PLY (Python Lex-Yacc)

PLY is a parsing tool written purely in Python. It is basically a re-implementation of Lex and Yacc originally in C-language. It was written by David M. Beazley.[1] Unlike Lex and Yacc in C which uses LALR parsing technique, PLY uses LR parsing which can incorporate large grammars easily. PLY also has extensive debugging and error reporting facilities.

PLY (Python Lex-Yacc)
Original author(s)David M. Beazley
Stable release
Written inPython


Beazley originally wrote PLY in 2001 for an introductory course on compilers. He created it to aid the students in creating compilers for simple Pascal-like languages.[2] Since then it has seen many improvements and has come out as one of the popular parsing tools.


Implemented in Python, it has almost all the features provided by Lex and Yacc. It includes support for empty productions, precedence rules, error recovery, and ambiguous grammars.

PLY-3.0 includes support for Python 3.0 and some other features.

Structure of a PLY file

PLY has the following two Python modules which are part of the ply package.[3]

  • ply.lex - This deals with the lexical analysis part.
  • ply.yacc - This module is for creating a parser.

These files contain the necessary modules.


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