Python data structure for a collection of objects with random access based on an attribute

Question

I need a collection of objects which can be looked up by a certain (unique) attribute common to each of the objects. Right now I am using a dicitionary assigning the dictionary

Answer 1

There is actually no duplication of information as you fear: the dict's key, and the object's .key attribute, are just two references to exactly the same object.

The only real problem is "what if the .key gets reassigned". Well then, clearly you must use a property that updates all the relevant dicts as well as the instance's attribute; so each object must know all the dicts in which it may be enregistered. Ideally one would want to use weak references for the purpose, to avoid circular dependencies, but, alas, you can't take a weakref.ref (or proxy) to a dict. So, I'm using normal references here, instead (the alternative is not to use dict instances but e.g. some special subclass -- not handy).

def enregister(d, obj):
  obj.ds.append(d)
  d[obj.key] = obj

class Item(object):
    def __init__(self, uniq_key, title=None):
        self._key = uniq_key
        self.title = title
        self.ds = []

    def adjust_key(self, newkey):
        newds = [d for d in self.ds if self._key in d]
        for d in newds:
          del d[self._key]
          d[newkey] = self
        self.ds = newds
        self._key = newkey

    def get_key(self):
        return self._key

    key = property(get_key, adjust_key)

Edit: if you want a single collection with ALL the instances of Item, that's even easier, as you can make the collection a class-level attribute; indeed it can be a WeakValueDictionary to avoid erroneously keeping items alive, if that's what you need. I.e.:

class Item(object):

    all = weakref.WeakValueDictionary()

    def __init__(self, uniq_key, title=None):
        self._key = uniq_key
        self.title = title
        # here, if needed, you could check that the key
        # is not ALREADY present in self.all
        self.all[self._key] = self

    def adjust_key(self, newkey):
        # "key non-uniqueness" could be checked here too
        del self.all[self._key]
        self.all[newkey] = self
        self._key = newkey

    def get_key(self):
        return self._key

    key = property(get_key, adjust_key)

Now you can use Item.all['akey'], Item.all.get('akey'), for akey in Item.all:, and so forth -- all the rich functionality of dicts.