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PLY (Python Lex-Yacc)                   Version 2.0  (September 8, 2006)

David M. Beazley (

Copyright (C) 2001-2006   David M. Beazley

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

See the file COPYING for a complete copy of the LGPL.


PLY is a 100% Python implementation of the common parsing tools lex
and yacc.   Although several other parsing tools are available for
Python, there are several reasons why you might want to consider PLY:

 -  The tools are 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 SLR algorithm which 
    is slightly weaker than LALR(1) used in traditional 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.

The original version of PLY was developed for an Introduction to
Compilers course where students used it to build a compiler for a
simple Pascal-like language.  Their compiler had to include lexical
analysis, parsing, type checking, type inference, and generation of
assembly code for the SPARC processor.  Because of this, the current
implementation has been extensively tested and debugged.  In addition,
most of the API and error checking steps have been adapted to address
common usability problems.

How to Use

PLY consists of two files : and  To use the system,
simply copy these files to your project and import them like standard
Python modules.

The file doc/ply.html contains complete documentation on how to use
the system.

The example directory contains several different examples including a
PLY specification for ANSI C as given in K&R 2nd Ed.   

A simple example is found at the end of this document

PLY requires the use of Python 2.0 or greater.  It should work on
just about any platform.


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

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.


A special thanks is in order for all of the students in CS326 who
suffered through about 25 different versions of these tools :-).

The CHANGES file acknowledges those who have contributed patches.

Elias Ioup did the first implementation of LALR(1) parsing in PLY-1.x. 
Andrew Waters and Markus Schoepflin were instrumental in reporting bugs
and testing a revised LALR(1) implementation for PLY-2.0.

Special Note for PLY-2.x
PLY-2.0 is the first in a series of PLY releases that will be adding a
variety of significant new features.  The first release in this series
(Ply-2.0) should be 100% compatible with all previous Ply-1.x releases
except for the fact that Ply-2.0 features a correct implementation of
LALR(1) table generation.  

If you have suggestions for improving PLY in future 2.x releases, please
contact me.   - Dave


Here is a simple example showing a PLY implementation of a calculator with variables.

# -----------------------------------------------------------------------------
# A simple calculator with variables.
# -----------------------------------------------------------------------------

tokens = (

# Tokens

t_PLUS    = r'\+'
t_MINUS   = r'-'
t_TIMES   = r'\*'
t_DIVIDE  = r'/'
t_EQUALS  = r'='
t_LPAREN  = r'\('
t_RPAREN  = r'\)'
t_NAME    = r'[a-zA-Z_][a-zA-Z0-9_]*'

def t_NUMBER(t):
        t.value = int(t.value)
    except ValueError:
        print "Integer value too large", t.value
        t.value = 0
    return t

# Ignored characters
t_ignore = " \t"

def t_newline(t):
    t.lineno += t.value.count("\n")
def t_error(t):
    print "Illegal character '%s'" % t.value[0]
# Build the lexer
import lex

# Precedence rules for the arithmetic operators
precedence = (

# dictionary of names (for storing variables)
names = { }

def p_statement_assign(p):
    'statement : NAME EQUALS expression'
    names[p[1]] = p[3]

def p_statement_expr(p):
    'statement : expression'
    print p[1]

def p_expression_binop(p):
    '''expression : expression PLUS expression
                  | expression MINUS expression
                  | expression TIMES expression
                  | expression DIVIDE expression'''
    if p[2] == '+'  : p[0] = p[1] + p[3]
    elif p[2] == '-': p[0] = p[1] - p[3]
    elif p[2] == '*': p[0] = p[1] * p[3]
    elif p[2] == '/': p[0] = p[1] / p[3]

def p_expression_uminus(p):
    'expression : MINUS expression %prec UMINUS'
    p[0] = -p[2]

def p_expression_group(p):
    'expression : LPAREN expression RPAREN'
    p[0] = p[2]

def p_expression_number(p):
    'expression : NUMBER'
    p[0] = p[1]

def p_expression_name(p):
    'expression : NAME'
        p[0] = names[p[1]]
    except LookupError:
        print "Undefined name '%s'" % p[1]
        p[0] = 0

def p_error(p):
    print "Syntax error at '%s'" % p.value

import yacc

while 1:
        s = raw_input('calc > ')
    except EOFError:

Bug Reports and Patches
Because of the extremely specialized and advanced nature of PLY, I
rarely spend much time working on it unless I receive very specific
bug-reports and/or patches to fix problems. I also try to incorporate
submitted feature requests and enhancements into each new version.  To
contact me about bugs and/or new features, please send email to

-- Dave