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POCan I change the base class value in a python subclass?
 

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  1. POCan I change the base class value in a python subclass?
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    <p>When subclassing a base type, like float, is it possible to "recalculate" or "reassign" the original value? If I have the following class definition,</p> <pre><code>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) </code></pre> <p>When I start with</p> <pre><code>&gt;&gt;&gt; ma = MovingAverage(10, 3) &gt;&gt;&gt; ma 10.0 </code></pre> <p>but</p> <pre><code>&gt;&gt;&gt; ma.add(3) 6.5 &gt;&gt;&gt; ma 10.0 </code></pre> <p>The other class definition I've tried is this:</p> <pre><code>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) </code></pre> <p>This time, when I start with</p> <pre><code>&gt;&gt;&gt; ma = MovingAverage([10], 3) &gt;&gt;&gt; ma 10.0 </code></pre> <p>and</p> <pre><code>&gt;&gt;&gt; ma.add(3) 6.5 &gt;&gt;&gt; ma 10.0 &gt;&gt;&gt; ma = ma.add(3) &gt;&gt;&gt; ma 5.333333333333333 </code></pre> <p>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 <code>ma</code> is the value that I'm looking for? Or do I need to define a <code>value</code> attribute, change the base class to <code>object</code>, and abandon my pretense that I have a chance of controlling the return value of the class?</p>
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    1. USJames C
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    1. COsubclassing from float for a moving average is B.A.D.
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      1. USfabrizioM
    2. COI don't think I've ever seen a case where it's actually a good idea to subclass a numerical type in Python.
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      1. USGlenn Maynard
 

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