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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