extrinsic_visitor_inheritance.py

"""
Implementation of the Extrinsic Visitor Pattern in Python with support
for Inheritance.

see: 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)


"""
generic_visit A
visit_B B
visit_B C
"""

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