Object vs. Dictionary: how to organise a data tree?

Question

I am programming some kind of simulation with its data organised in a tree. The main object is World which holds a bunch of methods and a list of City objects. Each City object in turn has a bunch of methods and a list of Population objects. Population objects have no method of their own, they merely hold attributes.

My question regards the latter Population objects, which I can either derive from object or create as dictionaries. What is the most efficient way to organise these?

Here are few cases which illustrate my hesitation:

Saving the Data
I need to be able to save and load the simulation, for which purpose I use the built-in json (I want the data to be human readable). Because of the program is organised in a tree, saving data at each level can be cumbersome. In this case, the population is best kept as a dictionary appended to a population list as an attribute of a City instance. This way, saving is a mere matter of passing the City instance's __dict__ into Json.

Using the Data
If I want to manipulate the population data, it is easier as a class instance than as a dictionary. Not only is the syntax simple, but I can also enjoy introspection features better while coding.

Performance
I am not sure, finally, as to what is the most efficient in terms of resources. An object and a dictionary have little difference in the end, since each object has a __dict__ attribute, which can be used to access all its attributes. If i run my simulation with large numbers of City and Population objects, what will be using the less resources: objects or dictionaries?

So again, what is the most efficient way to organise data in a tree? Are dictionaries or objects preferable? Or is there any secret to organising the data trees?

Answer 1

Why not a hybrid dict/object?

class Population(dict):
    def __getattr__(self, key):
        return self[key]
    def __setattr__(self, key, value):
        self[key] = value

Now you can easily access known names via attributes (foo.bar), while still having the dict functionality to easily access unknown names, iterate over them, etc. without the clunky getattr/setattr syntax.

If you want to always initialize them with particular fields, you can add an __init__ method:

def __init__(self, starting=0, birthrate=100, imrate=10, emrate=10, deathrate=100):
     self.update(n=starting, b=birthrate, i=imrate, e=emrate, d=deathrate)