[OpenSPARC-T2-SAM] / sam-t2 / devtools / amd64 / html / python / lib / module-compiler.visitor.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="next" href="node797.html" />
<link rel="prev" href="node792.html" />
<link rel="parent" href="compiler.html" />
<link rel="next" href="node797.html" />
<meta name='aesop' content='information' />
<title>19.4 Using Visitors to Walk ASTs</title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="19.3.3 Examples"
  href="node795.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="19. Python compiler package"
  href="compiler.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="19.5 Bytecode Generation"
  href="node797.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="node795.html">19.3.3 Examples</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="compiler.html">19. Python compiler package</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="node797.html">19.5 Bytecode Generation</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H1><A NAME="SECTION0021400000000000000000">
19.4 Using Visitors to Walk ASTs</A>
</H1>

<P>
<A NAME="module-compiler.visitor"></A>
<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 eliminate 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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5032' xml:id='l2h-5032' class="function">walk</tt></b>(</nobr></td>
  <td><var>tree, visitor</var><big>[</big><var>, verbose</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-5033' xml:id='l2h-5033' class="class">ASTVisitor</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<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><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5034' xml:id='l2h-5034' class="method">default</tt></b>(</nobr></td>
  <td><var>node</var><big>[</big><var>, ...</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5035' xml:id='l2h-5035' class="method">dispatch</tt></b>(</nobr></td>
  <td><var>node</var><big>[</big><var>, ...</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5036' xml:id='l2h-5036' class="method">preorder</tt></b>(</nobr></td>
  <td><var>tree, visitor</var>)</td></tr></table></dt>
<dd>
</dl>

