[C++-sig] Getting a b::p::object for a PyObject and vice-versa.

[Ralf W. Grosse-Kunstleve]

What you do looks right.

bp::object(bp::handle<>(c_object))

may be a little shorter in some places. (Watch out for compilers confusing your code with
function declarations.)

bpobj.ptr() will give you the PyObject* directly.
bphdl.get() does the same for handle<>.

(Improvements are always welcome, but a complete set of patches is a lot of work
and I doubt even wrapping the entire Python API will make much of a practical
difference.)

Ralf



----- Original Message ----
From: Murray Cumming <murrayc at murrayc.com>
To: cplusplus-sig at python.org
Sent: Thu, February 4, 2010 12:16:41 PM
Subject: [C++-sig] Getting a b::p::object for a PyObject and vice-versa.

Can someone confirm that this is correct:

1.
To get a boost::python::object that wraps an existing PyObject* (when
you get a PyObject from a C function not wrapped in boost::python), can
someone confirm that this is correct:
Either:
a)
  PyObject* c_object = get_the_c_object(); //returns a reference.
  boost::python::handle<> handle(c_object);
  boost::python::object cpp_object(handle);
or
b)
  PyObject* c_object = get_the_c_object(); //returns no extra reference.
  boost::python::handle<> handle(boost::python::borrowed(cObject));
  boost::python::object cpp_object(handle);

The code for b) can be simplified by doing 
  boost::python::object cpp_object(boost::python::borrowed(cObject))
but I don't see a way to simplify a)

I also see allow_null here, but like borrowed, it's not documented:
http://www.boost.org/doc/libs/1_41_0/libs/python/doc/v2/handle.html#allow_null-spec
so I can only guess at its purpose.

In general I wish this was much simpler and stated clearly somewhere.


2.
How can I get the underlying PyObject* from a boost::python::object
(when I need it to call a C function not wrapped in boost::python)?

I guessed at this, but it doesn't seem to be working for me:
  boost::python::extract<PyObject*> extractor(cpp_object);
  if(extractor.check())
  {
    PyObject* c_object = extractor;
  }