44ebb8d3e2
ext/ply/ply/lex.py:
ext/ply/ply/yacc.py:
ext/ply/CHANGES:
ext/ply/README:
ext/ply/TODO:
ext/ply/doc/ply.html:
ext/ply/example/ansic/clex.py:
ext/ply/example/ansic/cparse.py:
ext/ply/example/calc/calc.py:
ext/ply/example/hedit/hedit.py:
ext/ply/example/optcalc/calc.py:
ext/ply/test/README:
ext/ply/test/calclex.py:
ext/ply/test/lex_doc1.exp:
ext/ply/test/lex_doc1.py:
ext/ply/test/lex_dup1.exp:
ext/ply/test/lex_dup1.py:
ext/ply/test/lex_dup2.exp:
ext/ply/test/lex_dup2.py:
ext/ply/test/lex_dup3.exp:
ext/ply/test/lex_dup3.py:
ext/ply/test/lex_empty.py:
ext/ply/test/lex_error1.py:
ext/ply/test/lex_error2.py:
ext/ply/test/lex_error3.exp:
ext/ply/test/lex_error3.py:
ext/ply/test/lex_error4.exp:
ext/ply/test/lex_error4.py:
ext/ply/test/lex_hedit.exp:
ext/ply/test/lex_hedit.py:
ext/ply/test/lex_ignore.exp:
ext/ply/test/lex_ignore.py:
ext/ply/test/lex_re1.exp:
ext/ply/test/lex_re1.py:
ext/ply/test/lex_rule1.py:
ext/ply/test/lex_token1.py:
ext/ply/test/lex_token2.py:
ext/ply/test/lex_token3.py:
ext/ply/test/lex_token4.py:
ext/ply/test/lex_token5.exp:
ext/ply/test/lex_token5.py:
ext/ply/test/yacc_badargs.exp:
ext/ply/test/yacc_badargs.py:
ext/ply/test/yacc_badprec.exp:
ext/ply/test/yacc_badprec.py:
ext/ply/test/yacc_badprec2.exp:
ext/ply/test/yacc_badprec2.py:
ext/ply/test/yacc_badrule.exp:
ext/ply/test/yacc_badrule.py:
ext/ply/test/yacc_badtok.exp:
ext/ply/test/yacc_badtok.py:
ext/ply/test/yacc_dup.exp:
ext/ply/test/yacc_dup.py:
ext/ply/test/yacc_error1.exp:
ext/ply/test/yacc_error1.py:
ext/ply/test/yacc_error2.exp:
ext/ply/test/yacc_error2.py:
ext/ply/test/yacc_error3.exp:
ext/ply/test/yacc_error3.py:
ext/ply/test/yacc_inf.exp:
ext/ply/test/yacc_inf.py:
ext/ply/test/yacc_missing1.exp:
ext/ply/test/yacc_missing1.py:
ext/ply/test/yacc_nodoc.exp:
ext/ply/test/yacc_nodoc.py:
ext/ply/test/yacc_noerror.exp:
ext/ply/test/yacc_noerror.py:
ext/ply/test/yacc_nop.exp:
ext/ply/test/yacc_nop.py:
ext/ply/test/yacc_notfunc.exp:
ext/ply/test/yacc_notfunc.py:
ext/ply/test/yacc_notok.exp:
ext/ply/test/yacc_notok.py:
ext/ply/test/yacc_rr.exp:
ext/ply/test/yacc_rr.py:
ext/ply/test/yacc_simple.exp:
ext/ply/test/yacc_simple.py:
ext/ply/test/yacc_sr.exp:
ext/ply/test/yacc_sr.py:
ext/ply/test/yacc_term1.exp:
ext/ply/test/yacc_term1.py:
ext/ply/test/yacc_unused.exp:
ext/ply/test/yacc_unused.py:
ext/ply/test/yacc_uprec.exp:
ext/ply/test/yacc_uprec.py:
Import patch ply.diff
src/arch/isa_parser.py:
everything is now within the ply package
--HG--
rename : ext/ply/lex.py => ext/ply/ply/lex.py
rename : ext/ply/yacc.py => ext/ply/ply/yacc.py
extra : convert_revision : fca8deabd5c095bdeabd52a1f236ae1404ef106e
2910 lines
95 KiB
HTML
2910 lines
95 KiB
HTML
<html>
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<head>
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<title>PLY (Python Lex-Yacc)</title>
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</head>
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<body bgcolor="#ffffff">
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<h1>PLY (Python Lex-Yacc)</h1>
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<b>
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David M. Beazley <br>
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dave@dabeaz.com<br>
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</b>
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<p>
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<b>PLY Version: 2.3</b>
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<p>
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<!-- INDEX -->
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<div class="sectiontoc">
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<ul>
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<li><a href="#ply_nn1">Introduction</a>
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<li><a href="#ply_nn2">PLY Overview</a>
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<li><a href="#ply_nn3">Lex</a>
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<ul>
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<li><a href="#ply_nn4">Lex Example</a>
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<li><a href="#ply_nn5">The tokens list</a>
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<li><a href="#ply_nn6">Specification of tokens</a>
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<li><a href="#ply_nn7">Token values</a>
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<li><a href="#ply_nn8">Discarded tokens</a>
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<li><a href="#ply_nn9">Line numbers and positional information</a>
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<li><a href="#ply_nn10">Ignored characters</a>
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<li><a href="#ply_nn11">Literal characters</a>
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<li><a href="#ply_nn12">Error handling</a>
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<li><a href="#ply_nn13">Building and using the lexer</a>
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<li><a href="#ply_nn14">The @TOKEN decorator</a>
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<li><a href="#ply_nn15">Optimized mode</a>
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<li><a href="#ply_nn16">Debugging</a>
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<li><a href="#ply_nn17">Alternative specification of lexers</a>
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<li><a href="#ply_nn18">Maintaining state</a>
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<li><a href="#ply_nn19">Duplicating lexers</a>
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<li><a href="#ply_nn20">Internal lexer state</a>
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<li><a href="#ply_nn21">Conditional lexing and start conditions</a>
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<li><a href="#ply_nn21">Miscellaneous Issues</a>
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</ul>
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<li><a href="#ply_nn22">Parsing basics</a>
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<li><a href="#ply_nn23">Yacc reference</a>
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<ul>
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<li><a href="#ply_nn24">An example</a>
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<li><a href="#ply_nn25">Combining Grammar Rule Functions</a>
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<li><a href="#ply_nn26">Character Literals</a>
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<li><a href="#ply_nn26">Empty Productions</a>
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<li><a href="#ply_nn28">Changing the starting symbol</a>
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<li><a href="#ply_nn27">Dealing With Ambiguous Grammars</a>
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<li><a href="#ply_nn28">The parser.out file</a>
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<li><a href="#ply_nn29">Syntax Error Handling</a>
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<ul>
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<li><a href="#ply_nn30">Recovery and resynchronization with error rules</a>
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<li><a href="#ply_nn31">Panic mode recovery</a>
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<li><a href="#ply_nn32">General comments on error handling</a>
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</ul>
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<li><a href="#ply_nn33">Line Number and Position Tracking</a>
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<li><a href="#ply_nn34">AST Construction</a>
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<li><a href="#ply_nn35">Embedded Actions</a>
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<li><a href="#ply_nn36">Yacc implementation notes</a>
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</ul>
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<li><a href="#ply_nn37">Parser and Lexer State Management</a>
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<li><a href="#ply_nn38">Using Python's Optimized Mode</a>
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<li><a href="#ply_nn39">Where to go from here?</a>
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</ul>
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</div>
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<!-- INDEX -->
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<H2><a name="ply_nn1"></a>1. Introduction</H2>
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PLY is a pure-Python implementation of the popular compiler
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construction tools lex and yacc. The main goal of PLY is to stay
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fairly faithful to the way in which traditional lex/yacc tools work.
