throws error, try to compile with clang (cc=clang in ccc)

If you have other questions, please open issue.