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-PLY (Python Lex-Yacc) Version 3.4 |
- |
-Copyright (C) 2001-2011, |
-David M. Beazley (Dabeaz LLC) |
-All rights reserved. |
- |
-Redistribution and use in source and binary forms, with or without |
-modification, are permitted provided that the following conditions are |
-met: |
- |
-* Redistributions of source code must retain the above copyright notice, |
- this list of conditions and the following disclaimer. |
-* Redistributions in binary form must reproduce the above copyright notice, |
- this list of conditions and the following disclaimer in the documentation |
- and/or other materials provided with the distribution. |
-* Neither the name of the David Beazley or Dabeaz LLC may be used to |
- endorse or promote products derived from this software without |
- specific prior written permission. |
- |
-THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS |
-"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT |
-LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR |
-A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT |
-OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, |
-SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT |
-LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
-DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
-THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
-(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE |
-OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
- |
-Introduction |
-============ |
- |
-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. |
- |
-How to Use |
-========== |
- |
-PLY consists of two files : lex.py and yacc.py. These are contained |
-within the 'ply' directory which may also be used as a Python package. |
-To use PLY, simply copy the 'ply' directory to your project and import |
-lex and yacc from the associated 'ply' package. For example: |
- |
- import ply.lex as lex |
- import ply.yacc as yacc |
- |
-Alternatively, you can copy just the files lex.py and yacc.py |
-individually and use them as modules. For example: |
- |
- import lex |
- import yacc |
- |
-The file setup.py can be used to install ply using distutils. |
- |
-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 |
- |
-Requirements |
-============ |
-PLY requires the use of Python 2.2 or greater. However, you should |
-use the latest Python release if possible. It should work on just |
-about any platform. PLY has been tested with both CPython and Jython. |
-It also seems to work with IronPython. |
- |
-Resources |
-========= |
-More information about PLY can be obtained on the PLY webpage at: |
- |
- http://www.dabeaz.com/ply |
- |
-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 Google group for PLY can be found at |
- |
- http://groups.google.com/group/ply-hack |
- |
-Acknowledgments |
-=============== |
-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-3.0 |
-======================== |
-PLY-3.0 the first PLY release to support Python 3. However, backwards |
-compatibility with Python 2.2 is still preserved. PLY provides dual |
-Python 2/3 compatibility by restricting its implementation to a common |
-subset of basic language features. You should not convert PLY using |
-2to3--it is not necessary and may in fact break the implementation. |
- |
-Example |
-======= |
- |
-Here is a simple example showing a PLY implementation of a calculator |
-with variables. |
- |
-# ----------------------------------------------------------------------------- |
-# calc.py |
-# |
-# A simple calculator with variables. |
-# ----------------------------------------------------------------------------- |
- |
-tokens = ( |
- 'NAME','NUMBER', |
- 'PLUS','MINUS','TIMES','DIVIDE','EQUALS', |
- 'LPAREN','RPAREN', |
- ) |
- |
-# 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): |
- r'\d+' |
- t.value = int(t.value) |
- return t |
- |
-# Ignored characters |
-t_ignore = " \t" |
- |
-def t_newline(t): |
- r'\n+' |
- t.lexer.lineno += t.value.count("\n") |
- |
-def t_error(t): |
- print("Illegal character '%s'" % t.value[0]) |
- t.lexer.skip(1) |
- |
-# Build the lexer |
-import ply.lex as lex |
-lex.lex() |
- |
-# Precedence rules for the arithmetic operators |
-precedence = ( |
- ('left','PLUS','MINUS'), |
- ('left','TIMES','DIVIDE'), |
- ('right','UMINUS'), |
- ) |
- |
-# 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' |
- try: |
- 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 ply.yacc as yacc |
-yacc.yacc() |
- |
-while 1: |
- try: |
- s = raw_input('calc > ') # use input() on Python 3 |
- except EOFError: |
- break |
- yacc.parse(s) |
- |
- |
-Bug Reports and Patches |
-======================= |
-My goal with PLY is to simply have a decent lex/yacc implementation |
-for Python. As a general rule, I don't spend huge amounts of 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@dabeaz.com. |
- |
-In addition there is a Google group for discussing PLY related issues at |
- |
- http://groups.google.com/group/ply-hack |
- |
--- Dave |
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