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

        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