import sys
import ast
class ASTVisitor:
"""Performs a depth-first walk of the AST
The ASTVisitor will walk the AST, performing either a preorder or
postorder traversal depending on which method is called.
methods:
preorder(tree, visitor)
postorder(tree, visitor)
tree: an instance of ast.Node
visitor: an instance with visitXXX methods
The ASTVisitor is responsible for walking over the tree in the
correct order. For each node, it checks the visitor argument for
a method named 'visitNodeType' where NodeType is the name of the
node's class, e.g. Class. If the method exists, it is called
with the node as its sole argument.
The visitor method for a particular node type can control how
child nodes are visited during a preorder walk. (It can't control
the order during a postorder walk, because it is called _after_
the walk has occurred.) 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 the visitor method
returns a true value, the ASTVisitor will not traverse the child
nodes.
XXX The interface for controlling the preorder walk needs to be
re-considered. The current interface is convenient for visitors
that mostly let the ASTVisitor do everything. For something like
a code generator, where you want to walk to occur in a specific
order, it's a pain to add "return 1" to the end of each method.
"""
VERBOSE = 0
def __init__(self):
self.node = None
self._cache = {}
def default(self, node, *args):
for child in node.getChildren():
if isinstance(child, ast.Node):
apply(self._preorder, (child,) + args)
def dispatch(self, node, *args):
self.node = node
klass = node.__class__
meth = self._cache.get(klass, None)
if meth is None:
className = klass.__name__
meth = getattr(self.visitor, 'visit' + className, self.default)
self._cache[klass] = meth
if self.VERBOSE > 0:
className = klass.__name__
if self.VERBOSE == 1:
if meth == 0:
print "dispatch", className
else:
print "dispatch", className, (meth and meth.__name__ or '')
return meth(node, *args)
def preorder(self, tree, visitor, *args):
"""Do preorder walk of tree using visitor"""
self.visitor = visitor
visitor.visit = self._preorder
self._preorder(tree, *args) # XXX *args make sense?
_preorder = dispatch
class ExampleASTVisitor(ASTVisitor):
"""Prints examples of the nodes that aren't visited
This visitor-driver is only useful for development, when it's
helpful to develop a visitor incremently, and get feedback on what
you still have to do.
"""
examples = {}
def dispatch(self, node, *args):
self.node = node
meth = self._cache.get(node.__class__, None)
className = node.__class__.__name__
if meth is None:
meth = getattr(self.visitor, 'visit' + className, 0)
self._cache[node.__class__] = meth
if self.VERBOSE > 1:
print "dispatch", className, (meth and meth.__name__ or '')
if meth:
return apply(meth, (node,) + args)
elif self.VERBOSE > 0:
klass = node.__class__
if not self.examples.has_key(klass):
self.examples[klass] = klass
print
print self.visitor
print klass
for attr in dir(node):
if attr[0] != '_':
print "\t", "%-12.12s" % attr, getattr(node, attr)
print
return apply(self.default, (node,) + args)
_walker = ASTVisitor
def walk(tree, visitor, verbose=None):
w = _walker()
if verbose is not None:
w.VERBOSE = verbose
w.preorder(tree, visitor)
return w.visitor
def dumpNode(node):
print node.__class__
for attr in dir(node):
if attr[0] != '_':
print "\t", "%-10.10s" % attr, getattr(node, attr)
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