Tsize wrote:
Hello,
I am hoping for a little help. I have been playing with the python
ast module and have run into
an issue that I need a little push on. I would like to be able to
change a specific element in a
specific node in an ast then compile the resulting ast.
If you can identify the specific nodes you want to change, no problem
Consider the simplified example below
with its output. In this example I would like a way to change a
specific addition operation. With the NodeTransformer I see how to
change every addition operator but not how to change a specific one.
Which specific one or one?
I would like this to work on both the 2.6 and 3.1 branches. Ideally I
would like to read a file, count the instances of an operation of
interest and then use an index to make the changes.
If 'specific one' means number i, great. In not, not.
I am probably missing something simple but I am lost right now.
You have not said what 'specific one' means.
Nor what your general goal is, why you want to change asts.
import ast
class SwitchMinusPlus(ast.NodeTransformer):
def visit_BinOp(self, node):
node = self.generic_visit(node)
if isinstance(node.op, ast.Add):
if isinstance(node.op, ast.Add) and isspecificnode(node):
node.op = ast.Sub()
return node
myfile = open('trivial.py').read()
print myfile
tree = compile(myfile, '<string>', 'exec', ast.PyCF_ONLY_AST)
print ast.dump(tree, annotate_fields=False, include_attributes=False)
node = SwitchMinusPlus().visit(ast.parse(myfile))
print ast.dump(node, annotate_fields=False, include_attributes=False)
Which gives the following output: Note that this code changes the
addition operator to an
subtraction operator at the AST level for every instance.
a = 8
b = 6
c = b + a
d = c + a
Module([Assign([Name('a', Store())], Num(8)), Assign([Name('b', Store
())], Num(6)),
Assign([Name('c', Store())], BinOp(Name('b', Load()), Add(), Name('a',
Load()))),
Assign([Name('d', Store())], BinOp(Name('c', Load()), Add(), Name('a',
Load())))])
Module([Assign([Name('a', Store())], Num(8)), Assign([Name('b', Store
())], Num(6)),
Assign([Name('c', Store())], BinOp(Name('b', Load()), Sub(), Name('a',
Load()))),
Assign([Name('d', Store())], BinOp(Name('c', Load()), Sub(), Name('a',
Load())))])
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