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

Martin Teichmann lkb.teichmann at gmail.com
Sat Oct 14 10:37:13 EDT 2017

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.

When thinking about how an automatic metaclass combiner would look like, I realized that it should ideally just reproduce the class mro, just with metaclasses. So if a class has an mro of [A, B, C, object], its metaclass should have an mro of unique_everseen([type(A), type(B), type(C), type]). But in this case, why add this layer at all? Just give the class the ability to do everything a metaclass could do, using mechanisms like @classmethod, and we're done.

Greetings

Martin

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