Why not just have "a" return something containing "b"? In Python 3.8 for the brief syntax afforded by assignment expressions: def foo(o): if (b :=

Why not just have "a" return something containing "b"? In Python 3.8 for the brief syntax afforded by assignment expressions: def foo(o): if (b := o.a()) is not None: b() Because the original interface indicates that all "o"s must know about the link between "a" and "b", there's no loss of generality, but now there's a statically inferable structure: "o" must support an "a" that returns None or a callable. If you do a tiny modification to "def foo(o, a): ..." then this doesn't apply any more, and you're outside the realm of structural types and into the realm of dependent types, which means you must drink deeply of the static typing kool-aid and want to write type signatures that run the risk of being more complex than the code they describe. (But I like to quote Conor McBride: "if you're not going to write types your type system has to be stupid enough a computer can do it", from https://www.youtube.com/watch?v=3U3lV5VPmOU&feature=youtu.be&t=2369 https://www.youtube.com/watch?v=3U3lV5VPmOU&feature=youtu.be...).