As a point of interest, is get() considered an official part of the
mapping protocol, or just nice-to-have?
The docs don't seem to be very clear about that. There used to be
some tables listing the methods making up the core sequence and
mapping protocols, but I can't find them now. Have they been removed?
Are the ABCs now the definitions of the protocols?
I would also like to see an official answer to this. Are the mixin methods of the ABCs considered part of their contract? Is it documented somewhere? I think it should be.
On the one hand it sort of feels like the contract is just the abstract methods that users are supposed to implement themselves, and the mixin methods are just a convenient benefit of subclassing. It's hard to say where that feeling comes from, but I think the name "mixin method" is part of it. I'm glad to see I'm not the only one that gets this feeling.
On the other hand, if I have a variable of type Mapping, I'm usually going to assume it has a .get method that behaves like dict. Most tools will assume that too. If it doesn't, I'll be quite surprised and probably annoyed.