[C++-sig] howto convert a python list to a C++ matrix

[Stefan Seefeld]

hi there,

I'm trying to define a matrix constructor that
takes a python list. I want to be able to write

 > matrix = Matrix([[1,0,0,0],[0,1,0,0],[0,0,1,0],[0,0,0,1]])

where Matrix is a C++ type. Obviously I need a factory
such as 'Matrix factory(object);' or similar.

But how am I to extract the elements of the list(s) in
boost.python style ? And how should I raise a TypeError
if I don't find what I'm looking for ?

Here is a first attempt:

Matrix factory(python::list &l)
{
    float matrix[16];
    assert(l.length() == 4);
    for (size_t i = 0; i != 4; ++i)
    {
       python::list row = extract<python::list>(l[i]);
       assert(row.length() == 4);
       for (size_t j = 0; j != 4; ++j)
       {
          matrix[4*i + j] = row[j];
       }
    }
    return Matrix(matrix);
}

But 'length()' doesn't appear to exist, and the asserts
should be replaced with some (python) exceptions that
indicate a type error or similar.