The Boost Parameter Library Python Binding Documentation

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ The Boost Parameter Library Python Binding Documentation +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

:Authors: David Abrahams, Daniel Wallin :Contact: [email protected], [email protected] :organization: BoostPro Computing_ :date: $Date$

:copyright: Copyright David Abrahams, Daniel Wallin 2005-2009. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

:abstract: Makes it possible to bind Boost.Parameter-enabled functions, operators and constructors to Python.

|(logo)|__

.. |(logo)| image:: ../../../../boost.png :alt: Boost

__ ../../../../index.htm

.. _BoostPro Computing: http://www.boostpro.com

.. role:: class :class: class

.. role:: concept :class: concept

.. role:: function :class: function

.. |ParameterSpec| replace:: :concept:ParameterSpec

.. contents:: :depth: 1

Introduction

boost/parameter/python.hpp introduces a group of |def_visitors|_ that can be used to easily expose Boost.Parameter-enabled member functions to Python with Boost.Python. It also provides a function template def() that can be used to expose Boost.Parameter-enabled free functions.

.. |def_visitor| replace:: def_visitor .. |def_visitors| replace:: def_visitors

.. def_visitor: def_visitors .. _def_visitors: ../../../python/doc/v2/def_visitor.html

When binding a Boost.Parameter enabled function, the keyword tags must be specified. Additionally, because Boost.Parameter enabled functions are templates, the desired function signature must be specified.

.. The keyword tags are specified as an MPL Sequence, using the pointer qualifications described in |ParameterSpec| below. The signature is also specifid as an MPL sequence_ of parameter types. Additionally, boost::parameter::python::function and boost::parameter::python::def requires a class with forwarding overloads. We will take a closer look at how this is done in the tutorial section below.

The keyword tags and associated argument types are specified as an MPL Sequence, using the function type syntax described in |ParameterSpec| below. Additionally, boost::parameter::python::function and boost::parameter::python::def requires a class with forwarding overloads. We will take a closer look at how this is done in the tutorial section below.

.. The last two sentences are terribly vague. Which namespace is .. function in? Isn't the return type always needed? What .. else are we going to do other than pass these sequences to .. function?

.. _MPL Sequence: ../../../mpl/doc/refmanual/sequences.html .. parameterspec: concept ParameterSpec

Tutorial

In this section we will outline the steps needed to bind a simple Boost.Parameter-enabled member function to Python. Knowledge of the Boost.Parameter macros_ are required to understand this section.

.. _macros: index.html

The class and member function we are interested in binding looks like this:

.. parsed-literal::

#include <boost/parameter/keyword.hpp> #include <boost/parameter/preprocessor.hpp> #include <boost/parameter/python.hpp> #include <boost/python.hpp>

// First the keywords BOOST_PARAMETER_KEYWORD(tag, title) BOOST_PARAMETER_KEYWORD(tag, width) BOOST_PARAMETER_KEYWORD(tag, height)

class window { public: BOOST_PARAMETER_MEMBER_FUNCTION( (void), open, tag, (required (title, (std::string))) (optional (width, (unsigned), 400) (height, (unsigned), 400)) ) { … function implementation … } };

.. @example.prepend('#include <cassert>') .. @example.replace_emphasis(''' assert(title == "foo"); assert(height == 20); assert(width == 400); ''')

It defines a set of overloaded member functions called open with one required parameter and two optional ones. To bind this member function to Python we use the binding utility boost::parameter::python::function. boost::parameter::python::function is a |def_visitor|_ that we'll instantiate and pass to boost::python::class_::def().

To use boost::parameter::python::function we first need to define a class with forwarding overloads. This is needed because window::open() is a function template, so we can't refer to it in any other way.

struct open_fwd { template <class A0, class A1, class A2> void operator()( boost::type<void>, window& self , A0 const& a0, A1 const& a1, A2 const& a2 ) { self.open(a0, a1, a2); } };

The first parameter, boost::type<void>, tells the forwarding overload what the return type should be. In this case we know that it's always void but in some cases, when we are exporting several specializations of a Boost.Parameter-enabled template, we need to use that parameter to deduce the return type.

window::open() takes a total of 3 parameters, so the forwarding function needs to take three parameters as well.

