Encapsuler une classe Cplusplus dans Python

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Contexte

FreeCAD utilise un système personnalisé basé sur XML pour créer le wrapper Python d'une classe C++. Pour encapsuler une classe C++ afin de l'utiliser dans Python, deux fichiers doivent être créés manuellement, et deux fichiers sont automatiquement générés par le système de construction CMake (en plus des fichiers d'en-tête et d'implémentation C++ de la classe).

Vous devez créer :

  • [YourClass]Py.xml
  • [YourClass]PyImp.cpp

Editez le fichier approprié CMakeLists.txt pour ajouter des références à ces deux fichiers. A partir du fichier XML, le système de construction créera alors :

  • [YourClass]Py.cpp
  • [YourClass]Py.h

Fichier XML de description des classes

Le fichier XML [YourClass]Py.xml fournit des informations sur les fonctions et attributs que la classe Python implémente ainsi que la documentation utilisateur pour ces éléments qui s'affiche dans la Console Python de FreeCAD.

For this example, we will look at the wrapper for the Axis C++ class. The XML description file begins:

<?xml version="1.0" encoding="UTF-8"?>
<GenerateModel xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="generateMetaModel_Module.xsd">
	<PythonExport
		Father="PyObjectBase"
		Name="AxisPy"
		Twin="Axis"
		TwinPointer="Axis"
		Include="Base/Axis.h"
		FatherInclude="Base/PyObjectBase.h"
		Namespace="Base"
		Constructor="true"
		Delete="true"
		FatherNamespace="Base">
	<Documentation>
		<Author Licence="LGPL" Name="Juergen Riegel" EMail="FreeCAD@juergen-riegel.net" />
		<UserDocu>Axis
An defines a direction and a position (base) in 3D space.

The following constructors are supported:
Axis() -- empty constructor
Axis(Axis) -- copy constructor
Axis(Base, Direction) -- define position and direction
		</UserDocu>
		<DeveloperDocu>Axis</DeveloperDocu>
	</Documentation>

Following this preamble, a list of methods and attributes is given. The format of a method is:

<Methode Name="move">
      <Documentation>
        <UserDocu>
        move(Vector)
        Move the axis base along the vector
        </UserDocu>
      </Documentation>
    </Methode>

The format of an attribute is:

<Attribute Name="Direction" ReadOnly="false">
      <Documentation>
        <UserDocu>Direction vector of the Axis</UserDocu>
      </Documentation>
      <Parameter Name="Direction" Type="Object" />
    </Attribute>

For an attribute, if "ReadOnly" is false, you will provide both a getter and a setter function. If it is true, only a getter is allowed. In this case we will be required to provide two functions in the implementation C++ file:

Py::Object AxisPy::getDirection(void) const

and

void AxisPy::setDirection(Py::Object arg)

Implementation Cplusplus File

The implementation C++ file [YourClass]PyImp.cpp provides the "glue" that connects the C++ and Python structures together, effectively translating from one language to the other. The FreeCAD C++-to-Python system provides a number of C++ classes that map to their corresponding Python type. The most fundamental of these is the Py::Object class -- rarely created directly, this class provides the base of the inheritance tree, and is used as the return type for any function that is returning Python data.

Include Files

Your C++ implementation file will include the following files:

#include "PreCompiled.h"

#include "[YourClass].h"

// Inclusion of the generated files (generated out of [YourClass]Py.xml)
#include "[YourClass]Py.h"
#include "[YourClass]Py.cpp"

Of course, you may include whatever other C++ headers your code requires to function as well.

Constructor

Your C++ implementation must contain the definition of the PyInit function: for example, for the Axis class wrapper, this is

int AxisPy::PyInit(PyObject* args, PyObject* /*kwd*/)

Within this function you will most likely need to parse incoming arguments to the constructor: the most important function for this purpose is the Python-provided PyArg_ParseTuple. It takes in the passed argument list, a descriptor for the expected arguments that it should parse, and type information and storage locations for the parsed results. For example:

PyObject* d;
    if (PyArg_ParseTuple(args, "O!O", &(Base::VectorPy::Type), &o,
                                      &(Base::VectorPy::Type), &d)) {
        // NOTE: The first parameter defines the base (origin) and the second the direction.
        *getAxisPtr() = Base::Axis(static_cast<Base::VectorPy*>(o)->value(),
                                   static_cast<Base::VectorPy*>(d)->value());
        return 0;
    }

For a complete list of format specifiers see Python C API documentation. Note that several related functions are also defined which allow the use of keywords, etc. The complete set is:

PyAPI_FUNC(int) PyArg_Parse (PyObject *, const char *, ...);
PyAPI_FUNC(int) PyArg_ParseTuple (PyObject *, const char *, ...);
PyAPI_FUNC(int) PyArg_ParseTupleAndKeywords (PyObject *, PyObject *, const char *, char **, ...);
PyAPI_FUNC(int) PyArg_VaParse (PyObject *, const char *, va_list);
PyAPI_FUNC(int) PyArg_VaParseTupleAndKeywords (PyObject *, PyObject *, const char *, char **, va_list);