PLY is a 100% Python implementation of the common parsing tools lex and yacc.
Here are a few highlights:

 -  PLY is very closely modeled after traditional lex/yacc. If you know how to
    use these tools in C, you will find PLY to be similar.

 -  PLY provides *very* extensive error reporting and diagnostic information to
    assist in parser construction.  The original implementation was developed
    for instructional purposes.  As a result, the system tries to identify the
    most common types of errors made by novice users.

 -  PLY provides full support for empty productions, error recovery, precedence
    specifiers, and moderately ambiguous grammars.

 -  Parsing is based on LR-parsing which is fast, memory efficient, better
    suited to large grammars, and which has a number of nice properties when
    dealing with syntax errors and other parsing problems. Currently, PLY builds
    its parsing tables using the LALR(1) algorithm used in yacc.

 -  PLY uses Python introspection features to build lexers and parsers. This
    greatly simplifies the task of parser construction since it reduces the
    number of files and eliminates the need to run a separate lex/yacc tool
    before running your program.

 -  PLY can be used to build parsers for "real" programming languages. Although
    it is not ultra-fast due to its Python implementation, PLY can be used to
    parse grammars consisting of several hundred rules (as might be found for a
    language like C).  The lexer and LR parser are also reasonably efficient
    when parsing typically sized programs.  People have used PLY to build
    parsers for C, C++, ADA, and other real programming languages.