Question

When subclassing a base type, like float, is it possible to “recalculate” or “reassign” the original value? If I have the following class definition,

import collections

class MovingAverage(float):
    def __new__(self, initial_value, window):
        self.d = collections.deque([initial_value], window)
        return float.__new__(self, initial_value)
    def add(self, new_value):
        self.d.append(new_value)
        print sum(self.d) / len(self.d)
        # here, I want to _set_ the value of MovingAverage to
        #     sum(self.d) / len(self.d)

When I start with

>>> ma = MovingAverage(10, 3)
>>> ma
10.0

but

>>> ma.add(3)
6.5
>>> ma
10.0

The other class definition I’ve tried is this:

import collections

class MovingAverage(float):
    def __new__(self, iterable, window):
        self.d = collections.deque(iterable, window)
        initial_value = sum(iterable) / len(iterable)
        return float.__new__(self, initial_value)
    def add(self, new_value):
        self.d.append(new_value)
        return MovingAverage(self.d, self.d.maxlen)

This time, when I start with

>>> ma = MovingAverage([10], 3)
>>> ma
10.0

and

>>> ma.add(3)
6.5
>>> ma
10.0
>>> ma = ma.add(3)
>>> ma
5.333333333333333

However, I think (I haven’t tested to find out) it makes this significantly slower. So, can it be done? Can I somehow set it so that the return from ma is the value that I’m looking for? Or do I need to define a value attribute, change the base class to object, and abandon my pretense that I have a chance of controlling the return value of the class?

Answer 1

No. Since these types are immutable, you should be using encapsulation, not inheritance.