###  | # 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.