README.md
1PLY (Python Lex-Yacc) Version 3.10
2
3Copyright (C) 2001-2017
4David M. Beazley (Dabeaz LLC)
5All rights reserved.
6
7Redistribution and use in source and binary forms, with or without
8modification, are permitted provided that the following conditions are
9met:
10
11* Redistributions of source code must retain the above copyright notice,
12 this list of conditions and the following disclaimer.
13* Redistributions in binary form must reproduce the above copyright notice,
14 this list of conditions and the following disclaimer in the documentation
15 and/or other materials provided with the distribution.
16* Neither the name of the David Beazley or Dabeaz LLC may be used to
17 endorse or promote products derived from this software without
18 specific prior written permission.
19
20THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
21"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
22LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
23A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
24OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
25SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
26LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
27DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
28THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
29(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
30OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
31
32Introduction
33============
34
35PLY is a 100% Python implementation of the common parsing tools lex
36and yacc. Here are a few highlights:
37
38 - PLY is very closely modeled after traditional lex/yacc.
39 If you know how to use these tools in C, you will find PLY
40 to be similar.
41
42 - PLY provides *very* extensive error reporting and diagnostic
43 information to assist in parser construction. The original
44 implementation was developed for instructional purposes. As
45 a result, the system tries to identify the most common types
46 of errors made by novice users.
47
48 - PLY provides full support for empty productions, error recovery,
49 precedence specifiers, and moderately ambiguous grammars.
50
51 - Parsing is based on LR-parsing which is fast, memory efficient,
52 better suited to large grammars, and which has a number of nice
53 properties when dealing with syntax errors and other parsing problems.
54 Currently, PLY builds its parsing tables using the LALR(1)
55 algorithm used in yacc.
56
57 - PLY uses Python introspection features to build lexers and parsers.
58 This greatly simplifies the task of parser construction since it reduces
59 the number of files and eliminates the need to run a separate lex/yacc
60 tool before running your program.
61
62 - PLY can be used to build parsers for "real" programming languages.
63 Although it is not ultra-fast due to its Python implementation,
64 PLY can be used to parse grammars consisting of several hundred
65 rules (as might be found for a language like C). The lexer and LR
66 parser are also reasonably efficient when parsing typically
67 sized programs. People have used PLY to build parsers for
68 C, C++, ADA, and other real programming languages.
69
70How to Use
71==========
72
73PLY consists of two files : lex.py and yacc.py. These are contained
74within the 'ply' directory which may also be used as a Python package.
75To use PLY, simply copy the 'ply' directory to your project and import
76lex and yacc from the associated 'ply' package. For example:
77
78 import ply.lex as lex
79 import ply.yacc as yacc
80
81Alternatively, you can copy just the files lex.py and yacc.py
82individually and use them as modules. For example:
83
84 import lex
85 import yacc
86
87The file setup.py can be used to install ply using distutils.
88
89The file doc/ply.html contains complete documentation on how to use
90the system.
91
92The example directory contains several different examples including a
93PLY specification for ANSI C as given in K&R 2nd Ed.
94
95A simple example is found at the end of this document
96
97Requirements
98============
99PLY requires the use of Python 2.6 or greater. However, you should
100use the latest Python release if possible. It should work on just
101about any platform. PLY has been tested with both CPython and Jython.
102It also seems to work with IronPython.
103
104Resources
105=========
106More information about PLY can be obtained on the PLY webpage at:
107
108 http://www.dabeaz.com/ply
109
110For a detailed overview of parsing theory, consult the excellent
111book "Compilers : Principles, Techniques, and Tools" by Aho, Sethi, and
112Ullman. The topics found in "Lex & Yacc" by Levine, Mason, and Brown
113may also be useful.
114
115The GitHub page for PLY can be found at:
116
117 https://github.com/dabeaz/ply
118
119An old and relatively inactive discussion group for PLY is found at:
120
121 http://groups.google.com/group/ply-hack
122
123Acknowledgments
124===============
125A special thanks is in order for all of the students in CS326 who
126suffered through about 25 different versions of these tools :-).
127
128The CHANGES file acknowledges those who have contributed patches.
