<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <html> <head> <title>19.4 Using Visitors to Walk ASTs</title> <META NAME="description" CONTENT="19.4 Using Visitors to Walk ASTs"> <META NAME="keywords" CONTENT="lib"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <meta http-equiv="Content-Type" content="text/html; charset="> <link rel="STYLESHEET" href="lib.css"> <link rel="first" href="lib.html"> <link rel="contents" href="contents.html" title="Contents"> <link rel="index" href="genindex.html" title="Index"> <LINK REL="next" HREF="node579.html"> <LINK REL="previous" HREF="node574.html"> <LINK REL="up" href="compiler.html"> <LINK REL="next" HREF="node579.html"> </head> <body> <DIV CLASS="navigation"> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td><A HREF="node577.html"><img src="../icons/previous.gif" border="0" height="32" alt="Previous Page" width="32"></A></td> <td><A href="compiler.html"><img src="../icons/up.gif" border="0" height="32" alt="Up One Level" width="32"></A></td> <td><A HREF="node579.html"><img src="../icons/next.gif" border="0" height="32" alt="Next Page" width="32"></A></td> <td align="center" width="100%">Python Library Reference</td> <td><A href="contents.html"><img src="../icons/contents.gif" border="0" height="32" alt="Contents" width="32"></A></td> <td><a href="modindex.html" title="Module Index"><img src="../icons/modules.gif" border="0" height="32" alt="Module Index" width="32"></a></td> <td><A href="genindex.html"><img src="../icons/index.gif" border="0" height="32" alt="Index" width="32"></A></td> </tr></table> <b class="navlabel">Previous:</b> <a class="sectref" HREF="node577.html">19.3.3 Examples</A> <b class="navlabel">Up:</b> <a class="sectref" href="compiler.html">19. Python compiler package</A> <b class="navlabel">Next:</b> <a class="sectref" HREF="node579.html">19.5 Bytecode Generation</A> <br><hr> </DIV> <!--End of Navigation Panel--> <H1><A NAME="SECTION0021400000000000000000"> 19.4 Using Visitors to Walk ASTs</A> </H1> <P> <P> The visitor pattern is ... The <tt class="module"><a href="module-compiler.html">compiler</a></tt> 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. <P> 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. <P> XXX The magic <tt class="method">visit()</tt> method for visitors. <P> <dl><dt><b><a name="l2h-3849"><tt class="function">walk</tt></a></b>(<var>tree, visitor</var><big>[</big><var>, verbose</var><big>]</big>) <dd> </dl> <P> <dl><dt><b><span class="typelabel">class</span> <a name="l2h-3850"><tt class="class">ASTVisitor</tt></a></b>() <dd> <P> The <tt class="class">ASTVisitor</tt> is responsible for walking over the tree in the correct order. A walk begins with a call to <tt class="method">preorder()</tt>. For each node, it checks the <var>visitor</var> argument to <tt class="method">preorder()</tt> for a method named `visitNodeType,' where NodeType is the name of the node's class, e.g. for a <tt class="class">While</tt> node a <tt class="method">visitWhile()</tt> would be called. If the method exists, it is called with the node as its first argument. <P> The visitor method for a particular node type can control how child nodes are visited during the walk. The <tt class="class">ASTVisitor</tt> 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 <tt class="method">default()</tt> method is called. </dl> <P> <tt class="class">ASTVisitor</tt> objects have the following methods: <P> XXX describe extra arguments <P> <dl><dt><b><a name="l2h-3851"><tt class="method">default</tt></a></b>(<var>node</var><big>[</big><var>, ...</var><big>]</big>) <dd> </dl> <P> <dl><dt><b><a name="l2h-3852"><tt class="method">dispatch</tt></a></b>(<var>node</var><big>[</big><var>, ...</var><big>]</big>) <dd> </dl> <P> <dl><dt><b><a name="l2h-3853"><tt class="method">preorder</tt></a></b>(<var>tree, visitor</var>) <dd> </dl> <P> <DIV CLASS="navigation"> <p><hr> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td><A HREF="node577.html"><img src="../icons/previous.gif" border="0" height="32" alt="Previous Page" width="32"></A></td> <td><A href="compiler.html"><img src="../icons/up.gif" border="0" height="32" alt="Up One Level" width="32"></A></td> <td><A HREF="node579.html"><img src="../icons/next.gif" border="0" height="32" alt="Next Page" width="32"></A></td> <td align="center" width="100%">Python Library Reference</td> <td><A href="contents.html"><img src="../icons/contents.gif" border="0" height="32" alt="Contents" width="32"></A></td> <td><a href="modindex.html" title="Module Index"><img src="../icons/modules.gif" border="0" height="32" alt="Module Index" width="32"></a></td> <td><A href="genindex.html"><img src="../icons/index.gif" border="0" height="32" alt="Index" width="32"></A></td> </tr></table> <b class="navlabel">Previous:</b> <a class="sectref" HREF="node577.html">19.3.3 Examples</A> <b class="navlabel">Up:</b> <a class="sectref" href="compiler.html">19. Python compiler package</A> <b class="navlabel">Next:</b> <a class="sectref" HREF="node579.html">19.5 Bytecode Generation</A> <hr> <span class="release-info">Release 2.2, documentation updated on December 21, 2001.</span> </DIV> <!--End of Navigation Panel--> <ADDRESS> See <i><a href="about.html">About this document...</a></i> for information on suggesting changes. </ADDRESS> </BODY> </HTML>