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This includes supporting LALR(1) parsing as well as providing
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extensive input validation, error reporting, and diagnostics. Thus,
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if you've used yacc in another programming language, it should be
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relatively straightforward to use PLY.
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<p>
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Early versions of PLY were developed to support an Introduction to
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Compilers Course I taught in 2001 at the University of Chicago. In this course,
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students built a fully functional compiler for a simple Pascal-like
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language. Their compiler, implemented entirely in Python, had to
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include lexical analysis, parsing, type checking, type inference,
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nested scoping, and code generation for the SPARC processor.
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Approximately 30 different compiler implementations were completed in
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this course. Most of PLY's interface and operation has been influenced by common
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usability problems encountered by students.
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<p>
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Since PLY was primarily developed as an instructional tool, you will
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find it to be fairly picky about token and grammar rule
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specification. In part, this
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added formality is meant to catch common programming mistakes made by
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novice users. However, advanced users will also find such features to
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be useful when building complicated grammars for real programming
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languages. It should also be noted that PLY does not provide much in
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the way of bells and whistles (e.g., automatic construction of
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abstract syntax trees, tree traversal, etc.). Nor would I consider it
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to be a parsing framework. Instead, you will find a bare-bones, yet
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fully capable lex/yacc implementation written entirely in Python.
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<p>
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The rest of this document assumes that you are somewhat familar with
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parsing theory, syntax directed translation, and the use of compiler
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construction tools such as lex and yacc in other programming
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languages. If you are unfamilar with these topics, you will probably
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want to consult an introductory text such as "Compilers: Principles,
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Techniques, and Tools", by Aho, Sethi, and Ullman. O'Reilly's "Lex
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and Yacc" by John Levine may also be handy. In fact, the O'Reilly book can be
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used as a reference for PLY as the concepts are virtually identical.
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<H2><a name="ply_nn2"></a>2. PLY Overview</H2>
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PLY consists of two separate modules; <tt>lex.py</tt> and
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<tt>yacc.py</tt>, both of which are found in a Python package
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called <tt>ply</tt>. The <tt>lex.py</tt> module is used to break input text into a
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collection of tokens specified by a collection of regular expression
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rules. <tt>yacc.py</tt> is used to recognize language syntax that has
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been specified in the form of a context free grammar. <tt>yacc.py</tt> uses LR parsing and generates its parsing tables
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using either the LALR(1) (the default) or SLR table generation algorithms.
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<p>
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The two tools are meant to work together. Specifically,
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<tt>lex.py</tt> provides an external interface in the form of a
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<tt>token()</tt> function that returns the next valid token on the
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input stream. <tt>yacc.py</tt> calls this repeatedly to retrieve
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tokens and invoke grammar rules. The output of <tt>yacc.py</tt> is
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often an Abstract Syntax Tree (AST). However, this is entirely up to
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the user. If desired, <tt>yacc.py</tt> can also be used to implement
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simple one-pass compilers.
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<p>
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Like its Unix counterpart, <tt>yacc.py</tt> provides most of the
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features you expect including extensive error checking, grammar
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validation, support for empty productions, error tokens, and ambiguity
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resolution via precedence rules. In fact, everything that is possible in traditional yacc
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should be supported in PLY.
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<p>
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The primary difference between
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<tt>yacc.py</tt> and Unix <tt>yacc</tt> is that <tt>yacc.py</tt>
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doesn't involve a separate code-generation process.
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Instead, PLY relies on reflection (introspection)
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to build its lexers and parsers. Unlike traditional lex/yacc which
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require a special input file that is converted into a separate source
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file, the specifications given to PLY <em>are</em> valid Python
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programs. This means that there are no extra source files nor is
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there a special compiler construction step (e.g., running yacc to
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generate Python code for the compiler). Since the generation of the
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parsing tables is relatively expensive, PLY caches the results and
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saves them to a file. If no changes are detected in the input source,
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the tables are read from the cache. Otherwise, they are regenerated.
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<H2><a name="ply_nn3"></a>3. Lex</H2>
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<tt>lex.py</tt> is used to tokenize an input string. For example, suppose
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you're writing a programming language and a user supplied the following input string:
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<blockquote>
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<pre>
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x = 3 + 42 * (s - t)
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</pre>
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</blockquote>
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A tokenizer splits the string into individual tokens
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<blockquote>
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<pre>
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'x','=', '3', '+', '42', '*', '(', 's', '-', 't', ')'
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</pre>
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</blockquote>
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Tokens are usually given names to indicate what they are. For example:
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<blockquote>
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<pre>
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'ID','EQUALS','NUMBER','PLUS','NUMBER','TIMES',
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'LPAREN','ID','MINUS','ID','RPAREN'
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</pre>
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</blockquote>
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More specifically, the input is broken into pairs of token types and values. For example:
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<blockquote>
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<pre>
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('ID','x'), ('EQUALS','='), ('NUMBER','3'),
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('PLUS','+'), ('NUMBER','42), ('TIMES','*'),
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('LPAREN','('), ('ID','s'), ('MINUS','-'),
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('ID','t'), ('RPAREN',')'
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</pre>
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</blockquote>
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The identification of tokens is typically done by writing a series of regular expression
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rules. The next section shows how this is done using <tt>lex.py</tt>.
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<H3><a name="ply_nn4"></a>3.1 Lex Example</H3>
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The following example shows how <tt>lex.py</tt> is used to write a simple tokenizer.
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<blockquote>
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<pre>
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# ------------------------------------------------------------
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# calclex.py
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#
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# tokenizer for a simple expression evaluator for
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# numbers and +,-,*,/
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# ------------------------------------------------------------
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import ply.lex as lex
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# List of token names. This is always required
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tokens = (
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'NUMBER',
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'PLUS',
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'MINUS',
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'TIMES',
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'DIVIDE',
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'LPAREN',
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'RPAREN',
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)
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# Regular expression rules for simple tokens
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t_PLUS = r'\+'
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t_MINUS = r'-'
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t_TIMES = r'\*'
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t_DIVIDE = r'/'
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t_LPAREN = r'\('
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t_RPAREN = r'\)'
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# A regular expression rule with some action code
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def t_NUMBER(t):
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r'\d+'
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try:
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t.value = int(t.value)
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except ValueError:
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print "Line %d: Number %s is too large!" % (t.lineno,t.value)
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t.value = 0
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return t
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# Define a rule so we can track line numbers
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def t_newline(t):
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r'\n+'
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t.lexer.lineno += len(t.value)
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# A string containing ignored characters (spaces and tabs)
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t_ignore = ' \t'
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# Error handling rule
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def t_error(t):
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print "Illegal character '%s'" % t.value[0]
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t.lexer.skip(1)
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# Build the lexer
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lex.lex()
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</pre>
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</blockquote>
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To use the lexer, you first need to feed it some input text using its <tt>input()</tt> method. After that, repeated calls to <tt>token()</tt> produce tokens. The following code shows how this works:
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<blockquote>
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<pre>
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# Test it out
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data = '''
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3 + 4 * 10
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+ -20 *2
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'''
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# Give the lexer some input
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lex.input(data)
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# Tokenize
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while 1:
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tok = lex.token()
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if not tok: break # No more input
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print tok
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</pre>
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</blockquote>
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When executed, the example will produce the following output:
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<blockquote>
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<pre>
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$ python example.py
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LexToken(NUMBER,3,2,1)
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LexToken(PLUS,'+',2,3)
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LexToken(NUMBER,4,2,5)
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LexToken(TIMES,'*',2,7)
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LexToken(NUMBER,10,2,10)
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LexToken(PLUS,'+',3,14)
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LexToken(MINUS,'-',3,16)
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LexToken(NUMBER,20,3,18)
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LexToken(TIMES,'*',3,20)
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LexToken(NUMBER,2,3,21)
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</pre>
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</blockquote>
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The tokens returned by <tt>lex.token()</tt> are instances
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of <tt>LexToken</tt>. This object has
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attributes <tt>tok.type</tt>, <tt>tok.value</tt>,
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<tt>tok.lineno</tt>, and <tt>tok.lexpos</tt>. The following code shows an example of
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accessing these attributes:
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<blockquote>
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<pre>
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# Tokenize
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while 1:
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tok = lex.token()
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if not tok: break # No more input
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print tok.type, tok.value, tok.line, tok.lexpos
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</pre>
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</blockquote>
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The <tt>tok.type</tt> and <tt>tok.value</tt> attributes contain the
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type and value of the token itself.
