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Boost.Python

Header <boost/python/converter/pytype_function.hpp>

Contents

Introduction
Classes
Class wrap_pytype
Class wrap_pytype synopsis
Class registered_pytype
Class registered_pytype synopsis
Class expected_from_python_type
Class expected_from_python_type synopsis
Class to_python_target_type
Class to_python_target_type synopsis
Examples

Introduction

To support Pythonic signatures the converters should supply a get_pytype function returning a pointer to the associated PyTypeObject. See for example ResultConverter or to_python_converter. The classes in this header file are meant to be used when implmenting get_pytype. There are also _direct versions of the templates of class T which should be used with undecorated type parameter, expected to be in the conversion registry when the module loads.

Classes

Class wrap_pytype

This template generates a static get_pytype member returning the template parameter.

Class wrap_pytype synopsis

namespace boost { namespace python { namespace converter{

    template < PyTypeObject const *pytype >
    class wrap_pytype 
    {
      public:
          static PyTypeObject const *get_pytype(){return pytype; }
    };

}}}

Class registered_pytype

This template should be used with template parameters which are (possibly decorated) types exported to python using class_. The generated a static get_pytype member returns the corresponding python type.

Class registered_pytype synopsis

namespace boost { namespace python { namespace converter{

    template < class T >
    class registered_pytype 
    {
      public:
          static PyTypeObject const *get_pytype();
    };

}}}

Class expected_from_python_type

This template generates a static get_pytype member which inspects the registered from_python converters for the type T and returns a matching python type.

Class expected_from_python_type synopsis

namespace boost { namespace python { namespace converter{

    template < class T >
    class expected_from_python_type 
    {
      public:
          static PyTypeObject const *get_pytype();
    };

}}}

Class to_python_target_type

This template generates a static get_pytype member returning the python type to which T can be converted.

Class to_python_target_type synopsis

namespace boost { namespace python { namespace converter{

    template < class T >
    class to_python_target_type 
    {
      public:
          static PyTypeObject const *get_pytype();
    };

}}}

Examples

This example presumes that someone has implemented the standard noddy example module from the Python documentation, and placed the corresponding declarations in "noddy.h". Because noddy_NoddyObject is the ultimate trivial extension type, the example is a bit contrived: it wraps a function for which all information is contained in the type of its return value.

C++ module definition

#include <boost/python/reference.hpp>
#include <boost/python/module.hpp>
#include "noddy.h"

struct tag {};
tag make_tag() { return tag(); }

using namespace boost::python;

struct tag_to_noddy 
#if defined BOOST_PYTHON_SUPPORTS_PY_SIGNATURES //unnecessary overhead if py signatures are not supported
: wrap_pytype<&noddy_NoddyType> //inherits get_pytype from wrap_pytype
#endif
{
    static PyObject* convert(tag const& x)
    {
        return PyObject_New(noddy_NoddyObject, &noddy_NoddyType);
    }
};

BOOST_PYTHON_MODULE(to_python_converter)
{
    def("make_tag", make_tag);
    to_python_converter<tag, tag_to_noddy
#if defined BOOST_PYTHON_SUPPORTS_PY_SIGNATURES //invalid if py signatures are not supported
          , true
#endif
          >(); //"true" because tag_to_noddy has member get_pytype
}

The following example registers to and from python converters using the templates expected_from_python_type and to_pyhton_target_type. 

#include <boost/python/module.hpp>
#include <boost/python/def.hpp>
#include <boost/python/extract.hpp>
#include <boost/python/to_python_converter.hpp>
#include <boost/python/class.hpp>

using namespace boost::python;

struct A
{
};

struct B
{
  A a;
  B(const A& a_):a(a_){}
};

// Converter from A to python int
struct BToPython 
#if defined BOOST_PYTHON_SUPPORTS_PY_SIGNATURES //unnecessary overhead if py signatures are not supported
   : converter::to_python_target_type<A>  //inherits get_pytype
#endif
{
  static PyObject* convert(const B& b)
  {
    return incref(object(b.a).ptr());
  }
};

// Conversion from python int to A
struct BFromPython
{
  BFromPython()
  {
    boost::python::converter::registry::push_back
        ( &convertible
        , &construct
        , type_id< B >()
#if defined BOOST_PYTHON_SUPPORTS_PY_SIGNATURES //invalid if py signatures are not supported
        , &converter::expected_from_python_type<A>::get_pytype//convertible to A can be converted to B
#endif
        );
  }

  static void* convertible(PyObject* obj_ptr)
  {
      extract<const A&> ex(obj_ptr);
      if (!ex.check()) return 0;
      return obj_ptr;
  }

  static void construct(
      PyObject* obj_ptr,
      converter::rvalue_from_python_stage1_data* data)
  {
    void* storage = (
        (converter::rvalue_from_python_storage< B >*)data)-> storage.bytes;

    extract<const A&> ex(obj_ptr);
    new (storage) B(ex());
    data->convertible = storage;
  }
};


B func(const B& b) { return b ; }

BOOST_PYTHON_MODULE(pytype_function_ext)
{
  to_python_converter< B , BToPython
#if defined BOOST_PYTHON_SUPPORTS_PY_SIGNATURES //invalid if py signatures are not supported
             ,true 
#endif
             >(); //has get_pytype
  BFromPython();

  class_<A>("A") ;

  def("func", &func);

}



>>> from pytype_function_ext import *
>>> print func.__doc__
func( (A)arg1) -> A :
    C++ signature:
         struct B func(struct B)

© Copyright Nikolay Mladenov 2007.