Yup, defining

double Passer::val()
{
   object ret = m_f( m_x );
   return extract<double> ( ret );
}

works like a charm. Thanks! I didn't realise the boost API was
as clean and complete as it apparently is.


--- David Abrahams <dave@[...].com>  wrote:
>  "mghiggins <mghiggins@[...].com>" <mghiggins@[...].com>
>  writes:
>  
>  > Thanks, that helped a lot!
>  
>  As I asked you before, please post your Boost.Python questions
>  to the
>  C++-sig.
>  
>  > Another perhaps more basic question: how do I pass a python
>  function
>  > object to a C++ method and use it internally? e.g., if I
>  have a class
>  >
>  > class Passer
>  > {
>  >  public:
>  >    Passer() {};
>  >    void setx( dict x ) { this->m_x = x; };
>  >    void setf( object f ) { this->m_f = f; };
>  >    double val();
>  >    double xval() { return extract<double>( m_x['v'] ); };
>  >
>  >  private:
>  >    dict   m_x;
>  >    object m_f;
>  > };
>  >
>  > double Passer::val()
>  > {
>  >    PyObject * f   = m_f.ptr();
>  >    PyObject * ret = PyEval_CallObject( f, m_x.ptr() );
>  
>  
>  why not just
>  
>       object ret = m_f(m_x);
>  
>  ??
>  
>  >    double RetVal;
>  >    if( ret )
>  >       RetVal = 1;
>  >    else
>  >       RetVal = -1;
>  >
>  >    PyArg_Parse( ret, "(f)", &RetVal );
>  >    return RetVal;
>  > }
>  
>  I don't know what your PyArg_Parse thing is supposed to be
>  doing. I'm
>  not really sure what all of that 1/-1 stuff is about either. 
>  Is
>  RetVal a tuple of one element?  Maybe you want:
>  
>   double Passer::val()
>   {
>       return m_f(m_x)[0];
>   }
>  
>  > The setx method seems to work fine with a dictionary like
>  {'v':4} and
>  > the xval method returns its 'v' element properly.
>  >
>  > But the setf method doesn't seem to work correctly... when I
>  call
>  > val() from python after setting on a function, PyObject *
>  ret comes
>  > back NULL (which I learn from getting RetVal == -1 as a
>  return value).
>  
>  That might just mean that f raised a Python exception.  You're
>  masking the problem by returning -1 instead of using the
>  Boost.Python
>  facilities which propagate the Python exception through your
>  C++ code
>  and back to Python.
>  
>  > I'm sure I'm doing something dumb by making the type of
>  setf's arg be
>  > "object" rather than something specifying a function, but I
>  don't know
>  > what. :) Anything obvious I'm missing here?
>  
>  The only dumb-ish thing is using the raw Python 'C' API
>  instead of
>  what Boost.Python gives you.  Try the simple approach I
>  outlined
>  above; it might be revealing.
>  
>  -- 
>  Dave Abrahams
>  Boost Consulting
>  www.boost-consulting.com
>  


=====
-------------------------------------------------------
Mark Higgins, mghiggins@[...].com
http://www.molala.net

Opinions expressed herein do not reflect my own views;
I haven't had free will since last year when aliens ate
my brain.

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