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<tt>tok.line</tt> and <tt>tok.lexpos</tt> contain information about
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the location of the token. <tt>tok.lexpos</tt> is the index of the
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token relative to the start of the input text.
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<H3><a name="ply_nn5"></a>3.2 The tokens list</H3>
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All lexers must provide a list <tt>tokens</tt> that defines all of the possible token
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names that can be produced by the lexer. This list is always required
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and is used to perform a variety of validation checks. The tokens list is also used by the
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<tt>yacc.py</tt> module to identify terminals.
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<p>
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In the example, the following code specified the token names:
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<blockquote>
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<pre>
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tokens = (
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'NUMBER',
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'PLUS',
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'MINUS',
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'TIMES',
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'DIVIDE',
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'LPAREN',
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'RPAREN',
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)
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</pre>
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</blockquote>
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<H3><a name="ply_nn6"></a>3.3 Specification of tokens</H3>
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Each token is specified by writing a regular expression rule. Each of these rules are
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are defined by making declarations with a special prefix <tt>t_</tt> to indicate that it
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defines a token. For simple tokens, the regular expression can
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be specified as strings such as this (note: Python raw strings are used since they are the
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most convenient way to write regular expression strings):
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<blockquote>
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<pre>
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t_PLUS = r'\+'
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</pre>
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</blockquote>
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In this case, the name following the <tt>t_</tt> must exactly match one of the
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names supplied in <tt>tokens</tt>. If some kind of action needs to be performed,
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a token rule can be specified as a function. For example, this rule matches numbers and
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converts the string into a Python integer.
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<blockquote>
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<pre>
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def t_NUMBER(t):
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r'\d+'
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try:
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t.value = int(t.value)
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except ValueError:
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print "Number %s is too large!" % t.value
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t.value = 0
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return t
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</pre>
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</blockquote>
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When a function is used, the regular expression rule is specified in the function documentation string.
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The function always takes a single argument which is an instance of
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<tt>LexToken</tt>. This object has attributes of <tt>t.type</tt> which is the token type (as a string),
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<tt>t.value</tt> which is the lexeme (the actual text matched), <tt>t.lineno</tt> which is the current line number, and <tt>t.lexpos</tt> which
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is the position of the token relative to the beginning of the input text.
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By default, <tt>t.type</tt> is set to the name following the <tt>t_</tt> prefix. The action
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function can modify the contents of the <tt>LexToken</tt> object as appropriate. However,
|
|
when it is done, the resulting token should be returned. If no value is returned by the action
|
|
function, the token is simply discarded and the next token read.
|
|
|
|
<p>
|
|
Internally, <tt>lex.py</tt> uses the <tt>re</tt> module to do its patten matching. When building the master regular expression,
|
|
rules are added in the following order:
|
|
<p>
|
|
<ol>
|
|
<li>All tokens defined by functions are added in the same order as they appear in the lexer file.
|
|
<li>Tokens defined by strings are added next by sorting them in order of decreasing regular expression length (longer expressions
|
|
are added first).
|
|
</ol>
|
|
<p>
|
|
Without this ordering, it can be difficult to correctly match certain types of tokens. For example, if you
|
|
wanted to have separate tokens for "=" and "==", you need to make sure that "==" is checked first. By sorting regular
|
|
expressions in order of decreasing length, this problem is solved for rules defined as strings. For functions,
|
|
the order can be explicitly controlled since rules appearing first are checked first.
|
|
|
|
<p>
|
|
To handle reserved words, it is usually easier to just match an identifier and do a special name lookup in a function
|
|
like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
reserved = {
|
|
'if' : 'IF',
|
|
'then' : 'THEN',
|
|
'else' : 'ELSE',
|
|
'while' : 'WHILE',
|
|
...
|
|
}
|
|
|
|
def t_ID(t):
|
|
r'[a-zA-Z_][a-zA-Z_0-9]*'
|
|
t.type = reserved.get(t.value,'ID') # Check for reserved words
|
|
return t
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This approach greatly reduces the number of regular expression rules and is likely to make things a little faster.
|
|
|
|
<p>
|
|
<b>Note:</b> You should avoid writing individual rules for reserved words. For example, if you write rules like this,
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_FOR = r'for'
|
|
t_PRINT = r'print'
|
|
</pre>
|
|
</blockquote>
|
|
|
|
those rules will be triggered for identifiers that include those words as a prefix such as "forget" or "printed". This is probably not
|
|
what you want.
|
|
|
|
<H3><a name="ply_nn7"></a>3.4 Token values</H3>
|
|
|
|
|
|
When tokens are returned by lex, they have a value that is stored in the <tt>value</tt> attribute. Normally, the value is the text
|
|
that was matched. However, the value can be assigned to any Python object. For instance, when lexing identifiers, you may
|
|
want to return both the identifier name and information from some sort of symbol table. To do this, you might write a rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_ID(t):
|
|
...
|
|
# Look up symbol table information and return a tuple
|
|
t.value = (t.value, symbol_lookup(t.value))
|
|
...
|
|
return t
|
|
</pre>
|
|
</blockquote>
|
|
|
|
It is important to note that storing data in other attribute names is <em>not</em> recommended. The <tt>yacc.py</tt> module only exposes the
|
|
contents of the <tt>value</tt> attribute. Thus, accessing other attributes may be unnecessarily awkward.
|
|
|
|
<H3><a name="ply_nn8"></a>3.5 Discarded tokens</H3>
|
|
|
|
|
|
To discard a token, such as a comment, simply define a token rule that returns no value. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_COMMENT(t):
|
|
r'\#.*'
|
|
pass
|
|
# No return value. Token discarded
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Alternatively, you can include the prefix "ignore_" in the token declaration to force a token to be ignored. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_ignore_COMMENT = r'\#.*'
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Be advised that if you are ignoring many different kinds of text, you may still want to use functions since these provide more precise
|
|
control over the order in which regular expressions are matched (i.e., functions are matched in order of specification whereas strings are
|
|
sorted by regular expression length).
