In this final part of the series I’m going to look at one further way we can use Boost.Python to help when using a C++ class within Python.

In the previous part we built a wrapper for a C++ class, exposing a number of class methods to Python.

However, in Python, it’s quite typical for a class to use properties as well as methods.

So to begin – let’s take a moment to look at properties within the context of classes in Python.

For example, if we had a Python class which looks like this:

class simple:
    def __init__ (self):
        self._data = None

    def set_data(self, data):
        self._data = data

    def get_data(self):
        return self._data

We could then use it, something like this:

import example

s = example.simple()
s.set_data(7)

print(s.get_data())

Whilst this works, we might reasonably say that the necessity to use getter and setter functions directly is not especially Pythonic – as it’s not how most built-in objects behave.

Instead we’d expect to be able to use it more like this:

import example

s=example.simple()

s.data = 7

print(s.data)

We can do exactly this; if we use properties…

Read more “Advanced C++ / Python integration with Boost.Python (Part 3)”

Last time we looked at how to use Boost.Python to wrap a very simple piece of C++ code. This time we’re going to take that one step further along, and do the same thing for a more complex C++ example – which includes a C++ Class.

For the purposes of this example – let us assume that we have a “legacy” C++ class (i.e. one that we’re not going to change): which looks something like this:

class ExampleClass
{
public:

  ExampleClass();
  void print_state();
  unsigned char* exporter();
  void set_data(uint8_t, uint8_t, uint8_t);
  void importer(unsigned char*, int);

private:
  uint8_t a;
  uint8_t b;
  uint8_t c;

};

This (deliberately, slightly contrived) class has three methods (and a constructor). We can set the three 8-bit unsigned integers (uint8_t) directly using the set_data() method; we can print the data to the screen; or we can either import from or export to an unsigned char*.

In order to be able to use the importer & exporter methods with Python (via Boost.Python) we’ll need to further wrap these – as Boost.Python won’t let us use unsigned char* directly.

Read more “Advanced C++ / Python integration with Boost.Python (Part 2)”

In a previous post, I described how you could use the ctypes Python library to import a C++ class to make it usable from within Python. There is however, another way to do this; and that’s by using Boost.Python (which as the name suggests is a part of the Boost C++ library suite).

Whilst this method makes our code on the C++ side slightly more complicated; it significantly simplifies the Python code – and it let’s us use some more powerful features.

To begin let’s start with a simple example.

// hello.cpp

#include <boost/python.hpp>
#include <string>

char const* greet()
{
   return "hello, world";
}

std::string multi_bob(int n)
{
   std::string name = "Bob";
   std::string r = "";
   for (int i=0;i<n;i++)
      r += name;
   return r;
}

BOOST_PYTHON_MODULE(hello)
{
    using namespace boost::python;
    def("greet", greet);
    def("multibob", multi_bob);
}

Everything above the BOOST_PYTHON_MODULE line is perfectly ordinary C++ code. The clever bit comes in when we call the BOOST_PYTHON_MODULE macro (which is defined within the Boost.Python header)…

In this case we’re creating a Python module – with two methods: one based on our very simple greet() function (which we’re also going to call greet); and one based on the more complex multi_bob(int) function. Note that for this second function, to show the fact that the linkage between the C++ & Python code can be very flexible, we give it a different name on the Python side. Also note that we don’t need to tell the macro about the signature of the function as this is handled for us by Boost.Python.

Read more “Advanced C++ / Python integration with Boost.Python”

I recently found myself needing to call a C++ class from within Python. I didn’t want to call a separate process (as I had previously when using Thrift – see Using Apache Thrift for Python & C++); but rather to call a C++ library directly.

Before I go on I should say that there are a lot of different ways to do this is Python – and I’ve picked one that worked for me. Other techniques are available – and opinion seems quite divided as to which (if any) technique is ‘best’.

To start with we have our C++ class, written in the usual way.

Read more “Calling C++ Classes from Python, with ctypes…”