pet.py

#!/usr/bin/env python

"""Playing with pets.



http://stackoverflow.com/questions/1427479/managing-object-instances/1429344#1429344

"""





class Pet(object):

    """A pet with a name.



    """

    

    def __new__(cls, name):

        """Create a pet with a `name`.



        """

        return '<%s name="%s" xmlns="http://gist.github.com/187588"/>' % (

            cls.__name__,  name.replace('"', '\"'))



    name2pet = {} # combined register of all pets



    @classmethod

    def get(cls, name):

        """Return a pet with `name`.



        Try to get the pet registered by `name` 

        otherwise register a new pet and return it

        """

        register = cls.name2pet

        try: return register[name]

        except KeyError:

            pet = register[name] = cls(name) # no duplicates allowed

            Pet.name2pet.setdefault(name, []).append(pet)

            return pet

        



class Cat(Pet):

    name2pet = {} # each class has its own registry





class Dog(Pet):

    name2pet = {}





def test():

    assert eval(repr(Cat("Teal'c"))) == Cat("Teal'c")



    pets = [Pet('a'), Cat('a'), Dog('a'), Cat.get('a'), Dog.get('a')]

    assert all('name="a"' in pet for pet in pets)



    cat, dog = Cat.get('pet'), Dog.get('pet')

    assert cat == '<Cat name="pet" xmlns="http://gist.github.com/187588"/>'

    assert dog == '<Dog name="pet" xmlns="http://gist.github.com/187588"/>'

    assert dog is not cat

    assert dog != cat    

    

    assert all(v == [Cat(k), Dog(k)]

               for k, v in Pet.name2pet.items()), Pet.name2pet



    try: assert 0

    except AssertionError:

        return "OK"

    raise AssertionError("Assertions must be enabled during the test")



    

if __name__=="__main__":

    print test()