|
|
|
|
<H3><a name="ply_nn9"></a>3.6 Line numbers and positional information</H3>
|
|
|
|
|
|
<p>By default, <tt>lex.py</tt> knows nothing about line numbers. This is because <tt>lex.py</tt> doesn't know anything
|
|
about what constitutes a "line" of input (e.g., the newline character or even if the input is textual data).
|
|
To update this information, you need to write a special rule. In the example, the <tt>t_newline()</tt> rule shows how to do this.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# Define a rule so we can track line numbers
|
|
def t_newline(t):
|
|
r'\n+'
|
|
t.lexer.lineno += len(t.value)
|
|
</pre>
|
|
</blockquote>
|
|
Within the rule, the <tt>lineno</tt> attribute of the underlying lexer <tt>t.lexer</tt> is updated.
|
|
After the line number is updated, the token is simply discarded since nothing is returned.
|
|
|
|
<p>
|
|
<tt>lex.py</tt> does not perform and kind of automatic column tracking. However, it does record positional
|
|
information related to each token in the <tt>lexpos</tt> attribute. Using this, it is usually possible to compute
|
|
column information as a separate step. For instance, just count backwards until you reach a newline.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# Compute column.
|
|
# input is the input text string
|
|
# token is a token instance
|
|
def find_column(input,token):
|
|
i = token.lexpos
|
|
while i > 0:
|
|
if input[i] == '\n': break
|
|
i -= 1
|
|
column = (token.lexpos - i)+1
|
|
return column
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Since column information is often only useful in the context of error handling, calculating the column
|
|
position can be performed when needed as opposed to doing it for each token.
|
|
|
|
<H3><a name="ply_nn10"></a>3.7 Ignored characters</H3>
|
|
|
|
|
|
<p>
|
|
The special <tt>t_ignore</tt> rule is reserved by <tt>lex.py</tt> for characters
|
|
that should be completely ignored in the input stream.
|
|
Usually this is used to skip over whitespace and other non-essential characters.
|
|
Although it is possible to define a regular expression rule for whitespace in a manner
|
|
similar to <tt>t_newline()</tt>, the use of <tt>t_ignore</tt> provides substantially better
|
|
lexing performance because it is handled as a special case and is checked in a much
|
|
more efficient manner than the normal regular expression rules.
|
|
|
|
<H3><a name="ply_nn11"></a>3.8 Literal characters</H3>
|
|
|
|
|
|
<p>
|
|
Literal characters can be specified by defining a variable <tt>literals</tt> in your lexing module. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
literals = [ '+','-','*','/' ]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
or alternatively
|
|
|
|
<blockquote>
|
|
<pre>
|
|
literals = "+-*/"
|
|
</pre>
|
|
</blockquote>
|
|
|
|
A literal character is simply a single character that is returned "as is" when encountered by the lexer. Literals are checked
|
|
after all of the defined regular expression rules. Thus, if a rule starts with one of the literal characters, it will always
|
|
take precedence.
|
|
<p>
|
|
When a literal token is returned, both its <tt>type</tt> and <tt>value</tt> attributes are set to the character itself. For example, <tt>'+'</tt>.
|
|
|
|
<H3><a name="ply_nn12"></a>3.9 Error handling</H3>
|
|
|
|
|
|
<p>
|
|
Finally, the <tt>t_error()</tt>
|
|
function is used to handle lexing errors that occur when illegal
|
|
characters are detected. In this case, the <tt>t.value</tt> attribute contains the
|
|
rest of the input string that has not been tokenized. In the example, the error function
|
|
was defined as follows:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# Error handling rule
|
|
def t_error(t):
|
|
print "Illegal character '%s'" % t.value[0]
|
|
t.lexer.skip(1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, we simply print the offending character and skip ahead one character by calling <tt>t.lexer.skip(1)</tt>.
|
|
|
|
<H3><a name="ply_nn13"></a>3.10 Building and using the lexer</H3>
|
|
|
|
|
|
<p>
|
|
To build the lexer, the function <tt>lex.lex()</tt> is used. This function
|
|
uses Python reflection (or introspection) to read the the regular expression rules
|
|
out of the calling context and build the lexer. Once the lexer has been built, two functions can
|
|
be used to control the lexer.
|
|
|
|
<ul>
|
|
<li><tt>lex.input(data)</tt>. Reset the lexer and store a new input string.
|
|
<li><tt>lex.token()</tt>. Return the next token. Returns a special <tt>LexToken</tt> instance on success or
|
|
None if the end of the input text has been reached.
|
|
</ul>
|
|
|
|
If desired, the lexer can also be used as an object. The <tt>lex()</tt> returns a <tt>Lexer</tt> object that
|
|
can be used for this purpose. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lexer = lex.lex()
|
|
lexer.input(sometext)
|
|
while 1:
|
|
tok = lexer.token()
|
|
if not tok: break
|
|
print tok
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
This latter technique should be used if you intend to use multiple lexers in your application. Simply define each
|
|
lexer in its own module and use the object returned by <tt>lex()</tt> as appropriate.
|
|
|
|
<p>
|
|
Note: The global functions <tt>lex.input()</tt> and <tt>lex.token()</tt> are bound to the <tt>input()</tt>
|
|
and <tt>token()</tt> methods of the last lexer created by the lex module.
|
|
|
|
<H3><a name="ply_nn14"></a>3.11 The @TOKEN decorator</H3>
|
|
|
|
|
|
In some applications, you may want to define build tokens from as a series of
|
|
more complex regular expression rules. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
digit = r'([0-9])'
|
|
nondigit = r'([_A-Za-z])'
|
|
identifier = r'(' + nondigit + r'(' + digit + r'|' + nondigit + r')*)'
|
|
|
|
def t_ID(t):
|
|
# want docstring to be identifier above. ?????
|
|
...
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, we want the regular expression rule for <tt>ID</tt> to be one of the variables above. However, there is no
|
|
way to directly specify this using a normal documentation string. To solve this problem, you can use the <tt>@TOKEN</tt>
|
|
decorator. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
from ply.lex import TOKEN
|
|
|
|
@TOKEN(identifier)
|
|
def t_ID(t):
|
|
...
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This will attach <tt>identifier</tt> to the docstring for <tt>t_ID()</tt> allowing <tt>lex.py</tt> to work normally. An alternative
|
|
approach this problem is to set the docstring directly like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_ID(t):
|
|
...
|
|
|
|
t_ID.__doc__ = identifier
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<b>NOTE:</b> Use of <tt>@TOKEN</tt> requires Python-2.4 or newer. If you're concerned about backwards compatibility with older
|
|
versions of Python, use the alternative approach of setting the docstring directly.
|
|
|
|
<H3><a name="ply_nn15"></a>3.12 Optimized mode</H3>
|
|
|
|
|
|
For improved performance, it may be desirable to use Python's
|
|
optimized mode (e.g., running Python with the <tt>-O</tt>
|
|
option). However, doing so causes Python to ignore documentation
|
|
strings. This presents special problems for <tt>lex.py</tt>. To
|
|
handle this case, you can create your lexer using
|
|
the <tt>optimize</tt> option as follows:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lexer = lex.lex(optimize=1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Next, run Python in its normal operating mode. When you do
|
|
this, <tt>lex.py</tt> will write a file called <tt>lextab.py</tt> to
|
|
the current directory. This file contains all of the regular
|
|
expression rules and tables used during lexing. On subsequent
|
|
executions,
|
|
<tt>lextab.py</tt> will simply be imported to build the lexer. This
|
|
approach substantially improves the startup time of the lexer and it
|
|
works in Python's optimized mode.