.. Note::

We only need one overload in the forwarding class, despite the
fact that there are two optional parameters. There are special
circumstances when several overload are needed; see 
`special keywords`_.

Next we'll define the module and export the class:

BOOST_PYTHON_MODULE(my_module) { using namespace boost::python; namespace py = boost::parameter::python; namespace mpl = boost::mpl;

  class_<window>("window")
      .def(
          "open", py::function<
              open_fwd
            , mpl::vector<
                  void
                , tag::title(std::string)
                , tag::width*(unsigned)
                , tag::height*(unsigned)
              >
          >()
      );

}

.. @jam_prefix.append('import python ;') .. @jam_prefix.append('stage . : my_module /boost/python//boost_python ;') .. @my_module = build( output = 'my_module' , target_rule = 'python-extension' , input = '/boost/python//boost_python' , howmany = 'all' )

.. @del jam_prefix[:]

py::function is passed two parameters. The first one is the class with forwarding overloads that we defined earlier. The second one is an MPL Sequence_ with the keyword tag types and argument types for the function specified as function types. The pointer syntax used in tag::width* and tag::height* means that the parameter is optional. The first element of the MPL Sequence_ is the return type of the function, in this case void, which is passed as the first argument to operator() in the forwarding class.

.. The pointer syntax means that the parameter is optional, so in this case width and height are optional parameters. The third parameter is an MPL Sequence_ with the desired function signature. The return type comes first, and then the parameter types:

.. parsed-literal::

    mpl::vector<void,        std::string, unsigned, unsigned>
                *return type*  *title*        *width*     *height*

.. @ignore()

That's it! This class can now be used in Python with the expected syntax::

>>> w = my_module.window()
>>> w.open(title = "foo", height = 20)

.. @example.prepend('import my_module') .. @run_python(module_path = my_module)

.. Sorry to say this at such a late date, but this syntax really .. strikes me as cumbersome. Couldn't we do something like:

class_<window>("window")
      .def(
          "open", 
          (void (*)( 
              tag::title(std::string), 
              tag::width*(unsigned), 
              tag::height*(unsigned)) 
          )0
      );

or at least:

  class_<window>("window")
      .def(
          "open", 
          mpl::vector<
              void, 
              tag::title(std::string), 
              tag::width*(unsigned), 
              tag::height*(unsigned)
          >()
      );

assuming, that is, that we will have to repeat the tags (yes, users of broken compilers will have to give us function pointer types instead).

concept |ParameterSpec|

A |ParameterSpec| is a function type K(T) that describes both the keyword tag, K, and the argument type, T, for a parameter.

K is either:

A required keyword of the form Tag
or, an optional keyword of the form Tag*
or, a special keyword of the form Tag**

where Tag is a keyword tag type, as used in a specialization of |keyword|__.

.. |keyword| replace:: boost::parameter::keyword __ ../../../parameter/doc/html/reference.html#keyword

The arity range for an MPL Sequence_ of |ParameterSpec|'s is defined as the closed range:

.. parsed-literal::

[ mpl::size<S> - number of special keyword tags in S, mpl::size<S> ]

For example, the arity range of mpl::vector2<x(int),y(int)> is [2,2], the arity range of mpl::vector2<x(int),y*(int)> is [2,2] and the arity range of mpl::vector2<x(int),y**(int)> is [1,2].

special keywords

Sometimes it is desirable to have a default value for a parameter that differ in type from the parameter. This technique is useful for doing simple tag-dispatching based on the presence of a parameter. For example:

.. An example_ of this is given in the Boost.Parameter docs. The example uses a different technique, but could also have been written like this:

