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    <p>The answer is found in the two types of class that Python has.</p> <p>The first code-snippet you provided uses a legacy "old-style" class (you can tell because it doesn't subclass anything - there's nothing before the colon). Its semantics are peculiar. In particular, you can add a special method to an instance:</p> <pre><code>class Foo: def __init__(self, num): self.num = num def _fn(other): return self.num + other.num self.__add__ = _fn </code></pre> <p>and get a valid response:</p> <pre><code>&gt;&gt;&gt; f = Foo(2) &gt;&gt;&gt; g = Foo(1) &gt;&gt;&gt; f + g 3 </code></pre> <p>But, subclassing <code>dict</code> means you are generating a new-style class. And the semantics of operator overloading are different:</p> <pre><code>class Foo (object): def __init__(self, num): self.num = num def _fn(other): return self.num + other.num self.__add__ = _fn &gt;&gt;&gt; f = Foo(2) &gt;&gt;&gt; g = Foo(1) &gt;&gt;&gt; f + g Traceback ... TypeError: unsupported operand type(s) for +: 'Foo' and 'Foo' </code></pre> <p>To make this work with new-style classes (which includes subclasses of <code>dict</code> or just about any other type you will find), you have to make sure the special method is defined on the class. You can do this through a metaclass:</p> <pre><code>class _MetaFoo(type): def __init__(cls, name, bases, args): def _fn(self, other): return self.num + other.num cls.__add__ = _fn class Foo(object): __metaclass__ = _MetaFoo def __init__(self, num): self.num = num &gt;&gt;&gt; f = Foo(2) &gt;&gt;&gt; g = Foo(1) &gt;&gt;&gt; f+g 3 </code></pre> <p>Also, the semantic difference means that in the very first case I could define my local add method with one argument (the <code>self</code> it uses is captured from the surrounding scope in which it is defined), but with new-style classes, Python expects to pass in both values explicitly, so the inner function has two arguments.</p> <p>As a previous commenter mentioned, best to avoid old-style classes if possible and stick with new-style classes (old-style classes are removed in Python 3+). Its unfortunate that the old-style classes happened to work for you in this case, where new-style classes will require more code.</p> <hr> <p>Edit: </p> <p>You can also do this more in the way you originally tried by setting the method on the <em>class</em> rather than the <em>instance</em>:</p> <pre><code>class Foo(object): def __init__(self, num): self.num = num setattr(Foo, '__add__', (lambda self, other: self.num + other.num)) &gt;&gt;&gt; f = Foo(2) &gt;&gt;&gt; g = Foo(1) &gt;&gt;&gt; f+g 3 </code></pre> <p>I'm afraid I sometimes think in Metaclasses, where simpler solutions would be better :)</p>
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    1. COThanks. This explanation is what I needed. It also made it easy for me to accomplish what I wanted the WRONG way, too by only changing one line in the original code. :P I'm sure people will flame the crap out of me for that.
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    2. COYes, exactly, sorry I missed that simpler version first time round!
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    3. COFlame? Well, it may not be 'pythonic', but if it ships the code, hey...
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