[Python-Dev] PEP 557: Data Classes

[Martin Teichmann]

Hi list,

first, a big thanks to the authors of PEP 557! Great idea!

For me, the dataclasses were a typical example for inheritance, to be
more precise, for metaclasses. I was astonished to see them
implemented using decorators, and I was not the only one, citing
Guido:

> I think it would be useful to write 1-2 sentences about the problem with
> inheritance -- in that case you pretty much have to use a metaclass, and the
> use of a metaclass makes life harder for people who want to use their own
> metaclass (since metaclasses don't combine without some manual
> intervention).

Python is at a weird point here. At about every new release of Python,
a new idea shows up that could be easily solved using metaclasses, yet
every time we hesitate to use them, because of said necessary manual
intervention for metaclass combination.

So I think we have two options now: We could deprecate metaclasses,
going down routes like PEP 487's __init_subclass__. Unfortunately, for
data classes __init_subclass__ it is too late in the class creation
process for it to influence the __slots__ mechanism. A
__new_subclass__, that acts earlier, could do the job, but to me that
simply sounds like reinventing the wheel of metaclasses.

The other option would be to simply make metaclasses work properly. We
would just have to define a way to automatically combine metaclasses.
Guido once mention once (here:
https://mail.python.org/pipermail/python-dev/2017-June/148501.html)
that he left out automatic synthesis of combined metaclasses on
purpose, but given that this seems to be a major problem, I think it
is about time to overthink this decision.

So I propose to add such an automatic synthesis. My idea is that a
metaclass author can define the __or__ and __ror__ methods for
automatic metaclass synthesis. Then if a class C inherits from two
classes A and B with metaclasses MetaA and MetaB, the metaclass would
be MetaA | MetaB.

Greetings

Martin