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POComparison of object with built-in type
 

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    <p>I have a class called <code>IntField</code> which encapsulates a integer from a database (not really relevant). I'd like to use an instance of <code>IntField</code> for evaluating expressions using <code>eval</code>. </p> <p>The classes looks like:</p> <pre><code>class DbField(object): def __init__(self, value): self.value = value def __cmp__(self, other): print "comparing {} ({}) with {} ({})".format(self, type(self), other, type(other)) if type(self.value) == type(other): return self.value.__cmp__(other) elif type(self) == type(other): return self.value.__cmp__(other.value) raise ValueError("cannot compare {} and {}".format(type(self), type(other))) class IntField(DbField): def __init__(self, value): super(IntField, self).__init__(int(value)) a = IntField(-2) b = IntField(-2) print "a=", a print "b=", b print "b == -1 ", b == -1 print "b == a ", b == a print "a == b ", a == b print "-1 == b ", -1 == b </code></pre> <p>What I want to achieve in the base class is to allow the custom object to be comparable with another custom object of the same type, but also with the builtin type; I want to be able to do <code>IntField == IntField</code> or <code>IntField == int</code>. </p> <p>I expect that <code>IntField.__cmp__()</code> of the first object to be called for these comparisons, which is happening. But I'm being puzzled about what happens for <code>int == IntField</code>. Seems that in this case, the <code>IntField.__cmp__()</code> method is also being called, instead of <code>int</code>'s . Can someone explain how the comparison against built-in types works? </p>
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    1. COOk, it seems I found the answer, it's been asked before: http://stackoverflow.com/questions/2281222/why-when-in-python-does-x-y-call-y-eq-x
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