|
|
|
|
<p>
|
|
To change the name of the lexer-generated file, use the <tt>lextab</tt> keyword argument. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lexer = lex.lex(optimize=1,lextab="footab")
|
|
</pre>
|
|
</blockquote>
|
|
|
|
When running in optimized mode, it is important to note that lex disables most error checking. Thus, this is really only recommended
|
|
if you're sure everything is working correctly and you're ready to start releasing production code.
|
|
|
|
<H3><a name="ply_nn16"></a>3.13 Debugging</H3>
|
|
|
|
|
|
For the purpose of debugging, you can run <tt>lex()</tt> in a debugging mode as follows:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lexer = lex.lex(debug=1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This will result in a large amount of debugging information to be printed including all of the added rules and the master
|
|
regular expressions.
|
|
|
|
In addition, <tt>lex.py</tt> comes with a simple main function which
|
|
will either tokenize input read from standard input or from a file specified
|
|
on the command line. To use it, simply put this in your lexer:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
if __name__ == '__main__':
|
|
lex.runmain()
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<H3><a name="ply_nn17"></a>3.14 Alternative specification of lexers</H3>
|
|
|
|
|
|
As shown in the example, lexers are specified all within one Python module. If you want to
|
|
put token rules in a different module from the one in which you invoke <tt>lex()</tt>, use the
|
|
<tt>module</tt> keyword argument.
|
|
|
|
<p>
|
|
For example, you might have a dedicated module that just contains
|
|
the token rules:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# module: tokrules.py
|
|
# This module just contains the lexing rules
|
|
|
|
# List of token names. This is always required
|
|
tokens = (
|
|
'NUMBER',
|
|
'PLUS',
|
|
'MINUS',
|
|
'TIMES',
|
|
'DIVIDE',
|
|
'LPAREN',
|
|
'RPAREN',
|
|
)
|
|
|
|
# Regular expression rules for simple tokens
|
|
t_PLUS = r'\+'
|
|
t_MINUS = r'-'
|
|
t_TIMES = r'\*'
|
|
t_DIVIDE = r'/'
|
|
t_LPAREN = r'\('
|
|
t_RPAREN = r'\)'
|
|
|
|
# A regular expression rule with some action code
|
|
def t_NUMBER(t):
|
|
r'\d+'
|
|
try:
|
|
t.value = int(t.value)
|
|
except ValueError:
|
|
print "Line %d: Number %s is too large!" % (t.lineno,t.value)
|
|
t.value = 0
|
|
return t
|
|
|
|
# Define a rule so we can track line numbers
|
|
def t_newline(t):
|
|
r'\n+'
|
|
t.lexer.lineno += len(t.value)
|
|
|
|
# A string containing ignored characters (spaces and tabs)
|
|
t_ignore = ' \t'
|
|
|
|
# Error handling rule
|
|
def t_error(t):
|
|
print "Illegal character '%s'" % t.value[0]
|
|
t.lexer.skip(1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Now, if you wanted to build a tokenizer from these rules from within a different module, you would do the following (shown for Python interactive mode):
|
|
|
|
<blockquote>
|
|
<pre>
|
|
>>> import tokrules
|
|
>>> <b>lexer = lex.lex(module=tokrules)</b>
|
|
>>> lexer.input("3 + 4")
|
|
>>> lexer.token()
|
|
LexToken(NUMBER,3,1,1,0)
|
|
>>> lexer.token()
|
|
LexToken(PLUS,'+',1,2)
|
|
>>> lexer.token()
|
|
LexToken(NUMBER,4,1,4)
|
|
>>> lexer.token()
|
|
None
|
|
>>>
|
|
</pre>
|
|
</blockquote>
|
|
|
|
The <tt>object</tt> option can be used to define lexers as a class instead of a module. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
import ply.lex as lex
|
|
|
|
class MyLexer:
|
|
# List of token names. This is always required
|
|
tokens = (
|
|
'NUMBER',
|
|
'PLUS',
|
|
'MINUS',
|
|
'TIMES',
|
|
'DIVIDE',
|
|
'LPAREN',
|
|
'RPAREN',
|
|
)
|
|
|
|
# Regular expression rules for simple tokens
|
|
t_PLUS = r'\+'
|
|
t_MINUS = r'-'
|
|
t_TIMES = r'\*'
|
|
t_DIVIDE = r'/'
|
|
t_LPAREN = r'\('
|
|
t_RPAREN = r'\)'
|
|
|
|
# A regular expression rule with some action code
|
|
# Note addition of self parameter since we're in a class
|
|
def t_NUMBER(self,t):
|
|
r'\d+'
|
|
try:
|
|
t.value = int(t.value)
|
|
except ValueError:
|
|
print "Line %d: Number %s is too large!" % (t.lineno,t.value)
|
|
t.value = 0
|
|
return t
|
|
|
|
# Define a rule so we can track line numbers
|
|
def t_newline(self,t):
|
|
r'\n+'
|
|
t.lexer.lineno += len(t.value)
|
|
|
|
# A string containing ignored characters (spaces and tabs)
|
|
t_ignore = ' \t'
|
|
|
|
# Error handling rule
|
|
def t_error(self,t):
|
|
print "Illegal character '%s'" % t.value[0]
|
|
t.lexer.skip(1)
|
|
|
|
<b># Build the lexer
|
|
def build(self,**kwargs):
|
|
self.lexer = lex.lex(object=self, **kwargs)</b>
|
|
|
|
# Test it output
|
|
def test(self,data):
|
|
self.lexer.input(data)
|
|
while 1:
|
|
tok = lexer.token()
|
|
if not tok: break
|
|
print tok
|
|
|
|
# Build the lexer and try it out
|
|
m = MyLexer()
|
|
m.build() # Build the lexer
|
|
m.test("3 + 4") # Test it
|
|
</pre>
|
|
</blockquote>
|
|
|
|
For reasons that are subtle, you should <em>NOT</em> invoke <tt>lex.lex()</tt> inside the <tt>__init__()</tt> method of your class. If you
|
|
do, it may cause bizarre behavior if someone tries to duplicate a lexer object. Keep reading.
|
|
|
|
<H3><a name="ply_nn18"></a>3.15 Maintaining state</H3>
|
|
|
|
|
|
In your lexer, you may want to maintain a variety of state information. This might include mode settings, symbol tables, and other details. There are a few
|
|
different ways to handle this situation. First, you could just keep some global variables:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
num_count = 0
|
|
def t_NUMBER(t):
|
|
r'\d+'
|
|
global num_count
|
|
num_count += 1
|
|
try:
|
|
t.value = int(t.value)
|
|
except ValueError:
|
|
print "Line %d: Number %s is too large!" % (t.lineno,t.value)
|
|
t.value = 0
|
|
return t
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Alternatively, you can store this information inside the Lexer object created by <tt>lex()</tt>. To this, you can use the <tt>lexer</tt> attribute
|
|
of tokens passed to the various rules. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_NUMBER(t):
|
|
r'\d+'
|
|
t.lexer.num_count += 1 # Note use of lexer attribute
|
|
try:
|
|
t.value = int(t.value)
|
|
except ValueError:
|
|
print "Line %d: Number %s is too large!" % (t.lineno,t.value)
|
|
t.value = 0
|
|
return t
|
|
|
|
lexer = lex.lex()
|
|
lexer.num_count = 0 # Set the initial count
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This latter approach has the advantage of storing information inside
|
|
the lexer itself---something that may be useful if multiple instances
|
|
of the same lexer have been created. However, it may also feel kind
|
|
of "hacky" to the purists. Just to put their mind at some ease, all
|
|
internal attributes of the lexer (with the exception of <tt>lineno</tt>) have names that are prefixed
|
|
by <tt>lex</tt> (e.g., <tt>lexdata</tt>,<tt>lexpos</tt>, etc.). Thus,
|
|
it should be perfectly safe to store attributes in the lexer that
|
|
don't have names starting with that prefix.
