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#!/usr/bin/env python

# -*- coding: utf-8 -*-

"""

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)

### OUTPUT ###

# generic_visit A

# visit_B B

# visit_B C