Question

I have the following scenario:

I have abstract classes A and B, and A uses B to perform some tasks. In both classes, there are some “constant” parameters (right now implemented as class attributes) to be set by concrete classes extending those abstract classes, and some of the parameters are shared (they should have the same value in a derived “suite” of classes SubA and SubB).

The problem I face here is with how namespaces are organized in Python. The ideal solution if Python had dynamic scoping would be to declare those parameters as module variables, and then when creating a new suite of extending classes I could just overwrite them in their new module. But (luckily, because for most cases that is safer and more convenient) Python does not work like that.

To put it in a more concrete context (not my actual problem, and of course not accurate nor realistic), imagine something like a nbody simulator with:

 ATTRACTION_CONSTANT = NotImplemented # could be G or a Ke for example

 class NbodyGroup(object):
     def __init__(self):
         self.bodies = []

     def step(self):
         for a in self.bodies:
             for b in self.bodies:
                 f = ATTRACTION_CONSTANT * a.var * b.var / distance(a, b)**2
                 ...

 class Body(object):
     def calculate_field_at_surface(self):
         return ATTRACTION_CONSTANT * self.var / self.r**2

Then other module could implement a PlanetarySystem(NBodyGroup) and Planet(Body) setting ATTRACTION_CONSTANT to 6.67384E-11 and other module could implement MolecularAggregate(NBodyGroup) and Particle(Body) and set ATTRACTION_CONSTANT to 8.987E9.

In brief: what are good alternatives to emulate global constants at module level that can be “overwritten” in derived modules (modules that implement the abstract classes defined in the first module)?

Answer 1

How about using a mixin? You could define (based on your example) classes for PlanetarySystemConstants and MolecularAggregateConstants that hold the ATTRACTION_CONSTANT and then use class PlanetarySystem(NBodyGroup, PlanetarySystemConstants) and class MolecularAggregate(NBodyGroup, MolecularAggregateConstants) to define those classes.