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    <p>Assuming the other answers are correct, and overriding all the methods is the only way to do this, here's my attempt at a moderately elegant way of doing this. If more instance variables are added, only one piece of code needs to change. Unfortunately if a new binary operator is added to the set object, this code will break, but I don't think there's a way to avoid that. Comments welcome!</p> <pre><code>def foocopy(f): def cf(self, new): r = f(self, new) r.foo = self.foo return r return cf class Fooset(set): def __init__(self, s = []): set.__init__(self, s) if isinstance(s, Fooset): self.foo = s.foo else: self.foo = 'default' def copy(self): x = set.copy(self) x.foo = self.foo return x @foocopy def __and__(self, x): return set.__and__(self, x) @foocopy def __or__(self, x): return set.__or__(self, x) @foocopy def __rand__(self, x): return set.__rand__(self, x) @foocopy def __ror__(self, x): return set.__ror__(self, x) @foocopy def __rsub__(self, x): return set.__rsub__(self, x) @foocopy def __rxor__(self, x): return set.__rxor__(self, x) @foocopy def __sub__(self, x): return set.__sub__(self, x) @foocopy def __xor__(self, x): return set.__xor__(self, x) @foocopy def difference(self, x): return set.difference(self, x) @foocopy def intersection(self, x): return set.intersection(self, x) @foocopy def symmetric_difference(self, x): return set.symmetric_difference(self, x) @foocopy def union(self, x): return set.union(self, x) f = Fooset([1,2,4]) f.foo = 'bar' assert( (f | f).foo == 'bar') </code></pre>
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    1. POWhat is the correct (or best) way to subclass the Python set class, adding a new instance variable?
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    1. USrog
    1. USrog
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    1. COYou've got some infinite recursion in the copy method. x = self.copy() should be x = super(Fooset,self).copy()
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      1. USJohn Montgomery
    2. COyes, you're right. is using super() better than explicitly mentioning the superclass?
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      1. USrog
 

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