# 19.4 Using Visitors to Walk ASTs

The visitor pattern is ... The `compiler` package uses a variant on the visitor pattern that takes advantage of Python's introspection features to elminiate the need for much of the visitor's infrastructure.

The classes being visited do not need to be programmed to accept visitors. The visitor need only define visit methods for classes it is specifically interested in; a default visit method can handle the rest.

XXX The magic `visit()` method for visitors.

`walk`(tree, visitor[, verbose])

class `ASTVisitor`()

  The `ASTVisitor` is responsible for walking over the tree in the correct order. A walk begins with a call to `preorder()`. For each node, it checks the visitor argument to `preorder()` for a method named `visitNodeType,' where NodeType is the name of the node's class, e.g. for a `While` node a `visitWhile()` would be called. If the method exists, it is called with the node as its first argument.

  The visitor method for a particular node type can control how child nodes are visited during the walk. The `ASTVisitor` modifies the visitor argument by adding a visit method to the visitor; this method can be used to visit a particular child node. If no visitor is found for a particular node type, the `default()` method is called.

`ASTVisitor` objects have the following methods:

XXX describe extra arguments

`default`(node[, ...])

`dispatch`(node[, ...])

`preorder`(tree, visitor)