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I want to provide some functionality to classes using mixin . The functionality uses some additional per-object state. I was wondering what is the cleanest way to initialize this local state. Consider the example: class Mixin: items = [] def append(self, x): self.items.append(x) def display(self): print self.items class Foo(object, Mixin): pass class Bar(object, Mixin): pass foo = Foo() foo.append('foo') foo.display() >>> ['foo'] bar = Bar() bar.append('bar') bar.display() >>> ['foo', 'bar'] Here, the state is items list. Initializing it in the Mixin body is obviously wrong. Normally, I would initialize it in __init__ , but with Mixin I do not want to mess with __init__ . I could do the following: class Mixin: items = None def append(self, x): if self.items is None: self.items = [] self.items.append(x) But the condition is evaluated on each append and it does not seem to be the cleanest solution. Any alternatives? Or maybe adding __init__ to the mixin is The way? (It is a separate question if using mixins is OK or not) Related : Initializing instance variables in Mixins for Ruby

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