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592 lines
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ReStructuredText
Executable File
592 lines
18 KiB
ReStructuredText
Executable File
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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The Boost Parameter Library Python Binding Documentation
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+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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|(logo)|__
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.. |(logo)| image:: ../../../../boost.png
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:alt: Boost
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__ ../../../../index.htm
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:Authors: Daniel Wallin
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:Contact: dalwan01@student.umu.se
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:organization: `Boost Consulting`_
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:date: $Date$
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:copyright: Copyright David Abrahams, Daniel Wallin
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2005. Distributed under the Boost Software License,
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Version 1.0. (See accompanying file LICENSE_1_0.txt
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or copy at http://www.boost.org/LICENSE_1_0.txt)
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.. _`Boost Consulting`: http://www.boost-consulting.com
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.. role:: class
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:class: class
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.. role:: concept
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:class: concept
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.. role:: function
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:class: function
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.. |KeywordsSpec| replace:: :concept:`KeywordsSpec`
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.. contents::
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:depth: 1
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Introduction
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------------
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``boost/parameter/python.hpp`` introduces a group of def_visitors_ that can
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be used to easily expose Boost.Parameter-enabled member functions to Python with
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Boost.Python. It also provides a function template ``def()`` that can be used
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to expose Boost.Parameter-enabled free functions.
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.. _def_visitor: def_visitors_
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.. _def_visitors: ../../../python/doc/v2/def_visitor.html
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When binding a Boost.Parameter enabled function, the keyword tags
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must be specified. Additionally, because Boost.Parameter enabled
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functions are templates, the desired function signature must be
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specified.
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The keyword tags are specified as an `MPL Sequence`_, using the
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pointer qualifications described in |KeywordsSpec|_ below. The
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signature is also specifid as an `MPL sequence`_ of parameter
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types. Additionally, ``boost::parameter::python::function`` and
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``boost::parameter::python::def`` requires a class with forwarding
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overloads. We will take a closer look at how this is done in the
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tutorial section below.
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.. The last two sentences are terribly vague. Which namespace is
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.. ``function`` in? Isn't the return type always needed? What
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.. else are we going to do other than pass these sequences to
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.. function?
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.. _`MPL Sequence`: ../../../mpl/doc/refmanual/sequences.html
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.. _keywordsspec: `concept KeywordsSpec`_
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Tutorial
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--------
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In this section we will outline the steps needed to bind a simple
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Boost.Parameter-enabled member function to Python. Knowledge of the
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Boost.Parameter macros_ are required to understand this section.
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.. _macros: index.html
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The class and member function we are interested in binding looks
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like this::
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#include <boost/parameter/keyword.hpp>
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#include <boost/parameter/preprocessor.hpp>
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#include <boost/parameter/python.hpp>
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// First the keywords
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BOOST_PARAMETER_KEYWORD(tag, title)
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BOOST_PARAMETER_KEYWORD(tag, width)
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BOOST_PARAMETER_KEYWORD(tag, height)
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class window
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{
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public:
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BOOST_PARAMETER_MEMBER_FUNCTION(
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(void), open, tag,
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(required (title, (std::string)))
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(optional (width, (unsigned), 400)
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(height, (unsigned), 400))
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)
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{
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…
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}
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};
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It defines a set of overloaded member functions called ``open`` with one
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required parameter and two optional ones. To bind this member function to
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Python we use the binding utility ``boost::parameter::python::function``.
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``boost::parameter::python::function`` is a def_visitor_ that we'll instantiate
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and pass to ``boost::python::class_::def()``.
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To use ``boost::parameter::python::function`` we first need to define
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a class with forwarding overloads.
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::
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struct open_fwd
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{
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template <class A0, class A1, class A2>
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void operator()(
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boost::type<void>, window& self, A0 const& a0, A1 const& a1, A2 const& a2
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)
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{
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self.open(a0, a1, a2);
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}
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};
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The first parameter, ``boost::type<void>``, tells the forwarding overload
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what the return type should be. In this case we know that it's always void
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but in some cases, when we are exporting several specializations of a
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Boost.Parameter-enabled template, we need to use that parameter to
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deduce the return type.
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``window::open()`` takes a total of 3 parameters, so the forwarding function
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needs to take three parameters as well.
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.. Note::
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We only need one overload in the forwarding class, despite the
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fact that there are two optional parameters. There are special
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circumstances when several overload are needed; see
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`special keywords`_.
