Writing Python bindings of existing C libraries – (2)

Introduction

As I promised in the preceding post, I’ll provide a very easy example of a python binding. Let’s suppose we don’t want to use the methods included in Python to sum two integer values and we want to do it in C and then call the add method from a python script. I’ll write the complete source code first and then I’ll explain all the parts of it.

Source Code

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#include <Python.h>

static PyObject *foo_add(PyObject *self, PyObject *args)  
{  
    int a;  
    int b;

    if (!PyArg_ParseTuple(args, "ii", &a, &b))  
    {  
        return NULL;  
    }

    return Py_BuildValue("i", a + b);  
}

static PyMethodDef foo_methods[] = {  
    { "add", (PyCFunction)foo_add, METH_VARARGS, NULL },  
    { NULL, NULL, 0, NULL }  
};

PyMODINIT_FUNC initfoo()
{  
    Py_InitModule3("foo", foo_methods, "My first extension module.");  
}

How it works

First of all we have to include Python.h in our C file. This allows us to write an extension for Python language. To be able to include this header, we must have the python development packages installed in our system. For example in Debian based distributions we can install them with this command:

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sudo apt-get install python2.5-dev

Every module has at least three parts. In the first part we write methods we want to call from the final python module: in this case we have a method called foo_add where “foo” is the name of the module and “add” the name of the method. Every method is declared as static PyObject. The method does anything particular except calling PyArg_ParseTuple to validate the input (we’ll discuss this later), adding the two passed numbers and returning the result.

In the second part we have something like a dictionary, defined as static PyMethodDef and called foo_methods (where “foo” again is the name of the module). For each method we want to expose in our python module, we have to add something like this:

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{"add", (PyCFunction)foo_add, METH_VARARGS, NULL}

where “add” is the name of the method we want to be visible in our module, (PyCFunction)foo_add is a pointer to our foo_add method, implemented in the C module, METH_VARARGS means that we want to pass some parameters to the function and the last one would be the description of the method (we can leave it NULL if we want).

Third part allows us to register the defined method/s and the module:

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Py_InitModule3("foo", foo_methods, "My first extension module.");

Parsing Parameters

The PyArg_ParseTuple function extracts arguments from the PyObject passed as parameter to the current method and follows almost the sscanf syntax to parse parameters (in this case we had “ii” for two integers). You can fin the complete reference here: http://docs.python.org/c-api/arg.html

## Building and installing

To build the module, we have to be in the source directory and execute this command:

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gcc -shared -I/usr/include/python2.5 foo.c -o foo.so

then we’ve to copy the generated module to the python’s modules directory:

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cp foo.so /usr/lib/python2.5/site-packages/

## Testing our module

Testing the module is really easy. We’ve to start a python shell or create a python script with the following source code:

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import foo
print foo.add(2, 3)

if all is working fine, the printed result should be 5