Easy Python generators using SWIG

In my research code, I have a number of classes that act as wrappers for containers, and they provide begin and end functions. I want an easy way to provide a generator for looping over these embedded containers. Because of the way SWIG's STL wrappers work, it's not (easily?) possible to wrap a vector of pointers, and we'd like to avoid all the extra overhead those wrappers have anyway: all we need is a way to increment a pointer and to tell if it's at the end.

So, my solution is to create a thin wrapper for only the increment operator ++, and extend the class with a function to return the "beginning" iterator and to check whether the iterator is at the end position.

For this particular instance, I have a class Mesh that has methods for beginLeftBoundaryFaces and endLeftBoundaryFaces—which return STL vector iterators that point to a Face *.

%inline %{
//! Thin wrapper for ONLY the increment operator
void _bfiter_incr( std::vector<Face *>::const_iterator* iter )
{
    // increment the iterator
    ++(*iter);
}
%}

%extend Mesh {
%insert("python") %{
    def left_boundary_faces(self):
        "A generator to iterate through boundary faces."
        faceIter = self._beginLeftBoundaryFaces()
        keepLooping = True
        while keepLooping == True:
            face = self._bfiter_dereference_Left( faceIter )
            if face:
                _bfiter_incr( faceIter )
                yield face
            else:
                keepLooping = False
%}

//! get the first element in the vector
std::vector<Face *>::const_iterator* _beginLeftBoundaryFaces()
{
    return new std::vector<Face *>::const_iterator(
            ($self->beginLeftBoundaryFaces()) );
}

//! dereference the iterator; return NULL if at the end
const Face* _bfiter_dereference_Left(
        const std::vector<Face *>::const_iterator* iter )
{
    // if at the end, return NULL
    if (*iter == ($self)->endLeftBoundaryFaces() ) {
        return NULL;
    }
    // otherwise, return the face to which this iterator points
    return **iter;
}
}

So now I can do:

for f in mesh.left_boundary_faces():
    print f.area()

or, of course, anything else now that I'm using Python.

See further updates on the thin Python wrappers for C++ iterators.