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Next we'll define the module and export the class:
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::
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BOOST_PYTHON_MODULE(my_module)
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{
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using namespace boost::python;
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namespace py = boost::parameter::python;
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class_<window>("window")
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.def(
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"open", py::function<
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open_fwd
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, mpl::vector<tag::title, tag::width*, tag::height*>
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, mpl::vector<void, std::string, unsigned, unsigned>
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>()
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);
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}
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.. @jam_prefix.append('import python ;')
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.. @jam_prefix.append('stage . : my_module /boost/python//boost_python ;')
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.. @my_module = build(
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output = 'my_module'
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, target_rule = 'python-extension'
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, input = '/boost/python//boost_python'
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, howmany = 'all'
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)
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.. @del jam_prefix[-1:]
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``py::function`` is passed three parameters. The first one is the class
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with forwarding overloads that we defined earlier. The second one is
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an `MPL Sequence`_ with the keyword tag types for the function. The
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pointer syntax means that the parameter is optional, so in this case
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``width`` and ``height`` are optional parameters. The third parameter
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is an `MPL Sequence`_ with the desired function signature. The return type comes first, and
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then the parameter types:
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.. parsed-literal::
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mpl::vector<void, std::string, unsigned, unsigned>
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*return type* *title* *width* *height*
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.. @ignore()
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That's it! This class can now be used in Python with the expected syntax::
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>>> w = my_module.window()
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>>> w.open(title = "foo", height = 20)
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.. @example.prepend('import my_module')
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.. @run_python(module_path = my_module)
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.. Sorry to say this at such a late date, but this syntax really
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.. strikes me as cumbersome. Couldn't we do something like:
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class_<window>("window")
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.def(
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"open",
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(void (*)(
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tag::title(std::string),
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tag::width*(unsigned),
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tag::height*(unsigned))
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)0
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);
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or at least:
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class_<window>("window")
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.def(
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"open",
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mpl::vector<
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void,
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tag::title(std::string),
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tag::width*(unsigned),
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tag::height*(unsigned)
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>()
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);
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assuming, that is, that we will have to repeat the tags (yes,
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users of broken compilers will have to give us function pointer
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types instead).
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------------------------------------------------------------------------------
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concept |KeywordsSpec|
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----------------------
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A |KeywordsSpec| is an `MPL sequence`_ where each element is either:
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* A *required* keyword of the form ``K``
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* **or**, an *optional* keyword of the form ``K*``
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* **or**, a *special* keyword of the form ``K**``
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where ``K`` is a keyword tag type, as used in a specialization
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of boost::parameter::keyword__.
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__ ../../../parameter/doc/html/reference.html#keyword
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The **arity range** of a |KeywordsSpec| is defined as the closed
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range:
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.. parsed-literal::
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[ mpl::size<S> - number of *special* keyword tags in ``S`` , mpl::size<S> ]
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For example, the **arity range** of ``mpl::vector2<x,y>`` is [2,2], the **arity range** of
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``mpl::vector2<x,y*>`` is [2,2] and the **arity range** of ``mpl::vector2<x,y**>`` is [1,2].
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.. Don't optional keywords affect the arity range?
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*special* keywords
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---------------------------------
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Sometimes it is desirable to have a default value for a parameter that differ
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in type from the parameter. This technique is useful for doing simple tag-dispatching
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based on the presence of a parameter. An example_ of this is given in the Boost.Parameter
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docs. The example uses a different technique, but could also have been written like this:
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.. parsed-literal::
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template <class ArgumentPack>
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void dfs_dispatch(ArgumentPack& args, mpl::false\_)
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{
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*…compute and use default color map…*
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}
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template <class ArgumentPack, class ColorMap>
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void dfs_dispatch(ArgumentPack& args, ColorMap colormap)
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{
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*…use colormap…*
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}
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template <class ArgumentPack>
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void depth_first_search(ArgumentPack& args)
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{
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core::dfs_dispatch(args, args[color | mpl::false_()]);
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}
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.. _example: index.html#dispatching-based-on-the-presence-of-a-default
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In the above example the type of the default for ``color`` is ``mpl::false_``, a
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type that is distinct from any color map that the user might supply.
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When binding the case outlined above, the default type for ``color`` will not
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be convertible to the parameter type. Therefore we need to tag the ``color``
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keyword as a *special* keyword. By doing this we tell the binding functions
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that it needs to generate two overloads, one with the ``color`` parameter
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present and one without. Had there been two *special* keywords, four
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overloads would need to be generated. The number of generated overloads is
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equal to ``2^N``, where ``N`` is the number of *special* keywords.
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------------------------------------------------------------------------------
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class template ``init``
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-----------------------
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Defines a named parameter enabled constructor.
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.. parsed-literal::
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template <class Keywords, class Signature>
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struct init : python::def_visitor<init<Keywords, Signature> >
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{
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template <class Class>
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void def(Class& class\_);
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};
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``init`` requirements
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~~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence of parameter types,
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in the order dictated by ``Keywords``.
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* For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity
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range** of ``Keywords``, ``Class`` must support these
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expressions:
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======================= ============= =========================================
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Expression Return type Requirements
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======================= ============= =========================================
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``Class(a0, ..., aN)`` \- ``a0``..\ ``aN`` are tagged arguments.
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======================= ============= =========================================
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.. Limit the width of these table cells. Some rst backend
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.. processors actually produce different results depending on the
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.. distribution of width.
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Example
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~~~~~~~
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.. parsed-literal::
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struct base { /\* ... \*/ };
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class X : base
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{
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public:
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BOOST_PARAMETER_CONSTRUCTOR(X, (base),
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(required (x, \*))
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(optional (y, \*))
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)
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def(
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init<
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector2<int, int>
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>()
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);
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}
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------------------------------------------------------------------------------
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class template ``call``
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-----------------------
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Defines a ``__call__`` operator, mapped to ``operator()`` in C++.
