python global object cache

Question

Hello

Little question concerning app architecture:

I have a python script, running as a daemon.

Inside i have many objects, all inheriting from one class (let's name it 'entity')

I have also one main object, let it be 'topsys'

Entities are identified by pair (id, type (= class, roughly)), and they are connected in many wicked ways. They are also created and deleted all the time, and they are need to access other entities.

So, i need a kind of storage, basically dictionary of dictionaries (one for each type), holding all entities.

And the question is, what is better: attach this dictionary to 'topsys' as a object property or to class entity, as a property of the class? I would opt for the second (so entities does not need to know of existence of 'topsys'), but i am not feeling good about using properties directly in classes. Or maybe there is another way?

Answer 1

There is no problem with using properties on classes. Classes are just objects, too.

In your case, with this little information available, I would go for a class property, too, because not creating dependencies ist great and will be one worry less sometimes later.

Answer 2

There's not enough detail here to be certain of what's best, but in general I'd store the actual object registry as a module-level (global) variable in the top class, and have a method in the base class to access it.

_entities = []
class entity(object):
    @staticmethod
    def get_entity_registry(): 
         return _entities

Alternatively, hide _entites entirely and expose a few methods, eg. get_object_by_id, register_object, so you can change the storage of _entities itself more easily later on.

By the way, a tip in case you're not there already: you'll probably want to look into weakrefs (http://docs.python.org/library/weakref.html) when creating object registries like this.