By now you hopefully have the idea that typemaps are a powerful mechanism for building more specialized applications. While writing typemaps can be technical, many have already been written for you. See the Typemaps chapter for more information about using library files.

Implementing C callback functions in Python

Now that you’re an expert, we will implement simple C callback functions in Python and use them in a C++ code.

Let’s say that we wanted to write a simple C++ 2D plotting widget layered on top of the gd-1.2 library. A class definition might look like this :

//--------------------------------------------------------------------

//Create a C++ plotting "widget" using the gd-1.2 library by Thomas Boutell

//This example primarily illustrates how callback functions can be

//implemented in Python.

//--------------------------------------------------------------------

#include <stdio.h> extern "C" { #include "gd.h"

}

typedef double (*PLOTFUNC)(double, void *);

PlotWidget(int w, int h,double,double,double,double);

The widget class hides all of the underlying implementation details so this could have just as easily been implemented on top of OpenGL, X11 or some other kind of library. When used in C++, the widget works like this :

// Simple main program to test out our widget #include <stdio.h>

#include "widget.h" #include <math.h>

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// Callback function

double my_func(double a, void *clientdata) { return sin(a);

}

int main(int argc, char **argv) { PlotWidget *w;

FILE *f;

w = new PlotWidget(500,500,-6.3,-1.5,6.3,1.5);

}

Now suppose that we wanted to use our widget interactively from Python. While possible, it is going to be difficult because we would really like to implement the callback function in Python, not C++. We also don’t want to go in and hack or C++ code to support this. Fortunately, you can do it with SWIG using the following interface file :

//SWIG interface to our PlotWidget %module plotwidget

%{

#include "widget.h" %}

//Grab a Python function object as a Python object. %typemap(python,in) PyObject *pyfunc {

if (!PyCallable_Check($source)) { PyErr_SetString(PyExc_TypeError, "Need a callable object!"); return NULL;

}

$target = $source;

}

//Type mapping for grabbing a FILE * from Python %typemap(python,in) FILE * {

if (!PyFile_Check($source)) { PyErr_SetString(PyExc_TypeError, "Need a file!"); return NULL;

}

$target = PyFile_AsFile($source);

}

//Grab the class definition

%include widget.h

%{

/* This function matches the prototype of the normal C callback function for our widget. However, we use the clientdata pointer for holding a reference to a Python callable object. */

static double PythonCallBack(double a, void *clientdata)

{

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Py_XDECREF(result); return dres;

}

%}

// Attach a new method to our plot widget for adding Python functions %addmethods PlotWidget {

//Set a Python function object as a callback function

//Note : PyObject *pyfunc is remapped with a typempap void set_pymethod(PyObject *pyfunc) {

self->set_method(PythonCallBack, (void *) pyfunc); Py_INCREF(pyfunc);

}

}

While this is certainly not a trivial SWIG interface file, the results are quite cool. Let’s try out our new Python module :

# Now use our plotting widget in variety of ways

from plotwidget import * from math import *

# Make a plot using a normal Python function as a callback def func1(x):

return 0.5*sin(x)+0.25*sin(2*x)+0.125*cos(4*x)

print "Making plot1.gif..."

# Make a widget and set callback

w = PlotWidget(500,500,-10,-2,10,2)

w.set_pymethod(func1) # Register our Python function w.plot()

f = open("plot1.gif","w") w.save(f)

f.close()

# Make a plot using an anonymous function

print "Making plot2.gif..."

w1 = PlotWidget(500,500,-4,-1,4,16)

w1.set_pymethod(lambda x: x*x) # Register x^2 as a callback w1.plot()

f = open("plot2.gif","w") w1.save(f)

f.close()

# Make another plot using a built-in function

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