PLY is an implementation of lex and yacc parsing tools for Python.

In a nutshell, PLY is nothing more than a straightforward lex/yacc
implementation. Here is a list of its essential features:

* It's implemented entirely in Python.
* It uses LR-parsing which is reasonably efficient and well suited
for larger grammars.
* PLY provides most of the standard lex/yacc features including support
for empty productions, precedence rules, error recovery, and support
for ambiguous grammars.
* PLY is straightforward to use and provides very extensive error checking.
* PLY doesn't try to do anything more or less than provide the basic
lex/yacc functionality. In other words, it's not a large parsing
framework or a component of some larger system.

The original version of PLY was developed in 2001 for use in an
Introduction to Compilers course where students used it to build a
compiler for a simple Pascal-like language. Because of its use in an
instructional setting, a lot of work went into providing extensive
error checking. In addition, this experience was used to sort out
common usability problems. Since then, a variety of incremental
improvements have been made to the system. PLY-3.0 adds support for
Python 3.0 and gives PLY's internals a much needed overhaul.