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

[Nick Coghlan]

On 4 October 2017 at 20:22, Koos Zevenhoven <k7hoven at gmail.com> wrote:
> On Wed, Oct 4, 2017 at 8:07 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> On 3 October 2017 at 03:13, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>> > 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.
>>
>> This is a known problem, and one of the main reasons that having a
>> truly transparent object proxy like
>> https://wrapt.readthedocs.io/en/latest/wrappers.html#object-proxy as
>> part of the standard library would be highly desirable.
>>
>
> This is barely related to the problem I describe. The wrapper is not
> supposed to pretend to *be* the underlying object. It's just supposed to
> extend its functionality.

If a wrapper *isn't* trying to act as a transparent object proxy, and
is instead adapting it to a particular protocol, then yes, you'll need
to update the wrapper when the protocol is extended.

That's not a backwards compatibility problem, because the only way to
encounter it is to update your code to rely on the new extended
protocol - your *existing* code will continue to work fine, since
that, by definition, can't be relying on the new protocol extension.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia