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    <p>Python performs no implicit calls to the <code>__init__</code> methods of a class's super-class(es)—but it's possible to make it happen automatically. One way is by defining a metaclass for your mixed class(es) that creates or extends the mixed class's <code>__init__</code> method so that it calls to all the listed bases' <code>__init__</code> functions in the order they were listed.</p> <p>A second way is to do it is to use a class decorator—which is shown below in the section subtitled <strong>Edit</strong>.</p> <p>Using a metaclass:</p> <pre><code>class APIBaseClassOne(object): # API class (can't be changed) def __init__(self, *args, **kwargs): print ' APIBaseClassOne.__init__()' class SomeMixin(object): def __init__(self, *args, **kwargs): print ' SomeMixin.__init__()' class MixedClassMeta(type): def __new__(cls, name, bases, classdict): classinit = classdict.get('__init__') # Possibly None. # define an __init__ function for the new class def __init__(self, *args, **kwargs): # call the __init__ functions of all the bases for base in type(self).__bases__: base.__init__(self, *args, **kwargs) # also call any __init__ function that was in the new class if classinit: classinit(self, *args, **kwargs) # add the local function to the new class classdict['__init__'] = __init__ return type.__new__(cls, name, bases, classdict) class MixedClass(APIBaseClassOne, SomeMixin): __metaclass__ = MixedClassMeta # important # if exists, is called after the __init__'s of all the direct bases def __init__(self, *args, **kwargs): print ' MixedClass.__init__()' print('MixedClass():') MixedClass() </code></pre> <p>Output:</p> <pre class="lang-none prettyprint-override"><code>MixedClass(): APIBaseClassOne.__init__() SomeMixin.__init__() MixedClass.__init__() </code></pre> <p><strong>Edit</strong></p> <p>Here's how to accomplish the same thing with a class decorator (requires Python 2.6+):</p> <pre><code>class APIBaseClassOne(object): # API class (can't be changed) def __init__(self, *args, **kwargs): print)' APIBaseClassOne.__init__()') class SomeMixin(object): def __init__(self, *args, **kwargs): print(' SomeMixin.__init__()') def mixedomatic(cls): """ Mixed-in class decorator. """ classinit = cls.__dict__.get('__init__') # Possibly None. # define an __init__ function for the class def __init__(self, *args, **kwargs): # call the __init__ functions of all the bases for base in cls.__bases__: base.__init__(self, *args, **kwargs) # also call any __init__ function that was in the class if classinit: classinit(self, *args, **kwargs) # make the local function the class's __init__ setattr(cls, '__init__', __init__) return cls @mixedomatic class MixedClass(APIBaseClassOne, SomeMixin): # if exists, is called after the __init__'s of all the direct base classes def __init__(self, *args, **kwargs): print(' MixedClass.__init__()') print('MixedClass():') MixedClass() </code></pre> <p>(For Python &lt; 2.6, use <code>MixedClass = mixedomatic(MixedClass)</code> <em>following</em> the class definition.)</p>
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    1. POAre Mixin class __init__ functions not automatically called?
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    1. COThanks for demonstrating this. While the discussion in the other answer has made me rethink whether its a good idea to begin with, this implementation works and I've learned a lot.
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    2. CO@Ben: Yes, mixins are somewhat controversial, and even considered harmful by some, plus using them in Python is somewhat complicated by the fact that it doesn't call base class constructors by default (unlike, say, C++ would). However their use *may* be justified in a situation like yours where you can't modify the API's class -- but you might want to think more about alternatives to do what you need like subclassing or wrapping the API's class instead. Another possibility might be to make an instance of it an attribute of one of your own classes.
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    3. COI'd be interested in hearing the reason for the downvote...
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