<P>

<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="19.3.3 Examples"
  href="node795.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="19. Python compiler package"
  href="compiler.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="19.5 Bytecode Generation"
  href="node797.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="node795.html">19.3.3 Examples</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="compiler.html">19. Python compiler package</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="node797.html">19.5 Bytecode Generation</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</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>
Commit	Line	Data
920dae64 AT	1	<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
	2	<html>
	3	<head>
	4	<link rel="STYLESHEET" href="lib.css" type='text/css' />
	5	<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
	6	<link rel='start' href='../index.html' title='Python Documentation Index' />
	7	<link rel="first" href="lib.html" title='Python Library Reference' />
	8	<link rel='contents' href='contents.html' title="Contents" />
	9	<link rel='index' href='genindex.html' title='Index' />
	10	<link rel='last' href='about.html' title='About this document...' />
	11	<link rel='help' href='about.html' title='About this document...' />
	12	<link rel="next" href="node797.html" />
	13	<link rel="prev" href="node792.html" />
	14	<link rel="parent" href="compiler.html" />
	15	<link rel="next" href="node797.html" />
	16	<meta name='aesop' content='information' />
	17	<title>19.4 Using Visitors to Walk ASTs</title>
	18	</head>
	19	<body>
	20	<DIV CLASS="navigation">
	21	<div id='top-navigation-panel' xml:id='top-navigation-panel'>
	22	<table align="center" width="100%" cellpadding="0" cellspacing="2">
	23	<tr>
	24	<td class='online-navigation'><a rel="prev" title="19.3.3 Examples"
	25	href="node795.html"><img src='../icons/previous.png'
	26	border='0' height='32' alt='Previous Page' width='32' /></A></td>
	27	<td class='online-navigation'><a rel="parent" title="19. Python compiler package"
	28	href="compiler.html"><img src='../icons/up.png'
	29	border='0' height='32' alt='Up One Level' width='32' /></A></td>
	30	<td class='online-navigation'><a rel="next" title="19.5 Bytecode Generation"
	31	href="node797.html"><img src='../icons/next.png'
	32	border='0' height='32' alt='Next Page' width='32' /></A></td>
	33	<td align="center" width="100%">Python Library Reference</td>
	34	<td class='online-navigation'><a rel="contents" title="Table of Contents"
	35	href="contents.html"><img src='../icons/contents.png'
	36	border='0' height='32' alt='Contents' width='32' /></A></td>
	37	<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
	38	border='0' height='32' alt='Module Index' width='32' /></a></td>
	39	<td class='online-navigation'><a rel="index" title="Index"
	40	href="genindex.html"><img src='../icons/index.png'
	41	border='0' height='32' alt='Index' width='32' /></A></td>
	42	</tr></table>
	43	<div class='online-navigation'>
	44	<b class="navlabel">Previous:</b>
	45	<a class="sectref" rel="prev" href="node795.html">19.3.3 Examples</A>
	46	<b class="navlabel">Up:</b>
	47	<a class="sectref" rel="parent" href="compiler.html">19. Python compiler package</A>
	48	<b class="navlabel">Next:</b>
	49	<a class="sectref" rel="next" href="node797.html">19.5 Bytecode Generation</A>
	50	</div>
	51	<hr /></div>
	52	</DIV>
	53	<!--End of Navigation Panel-->
	54
	55	<H1><A NAME="SECTION0021400000000000000000">
	56	19.4 Using Visitors to Walk ASTs</A>
	57	</H1>
	58
	59	<P>
	60	<A NAME="module-compiler.visitor"></A>
	61	<P>
	62	The visitor pattern is ... The <tt class="module"><a href="module-compiler.html">compiler</a></tt> package uses a
	63	variant on the visitor pattern that takes advantage of Python's
	64	introspection features to eliminate the need for much of the visitor's
65	infrastructure.
66
67	<P>
68	The classes being visited do not need to be programmed to accept
69	visitors. The visitor need only define visit methods for classes it
70	is specifically interested in; a default visit method can handle the
71	rest.
72
73	<P>
74	XXX The magic <tt class="method">visit()</tt> method for visitors.
75
76	<P>
77	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
78	<td><nobr><b><tt id='l2h-5032' xml:id='l2h-5032' class="function">walk</tt></b>(</nobr></td>
79	<td><var>tree, visitor</var><big>[</big><var>, verbose</var><big>]</big><var></var>)</td></tr></table></dt>
80	<dd>
81	</dl>
82
83	<P>
84	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
85	<td><nobr><b><span class="typelabel">class</span> <tt id='l2h-5033' xml:id='l2h-5033' class="class">ASTVisitor</tt></b>(</nobr></td>
86	<td><var></var>)</td></tr></table></dt>
87	<dd>
88
89	<P>
90	The <tt class="class">ASTVisitor</tt> is responsible for walking over the tree in the
91	correct order. A walk begins with a call to <tt class="method">preorder()</tt>. For
92	each node, it checks the <var>visitor</var> argument to <tt class="method">preorder()</tt>
93	for a method named `visitNodeType,' where NodeType is the name of the
94	node's class, e.g. for a <tt class="class">While</tt> node a <tt class="method">visitWhile()</tt>
95	would be called. If the method exists, it is called with the node as
96	its first argument.
97
98	<P>
99	The visitor method for a particular node type can control how child
100	nodes are visited during the walk. The <tt class="class">ASTVisitor</tt> modifies
101	the visitor argument by adding a visit method to the visitor; this
102	method can be used to visit a particular child node. If no visitor is
103	found for a particular node type, the <tt class="method">default()</tt> method is
104	called.
105	</dl>
106
107	<P>
108	<tt class="class">ASTVisitor</tt> objects have the following methods:
109
110	<P>
111	XXX describe extra arguments
112
113	<P>
114	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
115	<td><nobr><b><tt id='l2h-5034' xml:id='l2h-5034' class="method">default</tt></b>(</nobr></td>
116	<td><var>node</var><big>[</big><var>, ...</var><big>]</big><var></var>)</td></tr></table></dt>
117	<dd>
118	</dl>
119
120	<P>
121	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
122	<td><nobr><b><tt id='l2h-5035' xml:id='l2h-5035' class="method">dispatch</tt></b>(</nobr></td>
123	<td><var>node</var><big>[</big><var>, ...</var><big>]</big><var></var>)</td></tr></table></dt>
124	<dd>
125	</dl>
126
127	<P>
128	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
129	<td><nobr><b><tt id='l2h-5036' xml:id='l2h-5036' class="method">preorder</tt></b>(</nobr></td>
130	<td><var>tree, visitor</var>)</td></tr></table></dt>
131	<dd>
132	</dl>
133
134	<P>
135
136	<DIV CLASS="navigation">
137	<div class='online-navigation'>
138	<p></p><hr />
139	<table align="center" width="100%" cellpadding="0" cellspacing="2">
140	<tr>
141	<td class='online-navigation'><a rel="prev" title="19.3.3 Examples"
142	href="node795.html"><img src='../icons/previous.png'
143	border='0' height='32' alt='Previous Page' width='32' /></A></td>
144	<td class='online-navigation'><a rel="parent" title="19. Python compiler package"
145	href="compiler.html"><img src='../icons/up.png'
146	border='0' height='32' alt='Up One Level' width='32' /></A></td>
147	<td class='online-navigation'><a rel="next" title="19.5 Bytecode Generation"
148	href="node797.html"><img src='../icons/next.png'
149	border='0' height='32' alt='Next Page' width='32' /></A></td>
150	<td align="center" width="100%">Python Library Reference</td>
151	<td class='online-navigation'><a rel="contents" title="Table of Contents"
152	href="contents.html"><img src='../icons/contents.png'
153	border='0' height='32' alt='Contents' width='32' /></A></td>
154	<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
155	border='0' height='32' alt='Module Index' width='32' /></a></td>
156	<td class='online-navigation'><a rel="index" title="Index"
157	href="genindex.html"><img src='../icons/index.png'
158	border='0' height='32' alt='Index' width='32' /></A></td>
159	</tr></table>
160	<div class='online-navigation'>
161	<b class="navlabel">Previous:</b>
162	<a class="sectref" rel="prev" href="node795.html">19.3.3 Examples</A>
163	<b class="navlabel">Up:</b>
164	<a class="sectref" rel="parent" href="compiler.html">19. Python compiler package</A>
165	<b class="navlabel">Next:</b>
166	<a class="sectref" rel="next" href="node797.html">19.5 Bytecode Generation</A>
167	</div>
168	</div>
169	<hr />
170	<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
171	</DIV>
172	<!--End of Navigation Panel-->
173	<ADDRESS>
174	See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
175	</ADDRESS>
176	</BODY>
177	</HTML>