The visitor pattern is ... The compiler package uses a
variant on the visitor pattern that takes advantage of Python's
introspection features to eliminate 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])
classASTVisitor(
)
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.
