[Swig-user] Constructor extension with type checking

[John M. <mcf...@gm...>]

Hi,

I am wrapping a C++ template class for python.  The C++ class defines a 
constructor and that gets wrapped fine.  However, I would like to add an 
additional constructor for use from python.  I've tried this two ways, 
but I run into a problem with each.

The first approach I tried was to use %extend and define an alternate 
constructor that accepts a PyObject*.  This actually does work, but the 
problem is I can't incorporate any type checking on the argument.  I can 
put in code to check for an incorrect type, set the python error string, 
and return NULL, but the NULL return value doesn't get propagated all 
the way through the wrappers and the result is that incorrect argument 
types don't actually trigger an error when used from python.

The second approach I tried was to use %pythonprepend to try and prepend 
some python code to implement the alternate constructor and do error 
checking.  The problem with this is that the python code is not getting 
inserted, and I think it's because I'm using a template class (I could 
successfully use pythonprepend with a non-template class).

Here is some (shortened) example code to demonstrate what I tried in 
each case:

// Here is how I tried the prepend.  This worked with a non-template 
class...
%pythonprepend Matrix<double>::Matrix<double>(int,int) %{
     # additional code...
%}

template <class T>
class Matrix{
   T *data;
public:
   Matrix(int m, int n) {}
}

// Instantiate template:
%template(DMatrix) Matrix<double>;

// Here is how I tried extending an alternate constructor with C++ code.
// This works except for the type checking.  The NULL return value 
doesn't get
// propagated through all of the Swig wrapper code
%extend Matrix<double> {
  Matrix<double>(PyObject* o) {
      // Do type check and try to set an error on failure:
      if ([incorrect type]) {
        PyErr_SetString(PyExc_TypeError,"Error string");
        return NULL;
      } else
      return new Matrix<double>(2,2); // Or something appropriate based 
on PyObject *o
    }
};

// ------End code

Hopefully it's somewhat clear what I'm trying to do.  Essentially I 
would like to add an additional constructor that allows a Python data 
structure (such as a sequence type) to be used as the constructor 
argument -- this works with %extend except for my type checking.  The 
other problem is that I can't seem to get %pythonprepend to work with 
the template class.  Any help is much appreciated, or maybe there's just 
a better way of doing this?

John