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    <p>Because <code>int</code> is a immutable, you cannot magically turn it into a mutable type.</p> <p>Your methods are no-ops. They change <code>self</code> in the local namespace, by reassigning it to something else. They no longer point to the instance. In other words, both methods leave the original instance <em>unaltered</em>.</p> <p>You cannot do what you want to do with a subclass of <code>int</code>. You'll have to create a custom class from scratch using the <a href="http://docs.python.org/2/reference/datamodel.html#emulating-numeric-types" rel="nofollow">numeric type hooks</a> instead.</p> <p>Background: <code>int</code> has a <code>__new__</code> method that assigns the actual value to <code>self</code>, and there is no <code>__iadd__</code> method to support in-place adding. The <code>__init__</code> method is not ignored, but you can leave it out altogether since <code>__new__</code> already did the work.</p> <p>Assigning to <code>self</code> means you just replaced the reference to the instance with something else, you didn't <em>alter</em> anything about <code>self</code>:</p> <pre><code>&gt;&gt;&gt; class Foo(int): ... def __init__(self, value=0): ... print self, type(self) ... self = 'foobar' ... print type(self) ... &gt;&gt;&gt; foo = Foo(10) 10 &lt;class '__main__.Foo'&gt; &lt;type 'str'&gt; &gt;&gt;&gt; print foo, type(foo) 10 &lt;class '__main__.Foo'&gt; </code></pre> <p>Because <code>int</code> does not have a <code>__iadd__</code> in-place add method, your <code>self += 2</code> is interpreted as <code>self = self + 2</code> instead; again, you are assigning to <code>self</code> and replacing it with a new value altogether.</p>
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    1. COI see. So it only works for things like lists and such? So if I wanted to have a function that modified it, I would have to call it like this? foo = foo.dostuff() and just have the dostuff method returning a new value
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      1. USGreg Hornby
    2. CO@GregHornby: No, you need to do something inside the method `dostuff()` that assigns to *attributes* of `self` or calls methods *on* `self`, not assign to `self` itself.
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    3. COExactly like that; but `int` objects have no such attribute I am afraid.
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