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    <p>Python's core types are immutable by design, as other users have pointed out:</p> <pre><code>&gt;&gt;&gt; int.frobnicate = lambda self: whatever() Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; TypeError: can't set attributes of built-in/extension type 'int' </code></pre> <p>You certainly <em>could</em> achieve the effect you describe by making a subclass, since user-defined types in Python are mutable by default.</p> <pre><code>&gt;&gt;&gt; class MyInt(int): ... def frobnicate(self): ... print 'frobnicating %r' % self ... &gt;&gt;&gt; five = MyInt(5) &gt;&gt;&gt; five.frobnicate() frobnicating 5 &gt;&gt;&gt; five + 8 13 </code></pre> <p>There's no need to make the <code>MyInt</code> subclass public, either; one could just as well define it inline directly in the function or method that constructs the instance.</p> <p>There are certainly a few situations where Python programmers who are fluent in the idiom consider this sort of subclassing the right thing to do. For instance, <code>os.stat()</code> returns a <code>tuple</code> subclass that adds named members, precisely in order to address the sort of readability concern you refer to in your example.</p> <pre><code>&gt;&gt;&gt; import os &gt;&gt;&gt; st = os.stat('.') &gt;&gt;&gt; st (16877, 34996226, 65024L, 69, 1000, 1000, 4096, 1223697425, 1223699268, 1223699268) &gt;&gt;&gt; st[6] 4096 &gt;&gt;&gt; st.st_size 4096 </code></pre> <p>That said, in the specific example you give, I don't believe that subclassing <code>float</code> in <code>item.price</code> (or elsewhere) would be very likely to be considered the Pythonic thing to do. I <em>can</em> easily imagine somebody deciding to add a <code>price_should_equal()</code> method to <code>item</code> if that were the primary use case; if one were looking for something more general, perhaps it might make more sense to use named arguments to make the intended meaning clearer, as in</p> <pre><code>should_equal(observed=item.price, expected=19.99) </code></pre> <p>or something along those lines. It's a bit verbose, but no doubt it could be improved upon. A possible advantage to such an approach over Ruby-style monkey-patching is that <code>should_equal()</code> could easily perform its comparison on any type, not just <code>int</code> or <code>float</code>. But perhaps I'm getting too caught up in the details of the particular example that you happened to provide.</p>
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