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PODifferent instance-method behavior between Python 2.5 and 2.6
 

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  1. PODifferent instance-method behavior between Python 2.5 and 2.6
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    <p>Trying to change the <code>__unicode__</code> method on an instance after it's created produces different results on Python 2.5 and 2.6.</p> <p>Here's a test script:</p> <pre><code>class Dummy(object): def __unicode__(self): return u'one' def two(self): return u'two' d = Dummy() print unicode(d) d.__unicode__ = d.two print unicode(d) print d.__unicode__() </code></pre> <p>On Python 2.5, this produces</p> <pre><code>one two two </code></pre> <p>That is, changing the instance's <code>__unicode__</code> also changes <code>unicode(instance)</code></p> <p>On Python 2.6, this produces</p> <pre><code>one one two </code></pre> <p>So, after a change, <code>unicode(instance)</code> and <code>instance.__unicode__()</code> return different results.</p> <p>Why? How can I get this working on Python 2.6?</p> <p>(For what it's worth, the use case here is that I want to append something to the output of <code>__unicode__</code> for all subclasses of a given class, without having to modify the code of the subclasses.)</p> <p><strong>Edit to make the use case a little clearer</strong></p> <p>I have Class A, which has many subclasses. Those subclasses define simple <code>__unicode__</code> methods. I want to add logic so that, for instances of a Class A subclass, unicode(instance) gets something tacked on to the end. To keep the code simple, and because there are many subclasses I don't want to change, I'd prefer to avoid editing subclass code.</p> <p>This is actually existing code that works in Python 2.5. It's something like this:</p> <pre><code>class A(object): def __init__(self): self._original_unicode = self.__unicode__ self.__unicode__ = self.augmented_unicode def augmented_unicode(self): return self._original_unicode() + u' EXTRA' </code></pre> <p>It's this code that no longer works on 2.6. Any suggestions on how to achieve this without modifying subclass code? (If the answer involves metaclasses, note that class A is itself a subclass of another class -- django.db.models.Model -- with a pretty elaborate metaclass.)</p>
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    1. COLooking at the Python 2.6 release notes, the item that seems like it might be causing this is a method cache on type objects --http://bugs.python.org/issue1700288 -- but if that is indeed the culprit, how do I clear the cache?
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