Example of changing the python AST

rewrite_strings.py

import ast

class StringWrapper(ast.NodeTransformer):
    """Wraps all strings in 'START ' + string + ' END'. """
    def visit_Str(self, node):
        return ast.Call(func=ast.Name(id='wrap_string', ctx=ast.Load()),
                        args=[node], keywords=[])

def wrap_string(s):
    return 'START ' + s + ' END'

code = "print('test string')"
print(code)
print()

print("Without AST transformation:")
exec(code)
print()

print("With AST transformation:")
tree = ast.parse(code)
tree = StringWrapper().visit(tree)

# Add lineno & col_offset to the nodes we created
ast.fix_missing_locations(tree)
co = compile(tree, "<ast>", "exec")
exec(co)