[Python-Dev] PEP 246, redux

[Roeland Rengelink]

I'm trying to understand the relation between Guido's posts on optional 
  static typing and PEP 245 (interfaces) and 246 (adaptation). I have a 
  couple of questions

PEP 245 proposes to introduce a fundamental distinction between type and 
  interface. However, 245 only introduces a syntax for interfaces, and 
says very little about the semantics of interfaces. (Basically only that 
  if X implements Y then implements(X, Y) will return True). The 
semantics of interfaces are currently only implied by PEP 246, and by 
Guido's  posts referring to 246.

Unfortunately PEP 246 explicitly refuses to decide that protocols are 
245-style interfaces. Therefore, it is not clear to me how acceptance of 
  245 would impact on 246?  Specifically, what would be the difference 
between:

x = adapt(obj, a_245_style_interface)

x = adapt(obj, a_protocol_type)

and, if there is no difference, what would the use-case of interfaces be?

Put another way: explicit interfaces and adaptation based typing seem to 
  be about introducing rigor (dynamic, not static) to Python. Yet, PEP 
245 and 246 seems to go out of their way to give interfaces and 
adaptation as little baggage as possible. So, where is the rigor going 
to come from?

On the one hand this seems very Pythonic - introduce a new feature with 
as little baggage as possible, and see where it evolves from there. Let 
the rigor flow, not from the restrictions of the language, but from the 
  expressive power of the language.

On the other hand: why not, at least:

- explore in 245 how the semantics of interfaces might introduce rigor 
into the language. It would be particularly illuminating to find out in 
what way implementing an interface differs from deriving from an ABC 
and  in what way an interface hierarchy differs semantically from a 
hierarchy  of ABCs

- rewrite 246 under the assumption that 245 (including semantics) has 
been accepted

I would volunteer, but, for those of you who hadn't noticed yet, I don't 
know what I'm talking about.

Cheers,

Roeland Rengelink