Visitor and Walkabout : http://therning.org/magnus/?p=1159

visitor.py

# A visitor pattern implementation in Python

class Visitor:
    def visit(self, obj):
        func_name = 'visit_' + obj.__class__.__name__
        visit_func = getattr(self, func_name)
        visit_func(obj)

    def visit_Tree(self, obj):
        pass


class Tree:
    def __init__(self, value, children=[]):
        self.value = value
        self.children = children

    def accept(self, visitor):
        visitor.visit(self)
        for c in self.children:
            c.accept(visitor)


class TreePrintVisitor(Visitor):
    def visit_Tree(self, obj):
        print('Tree (%s): %i' % (hex(id(obj)), obj.value))


def test_visitor():
    leaves = [Tree(42), Tree(17)]
    tree = Tree(1, leaves)
    printer = TreePrintVisitor()
    tree.accept(printer)

walkabout.py

# A walkabout pattern implementation in Python

class Walkabout:
    def visit(self, obj):
        func_name = 'visit_%s' % obj.__class__.__name__
        if hasattr(self, func_name):
            visit_func = getattr(self, func_name)
            visit_func(obj)
        elif hasattr(obj, '__dict__'):
            for m in obj.__dict__.keys():
                self.visit(getattr(obj, m))


class Tree:
    def __init__(self, value, children=[]):
        self.value = value
        self.children = children


class TreePrintWalkabout(Walkabout):
    def visit_Tree(self, tree):
        print('Tree (%s): %i' % (hex(id(tree)), tree.value))
        for c in tree.children:
            self.visit(c)


def test_walkabout():
    leaves = [Tree(42), Tree(17)]
    tree = Tree(1, leaves)
    printer = TreePrintWalkabout()
    printer.visit(tree)

walkabouttwo.py

# A modified walkabout pattern implementation in Python

class WalkaboutTwo:
    def visit(self, obj):
        func_name = 'visit_%s' % obj.__class__.__name__
        descend = True
        if hasattr(self, func_name):
            visit_func = getattr(self, func_name)
            descend = visit_func(obj)
        if descend and hasattr(obj, '__dict__'):
            for m in obj.__dict__.keys():
                self.visit(getattr(obj, m))
        if descend and hasattr(obj, '__iter__'):
            for o in obj:
                self.visit(o)


class Tree:
    def __init__(self, value, children=[]):
        self.value = value
        self.children = children


class TreePrintWalkabout(WalkaboutTwo):
    def visit_Tree(self, tree):
        print('Tree (%s): %i' % (hex(id(tree)), tree.value))
        return True


def test_walkabouttwo():
    leaves = [Tree(42), Tree(17)]
    tree = Tree(1, leaves)
    printer = TreePrintWalkabout()
    printer.visit(tree)