python-patterns/visitor.py

'''http://peter-hoffmann.com/2010/extrinsic-visitor-pattern-python-inheritance.html'''

class Node(object): 
    pass 
class A(Node): 
    pass 
class B(Node): 
    pass 
class C(A,B): 
    pass

class Visitor(object):     
    def visit(self, node, *args, **kwargs):         
        meth = None         
        for cls in node.__class__.__mro__:             
            meth_name = 'visit_'+cls.__name__             
            meth = getattr(self, meth_name, None)             

            if meth:                 
                break          

            meth = self.generic_visit         
            return meth(node, *args, **kwargs)      

    def visit_B(self, node, *args, **kwargs):         
        print 'visit_B '+node.__class__.__name__   

    def generic_visit(self, node, *args, **kwargs):         
        print 'generic_visit '+node.__class__.__name__      

if __name__ == '__main__':
    a = A() 
    b = B() 
    # c = C() 
    visitor = Visitor() 
    visitor.visit(a) 
    visitor.visit(b) 
    # visitor.visit(c)