Hmm., I was playing with it and I actually do not see a way to add custom formats
It's somewhat obscure. Archivarius could do with a bit of overhaul in the usability department, but it's simple once you know where to look
Start creating a new index and follow the wizard to the "Select documents for indexing" page. This is where you pick known file formats. Un
check the "Inherit settings" at bottom to enable the list. Select "plain text" here, and add anything else you need. Click Next and arrive at "Select file name masks". Again, un
check the "Inherit settings". There are several lists of extensions here, some very long, in desperate need of filters or incremenetal search, but *.py is not in any of them. So just check "Custom extensions", expand it, and you'll see "Add extension (double click)
". Take it from there.
Or, to make things quicker in the long run, in the main window click More -> Settings -> File extensions, and add anything you need. That way you won't have to uncheck the "Inherit settings" whenever defining a new index.