For me it's annotations. Looked at from a distance, what appears to have happened is that the implementation hit a certain level of complexity before class met

For me it's annotations. Looked at from a distance, what appears to have happened is that the implementation hit a certain level of complexity before class methods got implemented, and it ended up being easier to do them with "@classmethod" than with "def self.foo", or to fix whatever reason it is that makes "def foo(a,b,c)" not act like a class method if "a" isn't "self".