Question

I have several classes, and want to work with their collections like with a DB (or Django ORM, but simpler). Working with a DB would be a huge overhead, so I’d prefer having similar functionality in memory:

>>> node = Nodes.create(lat=X,lon=Y)
>>> node.id
1

>>> Nodes.all()
[<Node 1>]

>>> Nodes[1]  # by id
[<Node 1>]

>>> way_nodes = map(Nodes.create, ((X, Y), (Z, W)))
>>> way = Ways.create(way_nodes)
>>> way.nodes
[<Node 2>, <Node 3>]

>>> way.id
1

This is basically all what I need. Is there anything similar in Python or in custom packages?

If none, and I have to write my own, what’s the best choice to inherit from?

Answer 1

Those seem like pretty basic requirements, why not code them into your classes?

from itertools import count, starmap

class Node(object):
    nextid = count(1).next
    nodes = {}
    def __init__(self, lat, lon):
        self.lat, self.lon = lat, lon
        self.id = self.nextid()
        self.nodes[self.id] = self

    @classmethod
    def all(cls):
        return cls.nodes.values()

    def __repr__(self):
        return '<Node %s>' % self.id

    @classmethod
    def byid(cls, id):
        return cls.nodes[id]

class Way(object):
    nextid = count(1).next
    def __init__(self, nodes):
        self.nodes = nodes[:]
        self.id = self.nextid()

node = Node(lat='X',lon='Y')
print node.id
print Node.all()
print Node.byid(1)

way_nodes = list(starmap(Node, (('X', 'Y'), ('Z', 'W'))))
way = Way(way_nodes)
print way.nodes
print way.id