| 1 |
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| 2 | # XXX should probably rename ASTVisitor to ASTWalker
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| 3 | # XXX can it be made even more generic?
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| 4 |
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| 5 | class ASTVisitor(object):
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| 6 | """Performs a depth-first walk of the AST
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| 7 |
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| 8 | The ASTVisitor will walk the AST, performing either a preorder or
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| 9 | postorder traversal depending on which method is called.
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| 10 |
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| 11 | methods:
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| 12 | preorder(tree, visitor)
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| 13 | postorder(tree, visitor)
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| 14 | tree: an instance of ast.Node
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| 15 | visitor: an instance with visitXXX methods
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| 16 |
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| 17 | The ASTVisitor is responsible for walking over the tree in the
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| 18 | correct order. For each node, it checks the visitor argument for
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| 19 | a method named 'visitNodeType' where NodeType is the name of the
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| 20 | node's class, e.g. Class. If the method exists, it is called
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| 21 | with the node as its sole argument.
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| 22 |
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| 23 | The visitor method for a particular node type can control how
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| 24 | child nodes are visited during a preorder walk. (It can't control
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| 25 | the order during a postorder walk, because it is called _after_
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| 26 | the walk has occurred.) The ASTVisitor modifies the visitor
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| 27 | argument by adding a visit method to the visitor; this method can
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| 28 | be used to visit a child node of arbitrary type.
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| 29 | """
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| 30 |
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| 31 | VERBOSE = 0
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| 32 |
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| 33 | def __init__(self):
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| 34 | self._method_cache = {}
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| 35 |
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| 36 | def _Default(self, node, *args):
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| 37 | """If a visitClassName method isn't provided, visit children."""
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| 38 | for child in node.getChildNodes():
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| 39 | self.Dispatch(child, *args)
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| 40 |
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| 41 | def Dispatch(self, node, *args):
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| 42 | klass = node.__class__
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| 43 |
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| 44 | # TODO: Shouldn't it be keyed by string rather than class instance?
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| 45 | # This would probably change the bytecode order.
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| 46 | meth = self._method_cache.get(klass, None)
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| 47 | if meth is None:
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| 48 | className = klass.__name__
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| 49 | meth = getattr(self, 'visit' + className, self._Default)
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| 50 | self._method_cache[klass] = meth
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| 51 | return meth(node, *args)
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| 52 |
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| 53 | # Subclasses call self.visit(). TODO: Rename?
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| 54 | visit = Dispatch
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| 55 |
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| 56 |
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| 57 | class ExampleASTVisitor(ASTVisitor):
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| 58 | """Prints examples of the nodes that aren't visited
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| 59 |
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| 60 | This visitor-driver is only useful for development, when it's
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| 61 | helpful to develop a visitor incrementally, and get feedback on what
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| 62 | you still have to do.
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| 63 | """
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| 64 | examples = {}
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| 65 |
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| 66 | def Dispatch(self, node, *args):
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| 67 | self.node = node
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| 68 | meth = self._method_cache.get(node.__class__, None)
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| 69 | className = node.__class__.__name__
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| 70 | if meth is None:
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| 71 | meth = getattr(self.visitor, 'visit' + className, 0)
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| 72 | self._method_cache[node.__class__] = meth
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| 73 | if self.VERBOSE > 1:
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| 74 | print("Dispatch", className, (meth and meth.__name__ or ''))
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| 75 | if meth:
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| 76 | meth(node, *args)
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| 77 | elif self.VERBOSE > 0:
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| 78 | klass = node.__class__
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| 79 | if klass not in self.examples:
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| 80 | self.examples[klass] = klass
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| 81 | print()
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| 82 | print(self.visitor)
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| 83 | print(klass)
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| 84 | for attr in dir(node):
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| 85 | if attr[0] != '_':
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| 86 | print("\t", "%-12.12s" % attr, getattr(node, attr))
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| 87 | print()
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| 88 | return self._Default(node, *args)
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