[Python-Dev] PEP 544: Protocols - second round

[Ivan Levkivskyi]

On 28 May 2017 at 16:13, Kevin Conway <kevinjacobconway at gmail.com> wrote:

> > Some of the possible options for the title are
> It seems like you're talking about something most other languages would
> refer to as "Interfaces". What is unique about this proposal that would
> call for not using the industry standard language?
>

Well, I would say there is no "industry standard language" about structural
subtyping. There are interfaces, protocols,
traits, mixins, typeclasses, roles, and probably some other terms I am not
aware of - all with subtly different semantics
in different languages. There are several reasons why we use term protocol
and not interface.
Two important reasons for me are:
* The term protocol is already de-facto standard in Python for things like
sequence protocol, iterator protocol,
  descriptor protocol, etc.
* Protocols are very different from Java interfaces in one important
aspect: they don't require explicit
  declaration of implementation, they are mainly oriented on duck-typing.
Maybe we need to add a short section to rejected ideas?

> Type-hints should not have runtime semantics, beyond those that they have
> as classes
> >  lots of code uses isinstance(obj, collections.abc.Iterable) and
> similar checks with other ABCs
> Having interfaces defined as something extended from abc doesn't
> necessitate their use at runtime, but it does open up a great deal of
> options for those of us who want to do so. I've been leveraging abc for a
> few years now to implement a lightweight version of what this PEP is
> attempting to achieve
>

IIUC this is not the main goal of the PEP, the main goal is to provide
support/standard for _static_ structural subtyping.
Possibility to use protocols in runtime context is rather a minor bonus
that exists mostly to provide a seamless transition
for projects that already use ABCs.

--
Ivan
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