I had forgotten that decorators could take parameters. Something like that pretty much ticks the boxes for me.

I'd _prefer_ something that sits inside the method body rather than just outside it, and I'd probably _prefer_ something that wasn't quite so heavyweight at runtime (which may be an irrational concern on my part ;)),

but those aren't deal breakers, depending on the project - and the vast majority of what I do in Python is short-lived one-off projects and rapid prototyping for later implementation in another language, so I do seem to be fleshing out a set of classes from scratch and writing a bunch of __init__ methods far more of the time than people with long-lived projects would do. Perhaps that's why it irritates me more than it does some others ;)