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    <p>Using a <a href="http://en.wikipedia.org/wiki/Factory_method_pattern" rel="nofollow">factory method</a> is the usual way to solve this, <em>especially</em> since instantiating a class is indistinguishable from calling a function in Python.</p> <p>However, if you <em>really</em> want, you can assign to <code>self.__class__</code>:</p> <pre><code>THRESHOLD = 1000 class Small(object): def __init__(self, n): if n &lt; THRESHOLD: self.n = n else: self.__class__ = Big self.__init__(n) class Big(object): def __init__(self, n): if n &lt; THRESHOLD: self.__class__ = Small self.__init__(n) else: self.n = n </code></pre> <p>This works as expected:</p> <pre><code>&gt;&gt;&gt; a = Small(100) &gt;&gt;&gt; type(a) &lt;class 'Small'&gt; &gt;&gt;&gt; b = Small(1234) &gt;&gt;&gt; type(b) &lt;class 'Big'&gt; &gt;&gt;&gt; c = Big(2) &gt;&gt;&gt; type(c) &lt;class 'Small'&gt; </code></pre> <p>If assigning to <code>self.__class__</code> seems too strange, then <a href="http://docs.python.org/reference/datamodel.html#object.__new__" rel="nofollow">you can override <code>__new__</code></a> instead. This method is called before <code>__init__</code> is called and it can be used to pick the class to instantiate:</p> <pre><code>THRESHOLD = 1000 class Switcher(object): def __new__(cls, n): if n &lt; THRESHOLD: new_cls = Small else: new_cls = Big instance = super(Switcher, new_cls).__new__(new_cls, n) if new_cls != cls: instance.__init__(n) return instance class Small(Switcher): def __init__(self, n): self.n = n class Big(Switcher): def __init__(self, n): self.n = n </code></pre>
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    1. USMartin Geisler
    1. USMartin Geisler
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    1. COThis approach is very cool, but it doesn't work 100% of the time: you can get "TypeError: \_\_class\_\_ assignment: 'X' object layout differs from 'Y'" from time to time, if your class bases aren't carefully chosen.
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      1. USRoman Susi
    2. CO@RomanSusi: good point! I can see [in the Python source](http://hg.python.org/cpython/file/443772a82dcd/Objects/typeobject.c#l3146) that you will get this error if the classes use different `__slots__`. This is also mentioned at the [bottom of this section](http://docs.python.org/reference/datamodel.html#slots) of the manual.
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      1. USMartin Geisler
    3. COI think I got many good answers to this question, but I loved this one! especially the first half. I think it's wonderful solution and works just like I want it to. Btw this works without (object) for python 3
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