|
|
|
|
<p>
|
|
A third approach is to define the lexer as a class as shown in the previous example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
class MyLexer:
|
|
...
|
|
def t_NUMBER(self,t):
|
|
r'\d+'
|
|
self.num_count += 1
|
|
try:
|
|
t.value = int(t.value)
|
|
except ValueError:
|
|
print "Line %d: Number %s is too large!" % (t.lineno,t.value)
|
|
t.value = 0
|
|
return t
|
|
|
|
def build(self, **kwargs):
|
|
self.lexer = lex.lex(object=self,**kwargs)
|
|
|
|
def __init__(self):
|
|
self.num_count = 0
|
|
|
|
# Create a lexer
|
|
m = MyLexer()
|
|
lexer = lex.lex(object=m)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
The class approach may be the easiest to manage if your application is going to be creating multiple instances of the same lexer and
|
|
you need to manage a lot of state.
|
|
|
|
<H3><a name="ply_nn19"></a>3.16 Duplicating lexers</H3>
|
|
|
|
|
|
<b>NOTE: I am thinking about deprecating this feature. Post comments on <a href="http://groups.google.com/group/ply-hack">ply-hack@googlegroups.com</a> or send me a private email at dave@dabeaz.com.</b>
|
|
|
|
<p>
|
|
If necessary, a lexer object can be quickly duplicated by invoking its <tt>clone()</tt> method. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lexer = lex.lex()
|
|
...
|
|
newlexer = lexer.clone()
|
|
</pre>
|
|
</blockquote>
|
|
|
|
When a lexer is cloned, the copy is identical to the original lexer,
|
|
including any input text. However, once created, different text can be
|
|
fed to the clone which can be used independently. This capability may
|
|
be useful in situations when you are writing a parser/compiler that
|
|
involves recursive or reentrant processing. For instance, if you
|
|
needed to scan ahead in the input for some reason, you could create a
|
|
clone and use it to look ahead.
|
|
|
|
<p>
|
|
The advantage of using <tt>clone()</tt> instead of reinvoking <tt>lex()</tt> is
|
|
that it is significantly faster. Namely, it is not necessary to re-examine all of the
|
|
token rules, build a regular expression, and construct internal tables. All of this
|
|
information can simply be reused in the new lexer.
|
|
|
|
<p>
|
|
Special considerations need to be made when cloning a lexer that is defined as a class. Previous sections
|
|
showed an example of a class <tt>MyLexer</tt>. If you have the following code:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
m = MyLexer()
|
|
a = lex.lex(object=m) # Create a lexer
|
|
|
|
b = a.clone() # Clone the lexer
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Then both <tt>a</tt> and <tt>b</tt> are going to be bound to the same
|
|
object <tt>m</tt>. If the object <tt>m</tt> contains internal state
|
|
related to lexing, this sharing may lead to quite a bit of confusion. To fix this,
|
|
the <tt>clone()</tt> method accepts an optional argument that can be used to supply a new object. This
|
|
can be used to clone the lexer and bind it to a new instance. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
m = MyLexer() # Create a lexer
|
|
a = lex.lex(object=m)
|
|
|
|
# Create a clone
|
|
n = MyLexer() # New instance of MyLexer
|
|
b = a.clone(n) # New lexer bound to n
|
|
</pre>
|
|
</blockquote>
|
|
|
|
It may make sense to encapsulate all of this inside a method:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
class MyLexer:
|
|
...
|
|
def clone(self):
|
|
c = MyLexer() # Create a new instance of myself
|
|
# Copy attributes from self to c as appropriate
|
|
...
|
|
# Clone the lexer
|
|
c.lexer = self.lexer.clone(c)
|
|
return c
|
|
</pre>
|
|
</blockquote>
|
|
|
|
The fact that a new instance of <tt>MyLexer</tt> may be created while cloning a lexer is the reason why you should never
|
|
invoke <tt>lex.lex()</tt> inside <tt>__init__()</tt>. If you do, the lexer will be rebuilt from scratch and you lose
|
|
all of the performance benefits of using <tt>clone()</tt> in the first place.
|
|
|
|
<H3><a name="ply_nn20"></a>3.17 Internal lexer state</H3>
|
|
|
|
|
|
A Lexer object <tt>lexer</tt> has a number of internal attributes that may be useful in certain
|
|
situations.
|
|
|
|
<p>
|
|
<tt>lexer.lexpos</tt>
|
|
<blockquote>
|
|
This attribute is an integer that contains the current position within the input text. If you modify
|
|
the value, it will change the result of the next call to <tt>token()</tt>. Within token rule functions, this points
|
|
to the first character <em>after</em> the matched text. If the value is modified within a rule, the next returned token will be
|
|
matched at the new position.
|
|
</blockquote>
|
|
|
|
<p>
|
|
<tt>lexer.lineno</tt>
|
|
<blockquote>
|
|
The current value of the line number attribute stored in the lexer. This can be modified as needed to
|
|
change the line number.
|
|
</blockquote>
|
|
|
|
<p>
|
|
<tt>lexer.lexdata</tt>
|
|
<blockquote>
|
|
The current input text stored in the lexer. This is the string passed with the <tt>input()</tt> method. It
|
|
would probably be a bad idea to modify this unless you really know what you're doing.
|
|
</blockquote>
|
|
|
|
<P>
|
|
<tt>lexer.lexmatch</tt>
|
|
<blockquote>
|
|
This is the raw <tt>Match</tt> object returned by the Python <tt>re.match()</tt> function (used internally by PLY) for the
|
|
current token. If you have written a regular expression that contains named groups, you can use this to retrieve those values.
|
|
</blockquote>
|
|
|
|
<H3><a name="ply_nn21"></a>3.18 Conditional lexing and start conditions</H3>
|
|
|
|
|
|
In advanced parsing applications, it may be useful to have different
|
|
lexing states. For instance, you may want the occurrence of a certain
|
|
token or syntactic construct to trigger a different kind of lexing.
|
|
PLY supports a feature that allows the underlying lexer to be put into
|
|
a series of different states. Each state can have its own tokens,
|
|
lexing rules, and so forth. The implementation is based largely on
|
|
the "start condition" feature of GNU flex. Details of this can be found
|
|
at <a
|
|
href="http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html">http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html.</a>.
|
|
|
|
<p>
|
|
To define a new lexing state, it must first be declared. This is done by including a "states" declaration in your
|
|
lex file. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
states = (
|
|
('foo','exclusive'),
|
|
('bar','inclusive'),
|
|
)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This declaration declares two states, <tt>'foo'</tt>
|
|
and <tt>'bar'</tt>. States may be of two types; <tt>'exclusive'</tt>
|
|
and <tt>'inclusive'</tt>. An exclusive state completely overrides the
|
|
default behavior of the lexer. That is, lex will only return tokens
|
|
and apply rules defined specifically for that state. An inclusive
|
|
state adds additional tokens and rules to the default set of rules.
|
|
Thus, lex will return both the tokens defined by default in addition
|
|
to those defined for the inclusive state.
