[Python-ideas] A way out of Meta-hell (was: A (meta)class algebra)

[Martin Teichmann]

Hi,

>>> Yes, populating __class__ happens inside the interpreter only after the
>>> class is fully constructed, so even though the *class* is fully initialised
>>> at the end of __init__, a reference hasn't been inserted into the cell yet.
>>> (The __class__ cell doesn't actually live on the class object - it's created
>>> implicitly by the interpreter and then referenced from the individual
>>> methods as a closure variable for each method that looks up "__class__" or
>>> "super")
>>>
>>> Python 2 doesn't provide the implicitly populated __class___ cell at all
>>> though, so you have to refer to the class object by its name - zero argument
>>> super is a Python 3 only feature.
>>
>> In short: PEP 422 in the current form is not back-portable.
>> With my modifications it is portable all the way down to python 2.7.
>>
>> PEP 422 being back-portable => people will immediately start using it.
>> PEP 422 not back-portable => it won't be used for several years to come.
>
> You haven't really explained why you think it's not backportable.
> You've suggested a backport may require some workarounds in order to
> handle cooperative multiple inheritance, but that's not even close to
> being the same thing as not being able to backported at all.

Well, your explanation up there is actually very good at explaining why
it isn't back portable: at the time of metaclass.__init__ the __class__ has
not been initialized yet, so cooperative multiple inheritance will never
work. This holds even true for the two parameter version of super():
as the __class__ has not yet been bound to anything, there is no way
to access it from within __class_init__. You say that some workarounds
would have to be done to handle cooperative multiple inheritance, yet
given that there is no way to tamper with __class__ I cannot see how
such a workaround should look like.

I would love to be proven wrong on the above statements, as I
actually like PEP 422. But back-portability, in my opinion, is actually
very important. So, maybe someone out there manages to write a
nice (or even ugly) backport of PEP 422?

> That said, if you'd like to write a competing proposal for the
> community's consideration, please go ahead (details on the process are
> in PEP 1)

I'll write something.

Greetings

Martin