Wrapping a Cplusplus class in Python

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Background

FreeCAD uses a custom XML-based system to create the Python wrapper for a C++ class. To wrap a C++ class for use in Python, two files must be manually created, and two files are automatically generated by the CMake build system (in addition to the C++ header and implementation files for the class).

You must create:

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

Edit the appropriate CMakeLists.txt file to add references to these two files. From the XML file, the build system will then create:

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

Class Description XML File

The XML file [YourClass]Py.xml provides information about the functions and attributes that the Python class implements, as well as the user documentation for those items that displays in the FreeCAD Python console.

For this example, we will look at the wrapper for the Axis C++ class. The XML description file begins with: {{Code|lang=xml|code= <?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 And 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);