[Python-Dev] PEP 487 vs 422 (dynamic class decoration)

[PJ Eby]

On Thu, Apr 2, 2015 at 1:42 PM, PJ Eby <pje at telecommunity.com> wrote:
> If the PEP 487 metaclass library,
> however, were to just port some bits of my code to Python 3 this could
> be a done deal already and available in *all* versions of Python 3,
> not just the next one.

Just for the heck of it, here's an actual implementation and demo of
PEP 487, that I've tested with 3.1, 3.2, and 3.4 (I didn't have a copy
of 3.3 handy):

    https://gist.github.com/pjeby/75ca26f8d2a7a0c68e30

The first module is just a demo that shows the features in use.  The
second module is the implementation.

Notice that the actual *functionality* of PEP 487 is just *16 lines*
in Python 3...  including docstrings and an `__all__` definition.  ;-)

The other 90 lines of code are only there to implement the
`noconflict` feature for fixing metaclass conflicts... and quite a lot
of *those* lines are comments and docstrings.  ;-)

Anyway, I think this demo is a knockout argument for why PEP 487
doesn't need a language change: if you're writing an __init_subclass__
method you just include the `pep487.init_subclasses` base in your base
classes, and you're done.  It'll silently fail if you leave it out
(but you'll notice that right away), and it *won't* fail in
third-party subclasses because the *third party* didn't include it.

In contrast, PEP 422 provided a way to have both the features
contemplated by 487, *and* a way to allow method-level decorators to
discover the class at class creation time.  If there's going to be a
language change, it should include that latter feature from the
outset.