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POUsing Python descriptors: How best to write a general validation getter-setter
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copied!<p>And by best, I mean both Pythonically and with as few lines of code as possible.</p> <p>I am used to defining classes with attributes in Python like so.</p> <pre><code>class MyClass(object): def __init__(self): self.value = 5 </code></pre> <p>Often, however, I find myself needing to ensure that <code>self.value</code> can only take a specific type of value. In the past (coming from a PHP OOP background) I've done this with a getter-setter method similar to this:</p> <pre><code> def value(self, new_value = -1) if new_value == -1: return self.value if isinstance(new_value, int) and new_value > -1: self.value = new_value else: raise TypeError </code></pre> <p>I then access <code>self.value</code> with <code>self.value()</code>, and set it with <code>self.value(67)</code>. Usually I'll name-mangle <code>self.value</code> to <code>self.__value</code> to discourage its direct access.</p> <p>I've recently discovered (or am in the process of discovering) descriptors. Following closely the <a href="http://martyalchin.com/2007/nov/24/python-descriptors-part-2-of-2/" rel="nofollow">excellent discussion here</a> I've written something that looks like this:</p> <pre><code>class ValidatedAttribute(object): def __init__(self, key, kind): self.key = key self.kind = kind def __get__(self, instance, owner): if self.key not in instance.__dict__: raise AttributeError, self.key return instance.__dict__[self.key] def __set__(self, instance, value): if not isinstance(value, self.kind): raise TypeError, self.kind instance.__dict__[self.key] = value class MyUsefulClass(object): a = ValidatedAttribute("a", int) </code></pre> <p>But I have a few questions!</p> <p>Firstly, what exactly is <code>MyUsefulClass().a</code>? Is it an instance of a ValidatedAttribute class somehow bound to an instance of a <code>MyUsefulClass</code> class?</p> <p>And secondly, what I'd really like is something like,</p> <pre><code>class MyUsefulClass(object): def __init__(self): self.value = Checker(int, 99) def Checker(self, kind, default): # Code for verifying that associated attribute is of type kind. </code></pre> <p>and somehow bind an attribute access to some sort of intercept method, without having to use a whole other descriptor class.</p> <p>I'm sorry if my understanding is lacking - I'm still getting to grips with new-style Python classes. Any help or pointers in the right direction would be much appreciated.</p>

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