class Sub(AbstractClass):
    """Subtype template implements AbstractClass base type and adds
      its own 'foo' attribute. Note (though poor style, that __slots__
      and __dict__ style attributes may be mixed."""

def __init__(self):
    """Subtype initializer. Sets 'foo' attribute. """
self.foo = 'bar'
python: abstract base class' __init__(): initializion or validation?