.. parsed-literal::

namespace core { template <class ArgumentPack> void dfs_dispatch(ArgumentPack const& args, mpl::false_) { …compute and use default color map… }

template <class ArgumentPack, class ColorMap>
void dfs_dispatch(ArgumentPack const& args, ColorMap colormap)
{
    *…use colormap…*
}

}

template <class ArgumentPack> void depth_first_search(ArgumentPack const& args) { core::dfs_dispatch(args, args[color | mpl::false_()]); }

.. @example.prepend(''' #include <boost/parameter/keyword.hpp> #include <boost/parameter/parameters.hpp> #include <boost/mpl/bool.hpp> #include <cassert>

BOOST_PARAMETER_KEYWORD(tag, color);

typedef boost::parameter::parameterstag::color params;

namespace mpl = boost::mpl; ''')

.. @example.replace_emphasis(''' assert(args[color | 1] == 1); ''')

.. @example.replace_emphasis(''' assert(args[color | 1] == 0); ''')

.. @example.append(''' int main() { depth_first_search(params()()); depth_first_search(params()(color = 0)); }''')

.. @build()

.. .. _example: index.html#dispatching-based-on-the-presence-of-a-default

In the above example the type of the default for color is mpl::false_, a type that is distinct from any color map that the user might supply.

When binding the case outlined above, the default type for color will not be convertible to the parameter type. Therefore we need to tag the color keyword as a special keyword. This is done by specifying the tag as tag::color** when binding the function (see concept ParameterSpec_ for more details on the tagging). By doing this we tell the binding functions that it needs to generate two overloads, one with the color parameter present and one without. Had there been two special keywords, four overloads would need to be generated. The number of generated overloads is equal to 2
:sup:N, where N is the number of special keywords.

class template `init`

Defines a named parameter enabled constructor.

.. parsed-literal::

template <class ParameterSpecs>
struct init : python::def_visitor<init<ParameterSpecs> >
{
    template <class Class> 
    void def(Class& class\_);

    template <class CallPolicies>
    *def\_visitor* operator[](CallPolicies const& policies) const;
};

.. @ignore()

init requirements


* ``ParameterSpecs`` is an `MPL sequence`_ where each element is a
  model of |ParameterSpec|. 
* For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity
  range** of ``ParameterSpecs``, ``Class`` must support these
  expressions: 

  ======================= ============= =========================================
  Expression              Return type   Requirements
  ======================= ============= =========================================
  ``Class(a0, …, aN)``    \-            ``a0``\ …\ ``aN`` are tagged arguments.
  ======================= ============= =========================================



``template <class CallPolicies> operator[](CallPolicies const&)``

Returns a def_visitor equivalent to *this, except that it uses CallPolicies when creating the binding.

Example


.. parsed-literal::

    #include <boost/parameter/keyword.hpp>
    #include <boost/parameter/preprocessor.hpp>
    #include <boost/parameter/python.hpp>
    #include <boost/python.hpp>
    #include <boost/mpl/vector.hpp>

    BOOST_PARAMETER_KEYWORD(tag, x)
    BOOST_PARAMETER_KEYWORD(tag, y)

    struct base 
    { 
        template <class ArgumentPack>
        base(ArgumentPack const& args)
        {
            *… use args …*
        }
    };

    class X : base
    {
    public:
        BOOST_PARAMETER_CONSTRUCTOR(X, (base), tag,
            (required (x, \*))
            (optional (y, \*))
        )
    };

    BOOST_PYTHON_MODULE(*module name*)
    {
        using namespace boost::python;
        namespace py = boost::parameter::python;
        namespace mpl = boost::mpl;

        class_<X>("X", no_init)
            .def(
                py::init<
                    mpl::vector<tag::x(int), tag::y\*(int)>
                >()
            );
    }

.. @example.replace_emphasis('''
   assert(args[x] == 0);
   assert(args[y | 1] == 1);
   ''')

.. @example.replace_emphasis('my_module')

.. @jam_prefix.append('import python ;')
.. @jam_prefix.append('stage . : my_module /boost/python//boost_python ;')
.. @my_module = build(
        output = 'my_module'
      , target_rule = 'python-extension'
      , input = '/boost/python//boost_python'
    )

------------------------------------------------------------------------------

class template ``call``
-----------------------

Defines a ``__call__`` operator, mapped to ``operator()`` in C++.

