Metaclass magic registry pattern

The Registry Pattern is something I use relatively frequently. In django, for instance, we see it used for the admin interface, and I used very derivative code for my first API generation tool: django-rest-api. For our integration with external POS and other systems, we need to register importers, so that the automated stats fetching is able to look for units that need to fetch data from an external system’s website, or parse incoming email headers for matching delivered data.

I had been using something similar to:

from base import BaseStatsImporter, register

class FooStatsImporter(BaseStatsImporter):
    # ...

register(FooStatsImporter)

This is all well and good, but it is annoying. I need to remember to register each class after I declare it.

Then I discovered the magic of __metaclass_, used with __new__:

class RegistryMetaClass(type):
    def __new__(cls, clsname, bases, attrs):
        new_class = super(cls, RegistryMetaClass).__new__(cls, clsname, bases, attrs)
        register(new_class)
        return new_class
        
class BaseStatsImporter(object):
    __metaclass__ = RegistryMetaClass
    
    # ...

As long as your subclasses don’t override __metaclass__, then every new subclass will be added to the registry.

Obviously, this is magic, and in some cases the explicit way would be better.

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