Demonstrates how to use meta-classes to build a registry of sub-classes.

registrable.py

# -*- coding: utf-8 -*-
#
# registrable.py
#
"""Demonstrates how to use meta-classes to build a registry of sub-classes."""

__all__ = [
    'Registrable',
    'RegistrableMeta',
    'BlueColor',
    'GreenColor',
    'YellowColor',
    'RedColor',
    'BlackColor'
]

import collections


class RegistrableMeta(type):
    """Meta-class for registrable and sub-classes.

    Holds a registry of all sub-classes.

    """
    registry = collections.OrderedDict()

    def __init__(cls, clsname, bases, classdict):
        cls.registry[cls.tag] = cls


Registrable = RegistrableMeta('Registrable', (object,), {'tag': None})


class BlueColor(Registrable):
    tag = 'blue'
    rgb = (0, 0, 255)

class GreenColor(Registrable):
    tag = 'green'
    rgb = (0, 255, 0)

class YellowColor(Registrable):
    tag = 'yellow'
    rgb = (0, 255, 255)

class RedColor(Registrable):
    tag = 'red'
    rgb = (255, 0, 0)

class BlackColor(Registrable):
    tag = 'black'
    rgb = (0, 0, 0)


if __name__ == '__main__':
    cls = Registrable.registry.get('blue')

    if cls:
        instance = cls()
        print(instance.rgb)
    else:
        print("No class found for tag 'blue'.")