Yapps (Yet Another Python Parser System) is an easy to use parser generator that is written in Python and generates Python code. There are several parser generator systems already available for Python, including PyLR, kjParsing, PyBison, and mcf.pars, but I had different goals for my parser. Yapps is simple, is easy to use, and produces human-readable parsers. It is not fast, powerful, or particularly flexible. Yapps is designed to be used when regular expressions are not enough and other parser systems are too much: situations where you may write your own recursive descent parser.
This is a brief introduction to Python for Lisp programmers. (Although it wasn't my intent, Python programers have told me this page has helped them learn Lisp.) Basically, Python can be seen as a dialect of Lisp with "traditional" syntax (what Lisp people call "infix" or "m-lisp" syntax). One message on comp.lang.python said "I never understood why LISP was a good idea until I started playing with python." Python supports all of Lisp's essential features except macros, and you don't miss macros all that much because it does have eval, and operator overloading, and regular expression parsing, so you can create custom languages that way.