Lib/compiler/visitor.py

   1 from compiler import ast
   2
   3 # XXX should probably rename ASTVisitor to ASTWalker
   4 # XXX can it be made even more generic?
   5
   6 class ASTVisitor:
   7     """Performs a depth-first walk of the AST
   8
   9     The ASTVisitor will walk the AST, performing either a preorder or
  10     postorder traversal depending on which method is called.
  11
  12     methods:
  13     preorder(tree, visitor)
  14     postorder(tree, visitor)
  15         tree: an instance of ast.Node
  16         visitor: an instance with visitXXX methods
  17
  18     The ASTVisitor is responsible for walking over the tree in the
  19     correct order.  For each node, it checks the visitor argument for
  20     a method named 'visitNodeType' where NodeType is the name of the
  21     node's class, e.g. Class.  If the method exists, it is called
  22     with the node as its sole argument.
  23
  24     The visitor method for a particular node type can control how
  25     child nodes are visited during a preorder walk.  (It can't control
  26     the order during a postorder walk, because it is called _after_
  27     the walk has occurred.)  The ASTVisitor modifies the visitor
  28     argument by adding a visit method to the visitor; this method can
  29     be used to visit a child node of arbitrary type.
  30     """
  31
  32     VERBOSE = 0
  33
  34     def __init__(self):
  35         self.node = None
  36         self._cache = {}
  37
  38     def default(self, node, *args):
  39         for child in node.getChildNodes():
  40             self.dispatch(child, *args)
  41
  42     def dispatch(self, node, *args):
  43         self.node = node
  44         klass = node.__class__
  45         meth = self._cache.get(klass, None)
  46         if meth is None:
  47             className = klass.__name__
  48             meth = getattr(self.visitor, 'visit' + className, self.default)
  49             self._cache[klass] = meth
  50 ##        if self.VERBOSE > 0:
  51 ##            className = klass.__name__
  52 ##            if self.VERBOSE == 1:
  53 ##                if meth == 0:
  54 ##                    print "dispatch", className
  55 ##            else:
  56 ##                print "dispatch", className, (meth and meth.__name__ or '')
  57         return meth(node, *args)
  58
  59     def preorder(self, tree, visitor, *args):
  60         """Do preorder walk of tree using visitor"""
  61         self.visitor = visitor
  62         visitor.visit = self.dispatch
  63         self.dispatch(tree, *args) # XXX *args make sense?
  64
  65 class ExampleASTVisitor(ASTVisitor):
  66     """Prints examples of the nodes that aren't visited
  67
  68     This visitor-driver is only useful for development, when it's
  69     helpful to develop a visitor incrementally, and get feedback on what
  70     you still have to do.
  71     """
  72     examples = {}
  73
  74     def dispatch(self, node, *args):
  75         self.node = node
  76         meth = self._cache.get(node.__class__, None)
  77         className = node.__class__.__name__
  78         if meth is None:
  79             meth = getattr(self.visitor, 'visit' + className, 0)
  80             self._cache[node.__class__] = meth
  81         if self.VERBOSE > 1:
  82             print "dispatch", className, (meth and meth.__name__ or '')
  83         if meth:
  84             meth(node, *args)
  85         elif self.VERBOSE > 0:
  86             klass = node.__class__
  87             if not self.examples.has_key(klass):
  88                 self.examples[klass] = klass
  89                 print
  90                 print self.visitor
  91                 print klass
  92                 for attr in dir(node):
  93                     if attr[0] != '_':
  94                         print "\t", "%-12.12s" % attr, getattr(node, attr)
  95                 print
  96             return self.default(node, *args)
  97
  98 # XXX this is an API change
  99
 100 _walker = ASTVisitor
 101 def walk(tree, visitor, walker=None, verbose=None):
 102     if walker is None:
 103         walker = _walker()
 104     if verbose is not None:
 105         walker.VERBOSE = verbose
 106     walker.preorder(tree, visitor)
 107     return walker.visitor
 108
 109 def dumpNode(node):
 110     print node.__class__
 111     for attr in dir(node):
 112         if attr[0] != '_':
 113             print "\t", "%-10.10s" % attr, getattr(node, attr)