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copied!<h2>Emulating numeric types in Python (<a href="http://docs.python.org/2/reference/datamodel.html#emulating-numeric-types" rel="nofollow">docs</a>)</h2> <p>When the expression <code>a + b</code> is evaluated, first <code>a</code> is checked an <code>__add__</code> implementation; if it has one, <code>a.__add__(b)</code> is executed. If it doesn't, or running <code>a.__add__(b)</code> returns the special value <code>NotImplemented</code>, then <code>b</code> is checked for an <code>__radd__</code> implementation; if it doesn't have one or it returns <code>NotImplemented</code>, you can't add them.</p> <h2>What's going on in your code</h2> <p>A reminder of our objects:</p> <pre><code>>>> a = adder(adder2(10)) >>> a.value adder2 >>> a.value.value 10 >>> b = adder(adder2(12)) >>> b.value adder2 b.value.value 12 </code></pre> <p><code>print a + b</code> could be described as:</p> <pre><code>a.__add__(b) which adder.__add__ implements as a.value.__add__(b) which adder2.__add__ implements as a.value.value + b so first we try a.value.value.__add__(b) but int.__add__ fails (returns NotImplemented) so we try b.__radd__(a.value.value) which adder.__radd__ implemented as b.value.__radd__(a.value.value) which adder2.__radd__ implements as a.value.value + b.value.value which int.__add__ implements as addition </code></pre> <h2>What to do about it</h2> <p>I think you're trying to create a type which can be added to numbers, or other objects of its type. If so, you want</p> <pre><code>class Adder(object): def __init__(self, value): self.value def __add__(self, other): return self.value + other def __radd__(self, other): return self.value + other </code></pre> <p>which is equivalent to the version without <code>+</code>:</p> <pre><code>class Adder(object): def __init__(self, value): self.value def __add__(self, other): r = self.value.__add__(other) if r is NotImplemented: r = other.__radd__(self.value) # so if other is a adder, use its __radd__ return r # since our value's __add__ will return __radd__ = __add__ </code></pre>

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