.. parsed-literal::

    template <class ParameterSpecs>
    struct call : python::def_visitor<call<ParameterSpecs> >
    {
        template <class Class> 
        void def(Class& class\_);

        template <class CallPolicies>
        *def\_visitor* operator[](CallPolicies const& policies) const;
    };

.. @ignore()

``call`` requirements

ParameterSpecs is an MPL sequence_ where each element except the first models |ParameterSpec|. The first element is the result type of c(…).
Class must support these expressions, where c is an instance of Class:

=================== ==================== ======================================= Expression Return type Requirements =================== ==================== ======================================= c(a0, …, aN) Convertible to R a0\ …\ aN are tagged arguments. =================== ==================== =======================================

For every N in [U,V], where [U,V] is the arity range of ParameterSpecs.

template <class CallPolicies> operator[](CallPolicies const&)


Returns a ``def_visitor`` equivalent to ``*this``, except that it
uses CallPolicies when creating the binding.


Example
~~~~~~~

.. parsed-literal::

    #include <boost/parameter/keyword.hpp>
    #include <boost/parameter/preprocessor.hpp>
    #include <boost/parameter/python.hpp>
    #include <boost/python.hpp>
    #include <boost/mpl/vector.hpp>

    BOOST_PARAMETER_KEYWORD(tag, x)
    BOOST_PARAMETER_KEYWORD(tag, y)

    namespace parameter = boost::parameter;

    typedef parameter::parameters<
        parameter::required<tag::x>
      , parameter::optional<tag::y>
    > call_parameters;

    class X
    {
    public:
        template <class ArgumentPack>
        int call_impl(ArgumentPack const& args)
        {
            *… use args …*
        }

        template <class A0>
        int operator()(A0 const& a0)
        {
            return call_impl(call_parameters()(a0));
        }

        template <class A0, class A1>
        int operator()(A0 const& a0, A1 const& a1)
        {
            return call_impl(call_parameters()(a0,a1));
        }
    };

    BOOST_PYTHON_MODULE(*module name*)
    {
        using namespace boost::python;
        namespace py = parameter::python;
        namespace mpl = boost::mpl;

        class_<X>("X")
            .def(
                py::call<
                    mpl::vector<int, tag::x(int), tag::y\*(int)>
                >()
            );
    }    

.. @example.replace_emphasis('''
   assert(args[x] == 0);
   assert(args[y | 1] == 1);
   return 0;
   ''')

.. @example.replace_emphasis('my_module')

.. @my_module = build(
        output = 'my_module'
      , target_rule = 'python-extension'
      , input = '/boost/python//boost_python'
    )

------------------------------------------------------------------------------

class template ``function``
---------------------------

Defines a named parameter enabled member function.

.. parsed-literal::

    template <class Fwd, class ParameterSpecs>
    struct function : python::def_visitor<function<Fwd, ParameterSpecs> >
    {
        template <class Class, class Options> 
        void def(Class& class\_, char const* name, Options const& options);
    };

.. @ignore()

``function`` requirements 
~~~~~~~~~~~~~~~~~~~~~~~~~

* ``ParameterSpecs`` is an `MPL sequence`_ where each element
  except the first models |ParameterSpec|. The first element
  is the result type of ``c.f(…)``, where ``f`` is the member
  function.
* An instance of ``Fwd`` must support this expression:

  ============================================ ==================== =================================================
  Expression                                   Return type          Requirements
  ============================================ ==================== =================================================
  ``fwd(boost::type<R>(), self, a0, …, aN)``   Convertible to ``R`` ``self`` is a reference to the object on which
                                                                    the function should be invoked. ``a0``\ …\ ``aN``
                                                                    are tagged arguments.
  ============================================ ==================== =================================================

  For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``ParameterSpecs``.


