[Python-ideas] Structural type checking for PEP 484

[Jukka Lehtosalo]

On Wed, Sep 9, 2015 at 2:16 PM, Sven R. Kunze <srkunze at mail.de> wrote:

> Thanks for sharing, Guido. Some random thoughts:
>
> - "classes should need to be explicitly marked as protocols"
> If so, why are they classes in the first place? Other languages has
> dedicated keywords like "interface".
>

I want to preserve compatibility with earlier Python versions (down to
3.2), and this makes it impossible to add any new syntax. Also, there is no
need to add a keyword as there are other existing mechanisms which are good
enough, including base classes (as in the proposal) and class decorators. I
don't think that this will become a very commonly used language feature,
and thus adding special syntax for this doesn't seem very important. My
expectation is that structural subtyping would be primarily useful for
libraries and frameworks.

Jukka
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