|
|
|
|
<p>
|
|
Once a state has been declared, tokens and rules are declared by including the
|
|
state name in token/rule declaration. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_foo_NUMBER = r'\d+' # Token 'NUMBER' in state 'foo'
|
|
t_bar_ID = r'[a-zA-Z_][a-zA-Z0-9_]*' # Token 'ID' in state 'bar'
|
|
|
|
def t_foo_newline(t):
|
|
r'\n'
|
|
t.lexer.lineno += 1
|
|
</pre>
|
|
</blockquote>
|
|
|
|
A token can be declared in multiple states by including multiple state names in the declaration. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_foo_bar_NUMBER = r'\d+' # Defines token 'NUMBER' in both state 'foo' and 'bar'
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Alternative, a token can be declared in all states using the 'ANY' in the name.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_ANY_NUMBER = r'\d+' # Defines a token 'NUMBER' in all states
|
|
</pre>
|
|
</blockquote>
|
|
|
|
If no state name is supplied, as is normally the case, the token is associated with a special state <tt>'INITIAL'</tt>. For example,
|
|
these two declarations are identical:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_NUMBER = r'\d+'
|
|
t_INITIAL_NUMBER = r'\d+'
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
States are also associated with the special <tt>t_ignore</tt> and <tt>t_error()</tt> declarations. For example, if a state treats
|
|
these differently, you can declare:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
t_foo_ignore = " \t\n" # Ignored characters for state 'foo'
|
|
|
|
def t_bar_error(t): # Special error handler for state 'bar'
|
|
pass
|
|
</pre>
|
|
</blockquote>
|
|
|
|
By default, lexing operates in the <tt>'INITIAL'</tt> state. This state includes all of the normally defined tokens.
|
|
For users who aren't using different states, this fact is completely transparent. If, during lexing or parsing, you want to change
|
|
the lexing state, use the <tt>begin()</tt> method. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_begin_foo(t):
|
|
r'start_foo'
|
|
t.lexer.begin('foo') # Starts 'foo' state
|
|
</pre>
|
|
</blockquote>
|
|
|
|
To get out of a state, you use <tt>begin()</tt> to switch back to the initial state. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_foo_end(t):
|
|
r'end_foo'
|
|
t.lexer.begin('INITIAL') # Back to the initial state
|
|
</pre>
|
|
</blockquote>
|
|
|
|
The management of states can also be done with a stack. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_begin_foo(t):
|
|
r'start_foo'
|
|
t.lexer.push_state('foo') # Starts 'foo' state
|
|
|
|
def t_foo_end(t):
|
|
r'end_foo'
|
|
t.lexer.pop_state() # Back to the previous state
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
The use of a stack would be useful in situations where there are many ways of entering a new lexing state and you merely want to go back
|
|
to the previous state afterwards.
|
|
|
|
<P>
|
|
An example might help clarify. Suppose you were writing a parser and you wanted to grab sections of arbitrary C code enclosed by
|
|
curly braces. That is, whenever you encounter a starting brace '{', you want to read all of the enclosed code up to the ending brace '}'
|
|
and return it as a string. Doing this with a normal regular expression rule is nearly (if not actually) impossible. This is because braces can
|
|
be nested and can be included in comments and strings. Thus, simply matching up to the first matching '}' character isn't good enough. Here is how
|
|
you might use lexer states to do this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# Declare the state
|
|
states = (
|
|
('ccode','exclusive'),
|
|
)
|
|
|
|
# Match the first {. Enter ccode state.
|
|
def t_ccode(t):
|
|
r'\{'
|
|
t.lexer.code_start = t.lexer.lexpos # Record the starting position
|
|
t.lexer.level = 1 # Initial brace level
|
|
t.lexer.begin('ccode') # Enter 'ccode' state
|
|
|
|
# Rules for the ccode state
|
|
def t_ccode_lbrace(t):
|
|
r'\{'
|
|
t.lexer.level +=1
|
|
|
|
def t_ccode_rbrace(t):
|
|
r'\}'
|
|
t.lexer.level -=1
|
|
|
|
# If closing brace, return the code fragment
|
|
if t.lexer.level == 0:
|
|
t.value = t.lexer.lexdata[t.lexer.code_start:t.lexer.lexpos+1]
|
|
t.type = "CCODE"
|
|
t.lexer.lineno += t.value.count('\n')
|
|
t.lexer.begin('INITIAL')
|
|
return t
|
|
|
|
# C or C++ comment (ignore)
|
|
def t_ccode_comment(t):
|
|
r'(/\*(.|\n)*?*/)|(//.*)'
|
|
pass
|
|
|
|
# C string
|
|
def t_ccode_string(t):
|
|
r'\"([^\\\n]|(\\.))*?\"'
|
|
|
|
# C character literal
|
|
def t_ccode_char(t):
|
|
r'\'([^\\\n]|(\\.))*?\''
|
|
|
|
# Any sequence of non-whitespace characters (not braces, strings)
|
|
def t_ccode_nonspace(t):
|
|
r'[^\s\{\}\'\"]+'
|
|
|
|
# Ignored characters (whitespace)
|
|
t_ccode_ignore = " \t\n"
|
|
|
|
# For bad characters, we just skip over it
|
|
def t_ccode_error(t):
|
|
t.lexer.skip(1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this example, the occurrence of the first '{' causes the lexer to record the starting position and enter a new state <tt>'ccode'</tt>. A collection of rules then match
|
|
various parts of the input that follow (comments, strings, etc.). All of these rules merely discard the token (by not returning a value).
|
|
However, if the closing right brace is encountered, the rule <tt>t_ccode_rbrace</tt> collects all of the code (using the earlier recorded starting
|
|
position), stores it, and returns a token 'CCODE' containing all of that text. When returning the token, the lexing state is restored back to its
|
|
initial state.
|
|
|
|
<H3><a name="ply_nn21"></a>3.19 Miscellaneous Issues</H3>
|
|
|
|
|
|
<P>
|
|
<li>The lexer requires input to be supplied as a single input string. Since most machines have more than enough memory, this
|
|
rarely presents a performance concern. However, it means that the lexer currently can't be used with streaming data
|
|
such as open files or sockets. This limitation is primarily a side-effect of using the <tt>re</tt> module.
|
|
|
|
<p>
|
|
<li>The lexer should work properly with both Unicode strings given as token and pattern matching rules as
|
|
well as for input text.
|
|
|
|
<p>
|
|
<li>If you need to supply optional flags to the re.compile() function, use the reflags option to lex. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lex.lex(reflags=re.UNICODE)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
<li>Since the lexer is written entirely in Python, its performance is
|
|
largely determined by that of the Python <tt>re</tt> module. Although
|
|
the lexer has been written to be as efficient as possible, it's not
|
|
blazingly fast when used on very large input files. If
|
|
performance is concern, you might consider upgrading to the most
|
|
recent version of Python, creating a hand-written lexer, or offloading
|
|
the lexer into a C extension module.
|
|
|
|
<p>
|
|
If you are going to create a hand-written lexer and you plan to use it with <tt>yacc.py</tt>,
|
|
it only needs to conform to the following requirements:
|
|
|
|
<ul>
|
|
<li>It must provide a <tt>token()</tt> method that returns the next token or <tt>None</tt> if no more
|
|
tokens are available.
|
|
<li>The <tt>token()</tt> method must return an object <tt>tok</tt> that has <tt>type</tt> and <tt>value</tt> attributes.
