Exposing Cplusplus to Python

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How to expose c++ functionality to Python

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. In so doing, providing an ability to trigger deep internal functionality with Python in real-time instead of needing to compile.

Below is an explanation of how one can achieve this.

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

  • get the Py object parameters and convert them to c++ variables using PyArg_ParseTuple(),
  • use various c++ routines from OpenCascade and/or FreeCAD to produce the desired result,
  • convert the c++ result into Py object using routines like PyLong_AsLong(), Py::asObject(), etc,
  • return the Py object.

Further Elaboration

There are two source files required to implement a new Python binding. Assuming we wanted to expose some methods from FreeCAD/src/Mod/Part/TopoShape.cpp, we would need to make:

  • TopoShapePy.xml - definitions of the functions for be exposed in XML format. This file is used to generate header files for our next file...
  • TopoShapePyImp.cpp - the actual C++ code that bridges from Python to C++.

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

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


  1. xxxxxPyImp routines return a PyObject* pointer (see TopoShapePyImp.cpp)
  2. AppmyModulePy.cpp routines return a Py::Object (see FreeCAD/src/Mod/Part/AppPartPy.cpp).

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