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