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

 if not meth:
 meth = self.generic_visit
 return meth(node, *args, **kwargs)

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

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


a = A()
b = B()
c = C()
visitor = Visitor()
visitor.visit(a)
visitor.visit(b)
visitor.visit(c)