Posts

Showing posts from August, 2010

A Simple Python NodeVisitor Example

The Python ast module helps parsing python code into its abstract syntax tree and manipulating the tree. This is immensely useful, e.g., when you want to inspect code for safety or correctness. This page walks you through a very simple example to get you up and running quickly.

A Very Simple ParserThe first step to analyzing a program is parsing the textual source code into an in-memory walkable tree. Module ast takes away the hard task of implementing a generic lexer and python parser with the function ast.parse(). The module also takes care of walking the tree. All you have to do is implement analysis code for the type of statements you are want to analyze. To do this, you implement a class that derives from class ast.NodeVisitor and only override the member functions that are pertinent to your analysis.

NodeVisitor exposes callback member functions that the tree walker ("node visitor") calls when it encounters particular python statements, one for each type of statement in…