ast manipulation

[Terry Reedy]

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())))])