129
130Elias Ioup did the first implementation of LALR(1) parsing in PLY-1.x.
131Andrew Waters and Markus Schoepflin were instrumental in reporting bugs
132and testing a revised LALR(1) implementation for PLY-2.0.
133
134Special Note for PLY-3.0
135========================
136PLY-3.0 the first PLY release to support Python 3. However, backwards
137compatibility with Python 2.6 is still preserved. PLY provides dual
138Python 2/3 compatibility by restricting its implementation to a common
139subset of basic language features. You should not convert PLY using
1402to3--it is not necessary and may in fact break the implementation.
141
142Example
143=======
144
145Here is a simple example showing a PLY implementation of a calculator
146with variables.
147
148 # -----------------------------------------------------------------------------
149 # calc.py
150 #
151 # A simple calculator with variables.
152 # -----------------------------------------------------------------------------
153
154 tokens = (
155 'NAME','NUMBER',
156 'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
157 'LPAREN','RPAREN',
158 )
159
160 # Tokens
161
162 t_PLUS = r'\+'
163 t_MINUS = r'-'
164 t_TIMES = r'\*'
165 t_DIVIDE = r'/'
166 t_EQUALS = r'='
167 t_LPAREN = r'\('
168 t_RPAREN = r'\)'
169 t_NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
170
171 def t_NUMBER(t):
172 r'\d+'
173 t.value = int(t.value)
174 return t
175
176 # Ignored characters
177 t_ignore = " \t"
178
179 def t_newline(t):
180 r'\n+'
181 t.lexer.lineno += t.value.count("\n")
182
183 def t_error(t):
184 print("Illegal character '%s'" % t.value[0])
185 t.lexer.skip(1)
186
187 # Build the lexer
188 import ply.lex as lex
189 lex.lex()
190
191 # Precedence rules for the arithmetic operators
192 precedence = (
193 ('left','PLUS','MINUS'),
194 ('left','TIMES','DIVIDE'),
195 ('right','UMINUS'),
196 )
197
198 # dictionary of names (for storing variables)
199 names = { }
200
201 def p_statement_assign(p):
202 'statement : NAME EQUALS expression'
203 names[p[1]] = p[3]
204
205 def p_statement_expr(p):
206 'statement : expression'
207 print(p[1])
208
209 def p_expression_binop(p):
210 '''expression : expression PLUS expression
211 | expression MINUS expression
212 | expression TIMES expression
213 | expression DIVIDE expression'''
214 if p[2] == '+' : p[0] = p[1] + p[3]
215 elif p[2] == '-': p[0] = p[1] - p[3]
216 elif p[2] == '*': p[0] = p[1] * p[3]
217 elif p[2] == '/': p[0] = p[1] / p[3]
218
219 def p_expression_uminus(p):
220 'expression : MINUS expression %prec UMINUS'
221 p[0] = -p[2]
222
223 def p_expression_group(p):
224 'expression : LPAREN expression RPAREN'
225 p[0] = p[2]
226
227 def p_expression_number(p):
228 'expression : NUMBER'
229 p[0] = p[1]
230
231 def p_expression_name(p):
232 'expression : NAME'
233 try:
234 p[0] = names[p[1]]
235 except LookupError:
236 print("Undefined name '%s'" % p[1])
237 p[0] = 0
238
239 def p_error(p):
240 print("Syntax error at '%s'" % p.value)
241
242 import ply.yacc as yacc
243 yacc.yacc()
244
245 while True:
246 try:
247 s = raw_input('calc > ') # use input() on Python 3
248 except EOFError:
249 break
250 yacc.parse(s)
251
252
253Bug Reports and Patches
254=======================
255My goal with PLY is to simply have a decent lex/yacc implementation
256for Python. As a general rule, I don't spend huge amounts of time
257working on it unless I receive very specific bug reports and/or
258patches to fix problems. I also try to incorporate submitted feature
259requests and enhancements into each new version. Please visit the PLY
260github page at https://github.com/dabeaz/ply to submit issues and pull
261requests. To contact me about bugs and/or new features, please send
262email to dave@dabeaz.com.
263
264-- Dave
265
266
267
268
269
270
271
272
273
274