FYI: As far as I know, Python scripts do not have the capability to address Building material properties. This is because the return property value function requires a elementId and Building materials are a attribute and are addressed with a attributeId.
However, I have added the capability to my data_access.apx via the CommandHandler function. The Command is 'GetBMAtt' And the first and only parameter is the name of the Building Material.
I have posted the apx and a sample Python file on my GitHub site -- Release Nine
GitHub site for data_access release AddOn
If you have any questions or comments , just leave them below.
Windows 10 - Visual Studio 2019; ArchiCAD 26