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.. parsed-literal::
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template <class Keywords, class Signature>
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struct call : python::def_visitor<call<Keywords, Signature> >
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{
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template <class Class>
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void def(Class& class\_);
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};
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``call`` requirements
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~~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence with the types of the keyword parameters,
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in the order dictated by ``Keywords``, and the return type prepended.
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* ``Class`` must support these expressions, where ``c`` is an instance of ``Class``:
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=================== ==================== =======================================
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Expression Return type Requirements
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=================== ==================== =======================================
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``c(a0, ..., aN)`` Convertible to ``R`` ``a0``..\ ``aN`` are tagged arguments.
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=================== ==================== =======================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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Example
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~~~~~~~
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.. parsed-literal::
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typedef parameter::parameters<
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parameter::required<tag::x>
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, parameter::optional<tag::y>
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> call_parameters;
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class X
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{
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public:
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template <class Args>
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int call_impl(Args const& args)
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{
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/\* ... \*/
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}
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template <class A0>
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int operator()(A0 const& a0)
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{
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return call_impl(call_parameters()(a0));
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}
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template <class A0, class A1>
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int operator()(A0 const& a0, A1 const& a1)
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{
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return call_impl(call_parameters()(a0,a1));
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}
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def("f",
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call<
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector3<int, int, int>
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>()
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);
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}
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------------------------------------------------------------------------------
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class template ``function``
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---------------------------
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Defines a named parameter enabled member function.
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.. parsed-literal::
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template <class Fwd, class Keywords, class Signature>
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struct function : python::def_visitor<function<Fwd, Keywords, Signature> >
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{
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template <class Class, class Options>
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void def(Class& class\_, char const* name, Options const& options);
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};
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``function`` requirements
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~~~~~~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence with the types of the keyword parameters,
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in the order dictated by ``Keywords``, and the return type prepended.
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* An instance of ``Fwd`` must support this expression:
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============================================ ==================== ==============================================
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Expression Return type Requirements
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============================================ ==================== ==============================================
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``fwd(boost::type<R>(), self, a0, …, aN)`` Convertible to ``R`` ``self`` is a reference to the object on which
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the function should be invoked. ``a0``…``aN``
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are tagged arguments.
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============================================ ==================== ==============================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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Example
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~~~~~~~
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This example exports a member function ``f(int x, int y = …)`` to Python.
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The |KeywordsSpec| ``mpl::vector2<tag::x, tag::y*>`` has an **arity range**
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of [2,2], so we only need one forwarding overload.
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.. parsed-literal::
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class X
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{
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public:
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BOOST_PARAMETER_MEMBER_FUNCTION((void), f, tag,
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(required (x, \*))
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(optional (y, \*))
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)
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{
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/\* … \*/
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}
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};
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|
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struct f_fwd
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{
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template <class A0, class A1>
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void operator()(boost::type<void>, X& self, A0 const& a0, A1 const& a1)
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{
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self.f(a0, a1);
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}
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def("f",
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function<
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f_fwd
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector3<void, int, int>
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>()
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);
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}
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.. This example is not consistent with your definition of arity
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.. range, above. There are no special keywords in play here.
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------------------------------------------------------------------------------
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function template ``def``
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-------------------------
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Defines a named parameter enabled free function in the current Python scope.
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.. parsed-literal::
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template <class Fwd, class Keywords, class Signature>
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void def(char const* name);
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``def`` requirements
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~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence of parameters types,
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in the order dictated by ``Keywords``, with the return type
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prepended.
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* An instance of ``Fwd`` must support this expression:
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|
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====================================== ==================== ======================================
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Expression Return type Requirements
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====================================== ==================== ======================================
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``fwd(boost::type<R>(), a0, …, aN)`` Convertible to ``R`` ``a0``…``aN`` are tagged arguments.
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====================================== ==================== ======================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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|
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Example
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~~~~~~~
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This example exports a function ``f(int x, int y = …)`` to Python.
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The |KeywordsSpec| ``mpl::vector2<tag::x, tag::y*>`` has an **arity range**
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of [2,2], so we only need one forwarding overload.
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|
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.. parsed-literal::
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BOOST_PARAMETER_FUNCTION((void), f, tag,
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(required (x, \*))
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(optional (y, \*))
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)
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{
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/\* … \*/
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}
|
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|
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struct f_fwd
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{
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template <class A0, class A1>
|
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void operator()(boost::type<void>, A0 const& a0, A1 const& a1)
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{
|
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f(a0, a1);
|
|
}
|
|
};
|
|
|
|
BOOST_PYTHON_MODULE(…)
|
|
{
|
|
def<
|
|
f_fwd
|
|
, mpl::vector2<tag::x, tag::y\*>
|
|
, mpl::vector3<void, int, int>
|
|
>("f");
|
|
}
|
|
|
|
.. again, the undefined ``fwd`` identifier.
|
|
|
|
Portability
|
|
-----------
|
|
|
|
The Boost.Parameter Python binding library requires *partial template specialization*.
|
|
|
|
.. Oh. In that case, we don't have to worry so much about
|
|
.. compilers that can't parse function types.
|