[Python-Dev] What is the design purpose of metaclasses vs code generating decorators? (was Re: PEP 557: Data Classes) (original) (raw)

Ronald Oussoren ronaldoussoren at mac.com
Fri Oct 20 11:48:18 EDT 2017

On 14 Oct 2017, at 16:37, Martin Teichmann <lkb.teichmann at gmail.com> wrote:

Things that will not work if Enum does not have a metaclass:

list(EnumClass) -> list of enum members dir(EnumClass) -> custom list of "interesting" items len(EnumClass) -> number of members member in EnumClass -> True or False - protection from adding, deleting, and changing members - guards against reusing the same name twice - possible to have properties and members with the same name (i.e. "value" and "name") In current Python this is true. But if we would go down the route of PEP 560 (which I just found, I wasn't involved in its discussion), then we could just add all the needed functionality to classes. I would do it slightly different than proposed in PEP 560: classmethods are very similar to methods on a metaclass. They are just not called by the special method machinery. I propose that the following is possible:

class Spam: ... @classmethod ... def getitem(self, item): ... return "Ham" Spam[3] Ham this should solve most of your usecases.

Except when you want to implement getitem for instances as well :-). An important difference between @classmethod and methods on the metaclass is that @classmethod methods live in the same namespace as instance methods, while methods on the metaclass don’t.

I ran into similar problems in PyObjC: Apple’s Cocoa libraries use instance and class methods with the same name. That when using methods on a metaclass, but not when using something similar to @classmethod. Because of this PyObjC is a heavy user of metaclasses (generated from C for additional fun). A major disadvantage of this is that tends to confuse smart editors.

Ronald

More information about the Python-Dev mailing list