ANN: PLY-1.6 (Python Lex Yacc)

David Beazley dave-announce at
Mon May 30 03:28:45 CEST 2005

May 27, 2005

                  Announcing :  PLY-1.6 (Python Lex-Yacc)


I'm pleased to announce a new update to PLY---a 100% Python 
of the common parsing tools lex and yacc.  PLY was originally written to
support an introduction to compilers course and has been freely 
since 2001.   However, the 1.6 release expands PLY's capabilities to
include full LALR(1) parsing just like the popular yacc and bison tools 
versions of PLY only provided SLR parsing).

Here are a few PLY highlights:

  -  PLY is 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 rules, 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 
     Currently, PLY can build its parsing tables using either SLR or 
     algorithms.  Note: traditional yacc tools typically use LALR(1).

  -  PLY can be used to build parsers for large 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 normal
     sized programs.

More information about PLY can be obtained on the PLY webpage at:


PLY is freely available and is licensed under the terms of the Lesser
GNU Public License (LGPL).


David Beazley (

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