Class factory in Python

Question

I\'m new to Python and need some advice implementing the scenario below.

I have two classes for managing domains at two different registrars. Both have the same inte

Answer 1

You can create a 'wrapper' class and overload its __new__() method to return instances of the specialized sub-classes, e.g.:

class Registrar(object):
    def __new__(self, domain):
        if ...:
            return RegistrarA(domain)
        elif ...:
            return RegistrarB(domain)
        else:
            raise Exception()

Additionally, in order to deal with non-mutually exclusive conditions, an issue that was raised in other answers, the first question to ask yourself is whether you want the wrapper class, which plays the role of a dispatcher, to govern the conditions, or it will delegate it to the specialized classes. I can suggest a shared mechanism, where the specialized classes define their own conditions, but the wrapper does the validation, like this (provided that each specialized class exposes a class method that verifies whether it is a registrar for a particular domain, is_registrar_for(...) as suggested in other answers):

class Registrar(object):
    registrars = [RegistrarA, RegistrarB]
    def __new__(self, domain):
        matched_registrars = [r for r in self.registrars if r.is_registrar_for(domain)]

        if len(matched_registrars) > 1:
            raise Exception('More than one registrar matched!')
        elif len(matched_registrars) < 1:
            raise Exception('No registrar was matched!')
        else:
            return matched_registrars[0](domain)