I am writing a script for QGIS that will identify the perimeter of only external facing walls.

The script is probably a little long to post directly into this post, but it is accessible here.

The script works when I am in QGIS's script editor and hit run i.e. the dialog box comes up and all runs as expected.

However upon using the 'add script from file' button (Processing > Toolbox > Add script from file) I am greeted with the error:

The selected file does not contain a valid script 

Unfortunately I can't seem to get any break down or tracestack of why this is the case. I've tried examining the GitHub page for the function responsible but am unable to uncover any answers.