[Python-Dev] Intention to accept PEP 552 soon (deterministic pyc files)

Koos Zevenhoven k7hoven at gmail.com
Mon Oct 2 10:03:17 EDT 2017

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):
        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

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):
        print("Exited context")

Now the wrapper cleanly inherits the new optional __suspend__ and
__resume__ from the wrapped context manager type.



+ Koos Zevenhoven + http://twitter.com/k7hoven +
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