Example
~~~~~~~

This example exports a member function ``f(int x, int y = …)`` to Python. The
sequence of |ParameterSpec|'s ``mpl::vector2<tag::x(int), tag::y*(int)>`` has
an **arity range** of [2,2], so we only need one forwarding overload.

.. parsed-literal::

    #include <boost/parameter/keyword.hpp>
    #include <boost/parameter/preprocessor.hpp>
    #include <boost/parameter/python.hpp>
    #include <boost/python.hpp>
    #include <boost/mpl/vector.hpp>

    BOOST_PARAMETER_KEYWORD(tag, x)
    BOOST_PARAMETER_KEYWORD(tag, y)

    class X
    {
    public:
        BOOST_PARAMETER_MEMBER_FUNCTION((void), f, tag,
            (required (x, \*))
            (optional (y, \*, 1))
        )
        {
            *…*
        }
    };

    struct f_fwd
    {
        template <class A0, class A1>
        void operator()(boost::type<void>, X& self, A0 const& a0, A1 const& a1)
        {
            self.f(a0, a1);
        }
    };

    BOOST_PYTHON_MODULE(*module name*)
    {
        using namespace boost::python;
        namespace py = boost::parameter::python;
        namespace mpl = boost::mpl;

        class_<X>("X")
            .def("f",
                py::function<
                    f_fwd
                  , mpl::vector<void, tag::x(int), tag::y\*(int)>
                >()
            );
    }

.. @example.replace_emphasis('''
   assert(x == 0);
   assert(y == 1);
   ''')

.. @example.replace_emphasis('my_module')

.. @my_module = build(
        output = 'my_module'
      , target_rule = 'python-extension'
      , input = '/boost/python//boost_python'
    )

------------------------------------------------------------------------------

function template ``def``
-------------------------

Defines a named parameter enabled free function in the current Python scope.

.. parsed-literal::

    template <class Fwd, class ParameterSpecs>
    void def(char const* name);

.. @ignore()

``def`` requirements 
~~~~~~~~~~~~~~~~~~~~

* ``ParameterSpecs`` is an `MPL sequence`_ where each element
  except the first models |ParameterSpec|. The first element
  is the result type of ``f(…)``, where ``f`` is the function.
* An instance of ``Fwd`` must support this expression:

  ====================================== ==================== =======================================
  Expression                             Return type          Requirements
  ====================================== ==================== =======================================
  ``fwd(boost::type<R>(), a0, …, aN)``   Convertible to ``R`` ``a0``\ …\ ``aN`` are tagged arguments.
  ====================================== ==================== =======================================

  For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``ParameterSpecs``.


Example
~~~~~~~

This example exports a function ``f(int x, int y = …)`` to Python. The
sequence of |ParameterSpec|'s ``mpl::vector2<tag::x(int), tag::y*(int)>`` has
an **arity range** of [2,2], so we only need one forwarding overload.

.. parsed-literal::

    BOOST_PARAMETER_FUNCTION((void), f, tag,
        (required (x, \*))
        (optional (y, \*, 1))
    )
    {
        *…*
    }

    struct f_fwd
    {
        template <class A0, class A1>
        void operator()(boost::type<void>, A0 const& a0, A1 const& a1)
        {
            f(a0, a1);
        }
    };

    BOOST_PYTHON_MODULE(…)
    {
        def<
            f_fwd
          , mpl::vector<
                void, tag::\ x(int), tag::\ y\*(int)
            >
        >("f");
    }

.. @ignore()

.. again, the undefined ``fwd`` identifier.

Portability
-----------

The Boost.Parameter Python binding library requires *partial template
specialization*.

Introduction

Tutorial

concept |ParameterSpec|

special keywords

class template init

class template `init`