Re: Boost.Python: using methods on attributes of wrapped classes

From: mikeowen_at_[hidden]
Date: 2001-07-06 02:52:43

Hi David,

Thanks for the suggestion.  I've tried out using the 
python::python_extension_class_converters<Type>::smart_ptr_to_python 
method you descibe in the message, and it does work so long as the 
class I need a pointer for is defined in the module in question.  
However, I don't seem to be able to use this method when I want to 
access inner classes that are defined in different modules.  Is 
there a way to force the to_python converters for pointers to 
objects to be imported?  It's not clear to me how to do this with an 
"import_converters" statement, since I don't know what to call the 
python wrapped instance in the last argument.

Thanks!

Mike.

--- In boost_at_y..., "David Abrahams" <david.abrahams_at_r...> wrote:
> Hi Mike,
> 
> I proposed a way of addressing this in
> http://groups.yahoo.com/group/boost/message/10144, but in response 
there was
> a kind of deafening silence. In the meantime, I've been directing 
people at
> the section titled "If you can't (afford to) copy the referent, or 
the
> pointer
> is non-const" on this page:
> http://www.boost.org/libs/python/doc/pointers.html
> which describes a dangerous hack to work around the problem.
> 
> I would happily make the proposed change to Boost.Python, but had 
been
> hoping to hear more from the user community before undertaking any 
redesign.
> 
> Ralf Grosse-Kunstleve has been working on a set of changes that 
would be
> closely related to the ones proposed above; I'd like to hear from 
Ralf on
> the status of those also. Ralf?
> 
> -Dave
> 
> ----- Original Message -----
> From: "Mike Owen" <mikeowen_at_s...>
> To: <boost_at_y...>
> Sent: Tuesday, July 03, 2001 3:33 AM
> Subject: [boost] Boost.Python: using methods on attributes of 
wrapped
> classes
> 
> 
> > I'm getting unexpected behaviour when using methods on 
attributes of BPL
> > wrapped objects.  For instance, if I have a wrapped class A that 
has
> another
> > wrapped class B as a member, calling the methods of the B 
instance in A
> does
> > not affect this B instance.  It looks like temporaries are being
> constructed
> > and then destroyed when I do this.  Here is a concrete example:
> >
> > struct Interior {
> >   double x;
> > };
> >
> > struct Top {
> >   Interior interiorInstance;
> > };
> >
> > #include <boost/python/class_builder.hpp>
> > namespace python = boost::python;
> >
> > namespace {
> >   BOOST_PYTHON_MODULE_INIT(TestAttr) {
> >     try {
> >       python::module_builder module("TestAttr");
> >
> >       python::class_builder<Interior> InteriorClass(module, 
"Interior");
> >       InteriorClass.def(python::constructor<>());
> >       InteriorClass.def_read_write(&Interior::x, "x");
> >
> >       python::class_builder<Top> TopClass(module, "Top");
> >       TopClass.def(python::constructor<>());
> >       TopClass.def_read_write(&Top::interiorInstance, 
"interiorInstance");
> >     }
> >     catch(...) {
> >       python::handle_exception();
> >     }
> >   }
> > }
> >
> > So the class "Top" has a member instance of "Interior".  Both 
classes are
> > exposed to python with BPL.  However, when I try to set the "x" 
value of
> > Top::Interior, it always fails.  Here's the output that results 
from
> > compiling the above and trying the experiment:
> >
> > HodgePodge{owen}: python
> > Python 2.1 (#2, May  9 2001, 22:11:49)
> > [GCC 2.96 20000731 (Linux-Mandrake 8.0 2.96-0.48mdk)] on linux2
> > Type "copyright", "credits" or "license" for more information.
> > >>> import TestAttr
> > >>> a = TestAttr.Interior()
> > >>> a.x
> > 1.750725915200081e-313
> > >>> a.x = 100
> > >>> a.x
> > 100.0
> > >>> b = TestAttr.Top()
> > >>> b.interiorInstance
> > <Interior object at 0x8115c20>
> > >>> b.interiorInstance.x
> > 1.750725915200081e-313
> > >>> b.interiorInstance.x = 100
> > >>> b.interiorInstance.x
> > 1.750725915200081e-313
> > >>>
> >
> > This seems like incorrect behaviour to me, but am I wrong?  How 
should I
> be
> > constructing the BPL wrap code so that I can call methods on the 
member
> > class?  Thanks!
> >
> > Mike.
> >
> > --
> >  "Hey...where are the sunflower seeds?" |       J. Michael Owen
> >         o_o /                           |
> >         (")                             |       Email:
> mikeowen_at_s...
> >        \/'\/                            |
> >
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