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FreeCAD Developers Handbook

A handbook about FreeCAD development

Create a Python Binding for C++ #

Background #

It becomes necessary at times to expand the FreeCAD API further by exposing functions that are available in the source code in c++ to the python. This process is generally referred to as creating a binding.

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:

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

Class Description XML File #

The XML file YourClassPy.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 Base::Axis C++ class. The XML description file begins with:

<GenerateModel xmlns:xsi##"" xsi:noNamespaceSchemaLocation##"generateMetaModel_Module.xsd">
        <Author Licence##"LGPL" Name##"Juergen Riegel" EMail##"" />
        <UserDocu>User documentation here
          <!-- Lengthy comments removed. See source file. -->
        <DeveloperDocu>Developer documentation here</DeveloperDocu>

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

<Methode Name##"move">
        Move the axis base along the vector

The format of an attribute is:

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

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


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

Implementation C++ File #

The implementation C++ file YourClassPyImp.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 incode 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 [YourClassPy.xml)
 #include "YourClassPy.h"
 #include "YourClassPy.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 incode|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(),
        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:

Another Explanation #

The basic structure of a program to expose functionality to Python is something like this:

There are two source files required to implement a new Python binding. Assuming we wanted to expose some methods from, we would need to make:

These 2 files need to be added to ../Mod/yourModule/App/CMakeLists.txt. See and FreeCAD/src/Mod/Part/App/CMakeLists.txt for examples.

You can extend the Python version of your module by adding to ../Mod/yourModule/App/AppmyModulePy.cpp (see and FreeCAD/src/Mod/Part/AppPartPy.cpp]). The additions are accessed in Python by “import myModule”.


There is a convention for return values from our C++/Python connections:

See also #