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

Koos Zevenhoven k7hoven at gmail.com
Mon Oct 2 17:52:21 EDT 2017

On Oct 3, 2017 00:02, "Guido van Rossum" <guido at python.org> wrote:

On Mon, Oct 2, 2017 at 10:13 AM, Koos Zevenhoven <k7hoven at gmail.com> wrote:

> Hi all, It was suggested that I start a new thread, because the other
> thread drifted away from its original topic. So here, in case someone is
> interested:
> On Oct 2, 2017 17:03, "Koos Zevenhoven <k7hoven at gmail.com> wrote:
> On Mon, Oct 2, 2017 at 6:42 AM, Guido van Rossum <guido at python.org> wrote:
> On Sun, Oct 1, 2017 at 1:52 PM, Koos Zevenhoven <k7hoven at gmail.com> wrote:
> On Oct 1, 2017 19:26, "Guido van Rossum" <guido at python.org> wrote:
> Your PEP is currently incomplete. If you don't finish it, it is not even a
> contender. But TBH it's not my favorite anyway, so you could also just
> withdraw it.
> I can withdraw it if you ask me to, but I don't want to withdraw it
> without any reason. I haven't changed my mind about the big picture. OTOH,
> PEP 521 is elegant and could be used to implement PEP 555, but 521 is
> almost certainly less performant and has some problems regarding context
> manager wrappers that use composition instead of inheritance.
> It is my understanding that PEP 521 (which proposes to add optional
> __suspend__ and __resume__ methods to the context manager protocol, to be
> called whenever a frame is suspended or resumed inside a `with` block) is
> no longer a contender because it would be way too slow. I haven't read it
> recently or thought about it, so I don't know what the second issue you
> mention is about (though it's presumably about the `yield` in a context
> manager implemented using a generator decorated with
> `@contextlib.contextmanager`).
> ​Well, it's not completely unrelated to that. The problem I'm talking
> about is perhaps most easily seen from a simple context manager wrapper
> that uses composition instead of inheritance:
> class Wrapper:
>     def __init__(self):
>         self._wrapped = SomeContextManager()
>     def __enter__(self):
>         print("Entering context")
>         return self._wrapped.__enter__()
>     def __exit__(self):
>         self._wrapped.__exit__()
>         print("Exited context")
> Now, if the wrapped contextmanager becomes a PEP 521 one with __suspend__
> and __resume__, the Wrapper class is broken, because it does not respect
> __suspend__ and __resume__. So actually this is a backwards compatiblity
> issue.
Why is it backwards incompatible? I'd think that without PEP 521 it would
be broken in exactly the same way because there's no __suspend__/__resume__
at all.

The wrapper is (would be) broken because it depends on the internal
implementation of the wrapped CM.

Maybe the author of SomeContextManager wants to upgrade the CM to also work
in coroutines and generators. But it could be a more subtle change in the
CM implementation.

The problem becomes more serious and more obvious if you don't know which
context manager you are wrapping:

class Wrapper:
    def __init__(self, contextmanager):
        self._wrapped = contextmanager

    def __enter__(self):
        print("Entering context")
        return self._wrapped.__enter__()

    def __exit__(self):
        print("Exited context")

The wrapper is (would be) broken because it does not work for all CMs

But if the wrapper is made using inheritance, the problem goes away:
> class Wrapper(SomeContextManager):
>     def __enter__(self):
>         print("Entering context")
>         return super().__enter__()
>     def __exit__(self):
>         super().__exit__()
>         print("Exited context")
> Now the wrapper cleanly inherits the new optional __suspend__ and
> __resume__ from the wrapped context manager type.
In any case this is completely academic because PEP 521 is not going to
happen. Nathaniel himself has said so (I think in the context of discussing
PEP 550).

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).

-- Koos (mobile)
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