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

[PJ Eby]

I recently got an inquiry from some of my users about porting some of
my libraries to Python 3 that make use of the Python 2 __metaclass__
facility.  While checking up on the status of PEP 422 today, I found
out about its recently proposed replacement, PEP 487.

While PEP 487 is a generally fine PEP, it actually *rules out* the
specific use case that I wanted PEP 422 for in the first place:
dynamic addition of callbacks or decorators for use at class creation
time without requiring explicit inheritance or metaclass
participation.  (So that e.g. method decorators can access the
enclosing class at class definition time.)

As discussed previously prior to the creation of PEP 422, it is not
possible to port certain features of my libraries to work on Python 3
without some form of that ability, and the only thing that I know of
that could even *potentially* provide that ability outside of PEP 422
is monkeypatching __build_class__ (which might not even work).  That
is, the very thing that PEP 422 was created to avoid the need for.
;-)

One possible alteration would be to replace __init_subclass__ with
some sort of __init_class__ invoked on the class that provides it, not
just subclasses.  That would allow the kind of dynamic decoration that
PEP 422 allows.  However, this approach was rather specifically ruled
out in earlier consideration of PEP 422, so....

Another alternative would be to have the default __init_subclass__
look at a class-level __decorators__ attribute, as originally
discussed for PEP 422.  That would solve *my* problem, but feels too
much like adding more than One Way To Do It.

So...  honestly, I'm not sure where to go from here.  Is there any
chance that this is going to be changed, or revert to the PEP 422
approach, or...  something?  If so, what Python version will the
"something" be in?  Or is this use case just going to be a dead parrot
in Python 3, period?