[Python-Dev] Inheritance vs composition in backcompat (PEP521)

Koos Zevenhoven k7hoven at gmail.com
Mon Oct 2 18:19:50 EDT 2017

On Oct 3, 2017 01:11, "Koos Zevenhoven" <k7hoven at gmail.com> wrote:

On Oct 3, 2017 01:00, "Guido van Rossum" <guido at python.org> wrote:

 Mon, Oct 2, 2017 at 2:52 PM, Koos Zevenhoven <k7hoven at gmail.com> wrote

I don't mind this (or Nathaniel ;-) being academic. The backwards
> incompatibility issue I've just described applies to any extension via
> composition, if the underlying type/protocol grows new members (like the CM
> protocol would have gained __suspend__ and __resume__ in PEP521).

Since you seem to have a good grasp on this issue, does PEP 550 suffer from
the same problem? (Or PEP 555, for that matter? :-)

Neither has this particular issue, because they don't extend an existing
protocol. If this thread has any significance, it will most likely be

That said, I did come across this thought while trying to find flaws in my
own PEP ;)

-- Koos
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