PLY (Python Lex-Yacc)


PLY requires the use of Python 3.6 or greater. Older versions of Python are not supported.


PLY is a 100% Python implementation of the lex and yacc tools commonly used to write parsers and compilers. Parsing is based on the same LALR(1) algorithm used by many yacc tools. Here are a few notable features:

  • 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.
  • 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).


For a detailed overview of parsing theory, consult the excellent book “Compilers : Principles, Techniques, and Tools” by Aho, Sethi, and Ullman. The topics found in “Lex & Yacc” by Levine, Mason, and Brown may also be useful.

The GitHub page for PLY can be found at:

Please direct bug reports and pull requests to the GitHub page. To contact me directly, send email to or contact me on Twitter (@dabeaz).