self.name = "image"+string.zfill(self.imgno,4)+".gif" self.imgno = self.imgno+1

# Set up an initial condition def initcond(h):

h.set_temp(0.0)

nx = h.grid.xpoints for i in range(0,nx):

h.grid[i][0] = 1.0

#Set up a problem and run it h = Heat2d(50,50)

#Make an image object

img = HeatImg(h)

initcond(h) fileno = 1

# Run it

for i in range(0,25): h.solve(100) h.dump("Dat"+str(fileno)) img.show()

print "time = ", h.time fileno = fileno+1

# Calculate average temperature and exit sum = 0.0

for i in range(0,h.grid.xpoints):

for j in range(0,h.grid.ypoints): sum = sum + h.grid[i][j]

avg = sum/(h.grid.xpoints*h.grid.ypoints) print "Avg temperature = ",avg

When run, we now get a collection of datafiles and series of images like this :

Thus, we have a simple physics application that only takes about 1 page of Python code, runs a simulation, creates data files, and a movie of images. We can easily change any aspect of the simulation, interactively query variables and examine data. New procedures can be written and tested in Python and later implemented in C++ if needed. More importantly, we have an application that is actually fun to use and modify (well, at least I think so).

Exception handling

The SWIG %except directive can be used to create a user-definable exception handler in charge

Version 1.1, June 24, 1997

of converting exceptions in your C/C++ program into Python exceptions. The chapter on exception handling contains more details, but suppose you have a C++ class like the following :

class RangeError {}; // Used for an exception

class DoubleArray { private:

int n; double *ptr;

public:

//Create a new array of fixed size DoubleArray(int size) {

ptr = new double[size]; n = size;

}

//Destroy an array

~DoubleArray() { delete ptr;

}

//Return the length of the array int length() {

return n;

}

//Get an item from the array and perform bounds checking. double getitem(int i) {

if ((i >= 0) && (i < n)) return ptr[i];

else

throw RangeError();

}

//Set an item in the array and perform bounds checking. void setitem(int i, double val) {

if ((i >= 0) && (i < n)) ptr[i] = val;

else {

throw RangeError();

}

}

};

The functions associated with this class can throw a range exception for an out-of-bounds array access. We can catch this in our Python extension by specifying the following in an interface file

:

%except(python) { try {

$function

}

catch (RangeError) { PyErr_SetString(PyExc_IndexError,"index out-of-bounds"); return NULL;

}

}

When the C++ class throws a RangeError exception, our wrapper functions will catch it, turn it

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into a Python exception, and allow a graceful death as opposed to just having some sort of mysterious program crash. Since SWIG’s exception handling is user-definable, we are not limited to C++ exception handling. Please see the chapter on exception handling for more details and using the exception.i library for writing language-independent exception handlers.

Python exceptions can be raised using the PyErr_SetString() function as shown above. The following table provides a list of the different Python exceptions available.

Version 1.1, June 24, 1997