Sunday, October 4, 2020

Desktop Software API's in Python (KiCAD, FreeCAD, Blender, QGIS)

Python wraps around everything

For the last couple of years I have mostly written Satellite Data Processing code in Python and plenty of Flask/Django web services. However Python is also an excellent automation tool for GUI based applications allowing custom plugins to be written and functionality provided out of the box extended by users.

The first desktop application I seriously looked at Python plugins for was QGIS. It was early days of learning how to wrap C++ code using SWIG/SIP etc. In the old mailing list you can find a much younger me making inane comments about mixing wrapper metaphors in QGIS with SWIG + SIP. We have come a long way since then and SIP based bindings are the mainstay of QGIS plugins.


QGIS has so many Python plugins that they need a registry of their own. Occasionally QGIS Python gets twisted around itself due to multiple Pythons in the user enviroment. You can also flip the python API around and instead of building a plugin you can turn QGIS into a custom desktop application. Which is what I have done with my basic Airport Viewer demo.

QGIS being a fairly extensive and complex C++ application which takes hours to compile, being able to make small quick changes in python is invaluable.


At the time of writing KiCAD has an extensive Python API for processing the automating the PCB layout part of the workflow and this has lead to many innovations in automating traditionally laborious hand layout or even performing complex simulations / optimization to set trace lengths. For example Josh Johnson has one for laying parts out in a circle and Greg Davill has several for length matching and rendering file generation. My personal favourite among the KiCAD scripts is the one for generation of Interactive BOM.

I am really looking forward to Python script support in the Schematic Editor. Meanwhile programmatic Schematic generation tools like Skidl provide schematic oriented Python fun.

The rendering of the PCB's is often done in Blender. Which has its own set of Python nicities.


My first foray in creating a Blender API based application was during the Kinect USB protocol hacking days. The data stream had just been decoded and I wanted an easy pipeline to a commonly installed / open-source 3D display software. The Python API is mature enough for people these days to quickly put together motion capture plugins for Blender. This plugin however demonstrates the challenges for creating native plugins for blender, the .pyd files for Python have to be recreated for different versions of Blender for ABI compaitibility.

Getting the binaries working has had me thrashing about and posting in forums, then sticking to a working Blender build with Python 2.7 for about 5 years since I did not want to touch it and break it. My integration actually reversed the embedding process, i.e. instead of using additional modules in the Blender embedded python I embedded Blender in a 3D GIS automation.

Native plugin weirdness aside, Blender Python API is a really powerful tool for creating procedural objects from waves / fluid simulation to astrophysics with amuse.


FreeCAD is sort of the third part of my physical electrical / mechnical design triumvirate. I occasionally design parts for KiCAD in FreeCAD, or bring multiple boards together to test enclosure fit. FreeCAD also has an extensive python library which is leveraged by KiCAD part library maintainers to parametrically generate parts.

The scripting in FreeCAD can be used much like the PCB layout scripts in KiCAD to create this with circular symmetry, like ball bearings which are difficult and repetitive to do by hand.

Final words

There are lots of other pieces of desktop software I have used that have started shipping with Python API's to address the never ending demand from users to easily automate repeated tasks. The live process for making this blogpost in somewhat recursive fashion can be found here.

I have even made videos withs a proprietary one, I will live that here for anyone interested in my attempts at a voiceover.