> But Python also looks up methods at runtime. Does that mean Python also does message passing? Not quite. I don't think Ruby's "message passing" is fundamentally different from Python's "method calls". Ultimately, both languages implementations are very similar: both look up methods by name in a hash table and then call them. IMO "message passing" is just an alternative metaphor for what Ruby does when you type `obj…
Except Ruby doesn't? cue `method_missing`. If you take only trivial examples you're not going to see much difference, this starts to show when you involve more advanced situations e.g with inheritance, and then you're drilling into singleton classes.
> Ruby immediately calls the method once it's found, whereas Python (generally) doesn't - instead it returns a binding to be called later
Again incorrect, `foo.bar` in Ruby and Python are two very fundamentally different things.
Python returns a binding to a method because it's an attribute accessor; when you throw inheritance into the mix it ends up that that attribute is inherited from the parent class, bound to the instance (which really in python means pass `self` as first argument), and - schematically - adding `()` after that ends up calling that bound method. If there's no attribute that's a no method error. It's all very C-ish and make believe, barely a notch above Go structs and their functions. The closest parallel in Ruby would be `foo.method(:bar).call()`
By contrast Ruby is going to send the :bar message along the inheritance chain, and if someone can respond it's going to invoke the responder's code, and surprise surprise method_missing happens only if it has exhausted inheritance but it's itself a method-slash-message; Oh and by the way the message passing is naturally so lazy that you can actually modify the inheritance chain -in flight- and inject a parent responder right before calling `super`. The whole notion of `binding` is a very concrete construct, way more rich that simply "hey I'm passing self as first argument". It becomes even more strange to "C&al. folks" when you start to involve singleton classes and start to realise weird things like Ruby classes are merely instances of the class Class and it's all instance turtles all the way down and all stupidly simple but you gotta have to wrap your head around it.
I surmise that so many differences and surprises have with Ruby are because most languages have some ALGOL legacy and Ruby is a conceptual heir to Smalltalk (and LISP†); the whole concept of open classes being another one: nothing is ever "finished" in Ruby!
Most of the time you don't have to care about these differences, until you do.
† While code isn't quite S-expr data in Ruby, there are more than enough first-class facilities that you can create and inject code entirely dynamically without resorting to `eval`ing strings.