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copied!<p>You can use Python <a href="http://docs.python.org/library/functions.html#property" rel="noreferrer">properties</a> to cleanly apply rules to each field separately, and enforce them even when client code tries to change the field:</p> <pre><code>class Spam(object): def __init__(self, description, value): self.description = description self.value = value @property def description(self): return self._description @description.setter def description(self, d): if not d: raise Exception("description cannot be empty") self._description = d @property def value(self): return self._value @value.setter def value(self, v): if not (v > 0): raise Exception("value must be greater than zero") self._value = v </code></pre> <p>An exception will be thrown on any attempt to violate the rules, even in the <code>__init__</code> function, in which case object construction will fail.</p> <p><strong>UPDATE:</strong> Sometime between 2010 and now, I learned about <code>operator.attrgetter</code>:</p> <pre><code>import operator class Spam(object): def __init__(self, description, value): self.description = description self.value = value description = property(operator.attrgetter('_description')) @description.setter def description(self, d): if not d: raise Exception("description cannot be empty") self._description = d value = property(operator.attrgetter('_value')) @value.setter def value(self, v): if not (v > 0): raise Exception("value must be greater than zero") self._value = v </code></pre>

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