Type conversion of octave_value to/from Python representations

[David Grundberg]

Hi,

I'm working on a Python-to-Octave bridge in order to feval Octave 
functions, like this:

print = pytave.feval("sin", (Numeric.ones([3, 3]),))

I've gotten this far for real 2d matrices. I'm writing a Python module 
in C++, linking against Octave 3.0.2 and Python 2.4. However, I'm having 
trouble with the conversions back and forth Octave values and Python 
objects.


*Converting Python objects to octave-value:s*

Integers, floats and strings are converted with the octave_value 
constructor. Numeric arrays are copied to a Array<T> subclass of closest 
matching nature, and then wrapped into an octave_value. (i.e. support 
for int8, int16 etc) Any 3d double matrices are converted to NDArray 
instead of Matrix as a special case. The 1d cases are treated like 2d 
cases, i.e. the vector subclasses are not used at all.

I'm having trouble with creating any octave-value with a dimension 
higher than 3, how do I do that?


*Converting octave-value:s to Python objects*

This is when I really got stuck. I'm having trouble figuring out how to 
properly access the actual storage type (Array<T> subclass, or 
primitive, or?). I'm doing this right now (kind of):

octave_value ov = fromsomewhere;
if (ov.is_bool_scalar)) {
   pyobj = object(ov.bool_value());
} if // etc..
} else if (ov.is_matrix_type()) { // try to convert to numeric array
   if (ov.is_nd_real_array()) {
      // Try to use instantiate an NDArray with octave-value-argument 
constructor
   }
   else if (ov.is_double_type()) {
      // Try to use instantiate a Matrix with octave-value-argument 
constructor
   }
}

And this won't work, since octave-values with NDArray:s won't return 
true on is_nd_real_array(). The Matrix(ov) constructor won't accept a 3d 
real ov either. ("fatal: invalid conversion of NDArray to Matrix")

Oh, there is so much wrong with this type of checking, I don't know 
where to begin. If someone has a pointer as how to find the type, or a 
section of Octave code that does, or any hint on accomplishing the 
conversion, please let me know.

David

PS: Anyone interested in the module can download the early-stage code 
(with Autotools configuration) from here:
http://www.cs.umu.se/~c04dgg/pytave-0.1.tar.gz