Python binding of functions within a c++ program

Question

I have a program written in c++ that functions on it's own, however we want to make it accessible to Python. Specifically, we have several functions that are more efficient in c++, but we do a lot of other things with the output using Python scripts. I don't want to rewrite the whole of main() in Python as we make use of Boost's root finding algorithms and other functionalities that'd be a pain to do in Python.

Is it possible to add Python binding to these functions while keeping the c++ main()? I've never done Python binding before, but I've looked at Boost.python since we're already using Boost. Most of the examples use c++ functions/classes in a hpp file and embed them into a python program, which isn't exactly what we want.

What we want is to keep our c++ program as a standalone so it can run as it is if users want, and also allow users to call these functions from a Python program. Being able to use the same Makefile and exe would be great. We don't really want to make a separate c++ library containing the bound functions; we're not interested in making a pythonic version of the code, merely allowing access to these useful functions.

Thanks

At least with Cython it is possible to have call chain like 'C++ main -> C function implemented in cython -> python code -> python module implemented in cython -> C++ code...'. It is probably possible with Boost.python also. — J.J. Hakala
– J.J. Hakala, Commented Jul 27, 2016 at 21:28
It is worth pointing out that it is usually considered good practice for the C/C++ main to do very little other than parsing the parameters and then calling the actual processing with the parsed results - this makes what you are considering doing, or any bindings, much easier. — Steve Barnes
– Steve Barnes, Commented Jul 28, 2016 at 5:46

Kenny Ostrom · Accepted Answer · 2016-07-29 16:18:38Z

1

We have an extensive c++ library which we made available to python through use of a python wrapper class which calls an interface that we defined in boost python.

One python class handles all the queries in a pythonic manner, by calling a python extension module written in c++ with boost python. The python extension executes c++ code, so it can link and use anything from the original library.

You said your c++ is an executable, though. Why can't you use system calls to launch a shell process? You can do that in any language, including python. What I thought was you want to access individual functions, which means you need all your functions in a static library.

You build your c++ exe normally, linking the common code. You make a "boost python extension module" which links the common code, and can be imported by a python script. And of course a unit test executable, which links and tests the common code. My preference is that the common code be a stand-alone static lib (use -fPic if there's a posix gcc build).

edited Jul 29, 2016 at 16:18

answered Jul 27, 2016 at 23:42

Kenny Ostrom

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9 Comments

Steve Barnes Over a year ago

It might be worth mentioning the possibility of using a shared library, (.dll/.so), as a possibility as well as a static library.

Kenny Ostrom Over a year ago

The python extension module is (and must be) a shared library (.pyd/.so). The underlying code library has to be compatibile with that (-fPic) but making it also shared means you have to distribute it and not have any version/interface mismatches. Risk with no benefit.

Steve Barnes Over a year ago

Please note that the answer above, mentions linking a static library to the python code - this is wrong. You also have the possibility of building a static library for a C++ executable and a shared library to bundle into a wheel with the python interface.

Kenny Ostrom Over a year ago

I'm willing to correct any errors, but I think we just have a miscommunication. Python doesn't link, but it can import a shared library (python extension module). When this shared library was built from c++, other shared or static libraries may have been linked (and that's where your c++ code library came in).

Kenny Ostrom Over a year ago

I edited my answer to separate my recommendation from the points we agree on. And I do grant that makefile achieves the same thing of building the source once.

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