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

Koos Zevenhoven k7hoven at gmail.com
Mon Oct 2 13:13:54 EDT 2017

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.

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.

––Koos

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