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PODescriptors as instance attributes in python
 

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    <p>To the question:</p> <blockquote> <p>Why can't descriptors be instance attributes?</p> </blockquote> <p>it has been <a href="https://stackoverflow.com/questions/2954331/dynamically-adding-property-in-python">answered</a> that:</p> <blockquote> <p>descriptor objects needs to live in the class, not in the instance</p> </blockquote> <p>because that is the way that the <code>__getattribute__</code> is implemented.</p> <p>A simple example. Consider a descriptor:</p> <pre><code>class Prop(object): def __get__(self, obj, objtype=None): if obj is None: return self return obj._value * obj._multiplier def __set__(self, obj, value): if obj is None: return self obj._value = value class Obj(object): val = Prop() def __init__(self): self._value = 1 self._multiplier = 0 </code></pre> <p>Consider the case in which each obj has multiple Prop: I would need to use unique names to identify the values and multipliers (Like <a href="https://stackoverflow.com/questions/2954331/dynamically-adding-property-in-python">here</a>. Having a per instance descriptor object would allow to store the <code>_multiplier</code> (and the <code>_value</code>) in the descriptor itself, simplifying a few things.</p> <p>To implement per instance descriptor attributes you need to either:</p> <ol> <li>create a per instance class <a href="https://stackoverflow.com/questions/2954331/dynamically-adding-property-in-python">See here</a></li> <li>override <code>__getattribute__</code> <a href="https://stackoverflow.com/questions/10232174/can-a-python-descriptor-be-used-to-instantiate-an-attribute-in-the-init-of-a">See here</a></li> </ol> <p>I am aware that similar questions have been raised before, but I have not found a real explanation:</p> <ol> <li>Why Python is designed this way?</li> <li>What is the suggested way to store information that the descriptor needs but is per instance?</li> </ol>
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