On Mon, Oct 2, 2017 at 6:42 AM, Guido van Rossum <gu...@python.org> wrote:
> On Sun, Oct 1, 2017 at 1:52 PM, Koos Zevenhoven <k7ho...@gmail.com> wrote:
>
>> On Oct 1, 2017 19:26, "Guido van Rossum" <gu...@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
>
--
+ Koos Zevenhoven + http://twitter.com/k7hoven +
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