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

Nick Coghlan ncoghlan at gmail.com
Thu Apr 2 04:39:01 CEST 2015

On 2 April 2015 at 07:35, PJ Eby <pje at telecommunity.com> wrote:

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.)

How hard is the requirement against relying on a mixin class or class decorator to request the defining class aware method decorator support? Is the main concern with the fact that failing to apply the right decorator/mixin at the class level becomes a potentially silent failure where the class aware method decorators aren't invoked properly?

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?

My preference at this point would definitely be to introduce a mixin class into the affected libraries and frameworks with an appropriate PEP 487 style init_subclass that was a noop in Python 2 (which would rely on metaclass injection instead), but implemented the necessary "defining class aware" method decorator support in Python 3.

The question of dynamically injecting additional base classes from the class body to allow the use of certain method decorators to imply specific class level behaviour could then be addressed as a separate proposal (e.g. making the case for an "append_mixins" attribute), rather than being linked directly to the question of how we going about defining inherited creation time behaviour without needing a custom metaclass.

Regards, Nick.

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

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