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  1. PO
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    <p>Initially I've supplied an answer which is based on changing <code>self.__class__</code> during <code>__init__()</code>. It was explained to me in the comments that this should be avoided.</p> <p>Now I am providing an answer that that is based on redefining the base class' <code>__new__</code>:</p> <pre><code>class Car(object): """The base class.""" def __new__(cls, max_speed): if cls is Car and max_speed &gt; 100: return object.__new__(SuperCar, max_speed) else: return object.__new__(cls, max_speed) def __init__(self, max_speed): self.max_speed = max_speed class SuperCar(Car): """The sub class.""" pass </code></pre> <p>The code responsible for the creation of an instance is in the built in(1) <code>__new__</code> method. It is called when a class is called (e.g. <code>my_car = Car(100)</code>) and passed the arguments that were given in that call.</p> <p>When the instanciation of a <code>Car</code> or a <code>SuperCar</code> (due to the inheritance of <code>__new__</code>) is called, the redefined <code>__new__</code> checks whether the instance to be created is of <code>cls</code> <code>Car</code> and also has a <code>max_speed</code> greater than 100.</p> <p>If so, it creates an instance of <code>SuperCar</code>, instead of <code>Car</code>, using <code>object.__new__(SuperCar, max_speed)</code> and returns that.</p> <p>Otherwise, it calls upon the normal, default, built in, <code>__new__</code> for the creation of this instance, using <code>object.__new__(cls, max_speed)</code> and returns that. This catches the expected case where <code>not max_speed &gt; 100</code> and also the case where this redefined <code>__new__</code> is called by the subclass <code>SuperCar</code> (like in <code>my_ferrari = SuperCar(250)</code>).</p> <p>For the record, here's my former answer:</p> <blockquote> <p>When you want an instance's class to be changed to one of it's subclasses, depending on some condition, upon creation of that instance, you can use some logic in <code>__init__</code>, like so:</p> <pre><code>class Car(object): def __init__(self, max_speed_kph): if self.__class__ == Car: if max_speed_kph &gt; 230: self.__class__ = SuperCar self.__init__() class SuperCar(Car): def __init__(self): print 'this car is fast!' </code></pre> <p>The <code>self.__init__()</code> is for re-initialisation of the instance as it's new class.</p> </blockquote> <p>Footnotes:</p> <ol> <li>Is this really a built-in? I can't see it in the "Built-in functions" topic of Python Documentation.</li> </ol>
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    1. POCreate a subclass, instead, when calling the creation of a class, depending on some logic
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    1. USmightyiam
    1. USmightyiam
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    1. COChecking `self.__class__.__name__` is a really bad idea. Why not just check `self.__class__ == Car`?
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    2. COAnd your second half is also a bad idea. Typically you expect the base class `__init__` to call the parent class, but in this case that would cause infinite recursion. If you really want to construct a `SuperCar`, just construct a `SuperCar`; don't construct a `Car`, turn it into a `SuperCar`, and then manually `__init__` it. The only reason re-classing really makes sense is when you want to _avoid_ the subclass `__init__`.
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      1. USabarnert
    3. CO@abarnert I've edited to check `self.__class__` and not `self.__class__.__name__` . Can you please rephrase 'Typically you expect the base class `__init__` to call the parent class' for me? I don't get it because `__init__` is not a class.
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