|
|
</ul>
|
|
|
|
<H2><a name="ply_nn22"></a>4. Parsing basics</H2>
|
|
|
|
|
|
<tt>yacc.py</tt> is used to parse language syntax. Before showing an
|
|
example, there are a few important bits of background that must be
|
|
mentioned. First, <em>syntax</em> is usually specified in terms of a BNF grammar.
|
|
For example, if you wanted to parse
|
|
simple arithmetic expressions, you might first write an unambiguous
|
|
grammar specification like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
expression : expression + term
|
|
| expression - term
|
|
| term
|
|
|
|
term : term * factor
|
|
| term / factor
|
|
| factor
|
|
|
|
factor : NUMBER
|
|
| ( expression )
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In the grammar, symbols such as <tt>NUMBER</tt>, <tt>+</tt>, <tt>-</tt>, <tt>*</tt>, and <tt>/</tt> are known
|
|
as <em>terminals</em> and correspond to raw input tokens. Identifiers such as <tt>term</tt> and <tt>factor</tt> refer to more
|
|
complex rules, typically comprised of a collection of tokens. These identifiers are known as <em>non-terminals</em>.
|
|
<P>
|
|
The semantic behavior of a language is often specified using a
|
|
technique known as syntax directed translation. In syntax directed
|
|
translation, attributes are attached to each symbol in a given grammar
|
|
rule along with an action. Whenever a particular grammar rule is
|
|
recognized, the action describes what to do. For example, given the
|
|
expression grammar above, you might write the specification for a
|
|
simple calculator like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
Grammar Action
|
|
-------------------------------- --------------------------------------------
|
|
expression0 : expression1 + term expression0.val = expression1.val + term.val
|
|
| expression1 - term expression0.val = expression1.val - term.val
|
|
| term expression0.val = term.val
|
|
|
|
term0 : term1 * factor term0.val = term1.val * factor.val
|
|
| term1 / factor term0.val = term1.val / factor.val
|
|
| factor term0.val = factor.val
|
|
|
|
factor : NUMBER factor.val = int(NUMBER.lexval)
|
|
| ( expression ) factor.val = expression.val
|
|
</pre>
|
|
</blockquote>
|
|
|
|
A good way to think about syntax directed translation is to simply think of each symbol in the grammar as some
|
|
kind of object. The semantics of the language are then expressed as a collection of methods/operations on these
|
|
objects.
|
|
|
|
<p>
|
|
Yacc uses a parsing technique known as LR-parsing or shift-reduce parsing. LR parsing is a
|
|
bottom up technique that tries to recognize the right-hand-side of various grammar rules.
|
|
Whenever a valid right-hand-side is found in the input, the appropriate action code is triggered and the
|
|
grammar symbols are replaced by the grammar symbol on the left-hand-side.
|
|
|
|
<p>
|
|
LR parsing is commonly implemented by shifting grammar symbols onto a stack and looking at the stack and the next
|
|
input token for patterns. The details of the algorithm can be found in a compiler text, but the
|
|
following example illustrates the steps that are performed if you wanted to parse the expression
|
|
<tt>3 + 5 * (10 - 20)</tt> using the grammar defined above:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
Step Symbol Stack Input Tokens Action
|
|
---- --------------------- --------------------- -------------------------------
|
|
1 $ 3 + 5 * ( 10 - 20 )$ Shift 3
|
|
2 $ 3 + 5 * ( 10 - 20 )$ Reduce factor : NUMBER
|
|
3 $ factor + 5 * ( 10 - 20 )$ Reduce term : factor
|
|
4 $ term + 5 * ( 10 - 20 )$ Reduce expr : term
|
|
5 $ expr + 5 * ( 10 - 20 )$ Shift +
|
|
6 $ expr + 5 * ( 10 - 20 )$ Shift 5
|
|
7 $ expr + 5 * ( 10 - 20 )$ Reduce factor : NUMBER
|
|
8 $ expr + factor * ( 10 - 20 )$ Reduce term : factor
|
|
9 $ expr + term * ( 10 - 20 )$ Shift *
|
|
10 $ expr + term * ( 10 - 20 )$ Shift (
|
|
11 $ expr + term * ( 10 - 20 )$ Shift 10
|
|
12 $ expr + term * ( 10 - 20 )$ Reduce factor : NUMBER
|
|
13 $ expr + term * ( factor - 20 )$ Reduce term : factor
|
|
14 $ expr + term * ( term - 20 )$ Reduce expr : term
|
|
15 $ expr + term * ( expr - 20 )$ Shift -
|
|
16 $ expr + term * ( expr - 20 )$ Shift 20
|
|
17 $ expr + term * ( expr - 20 )$ Reduce factor : NUMBER
|
|
18 $ expr + term * ( expr - factor )$ Reduce term : factor
|
|
19 $ expr + term * ( expr - term )$ Reduce expr : expr - term
|
|
20 $ expr + term * ( expr )$ Shift )
|
|
21 $ expr + term * ( expr ) $ Reduce factor : (expr)
|
|
22 $ expr + term * factor $ Reduce term : term * factor
|
|
23 $ expr + term $ Reduce expr : expr + term
|
|
24 $ expr $ Reduce expr
|
|
25 $ $ Success!
|
|
</pre>
|
|
</blockquote>
|
|
|
|
When parsing the expression, an underlying state machine and the current input token determine what to do next.
|
|
If the next token looks like part of a valid grammar rule (based on other items on the stack), it is generally shifted
|
|
onto the stack. If the top of the stack contains a valid right-hand-side of a grammar rule, it is
|
|
usually "reduced" and the symbols replaced with the symbol on the left-hand-side. When this reduction occurs, the
|
|
appropriate action is triggered (if defined). If the input token can't be shifted and the top of stack doesn't match
|
|
any grammar rules, a syntax error has occurred and the parser must take some kind of recovery step (or bail out).
|
|
|
|
<p>
|
|
It is important to note that the underlying implementation is built around a large finite-state machine that is encoded
|
|
in a collection of tables. The construction of these tables is quite complicated and beyond the scope of this discussion.
|
|
However, subtle details of this process explain why, in the example above, the parser chooses to shift a token
|
|
onto the stack in step 9 rather than reducing the rule <tt>expr : expr + term</tt>.
|
|
|
|
<H2><a name="ply_nn23"></a>5. Yacc reference</H2>
|
|
|
|
|
|
This section describes how to use write parsers in PLY.
|
|
|
|
<H3><a name="ply_nn24"></a>5.1 An example</H3>
|
|
|
|
|
|
Suppose you wanted to make a grammar for simple arithmetic expressions as previously described. Here is
|
|
how you would do it with <tt>yacc.py</tt>:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# Yacc example
|
|
|
|
import ply.yacc as yacc
|
|
|
|
# Get the token map from the lexer. This is required.
|
|
from calclex import tokens
|
|
|
|
def p_expression_plus(p):
|
|
'expression : expression PLUS term'
|
|
p[0] = p[1] + p[3]
|
|
|
|
def p_expression_minus(p):
|
|
'expression : expression MINUS term'
|
|
p[0] = p[1] - p[3]
|
|
|
|
def p_expression_term(p):
|
|
'expression : term'
|
|
p[0] = p[1]
|
|
|
|
def p_term_times(p):
|
|
'term : term TIMES factor'
|
|
p[0] = p[1] * p[3]
|
|
|
|
def p_term_div(p):
|
|
'term : term DIVIDE factor'
|
|
p[0] = p[1] / p[3]
|
|
|
|
def p_term_factor(p):
|
|
'term : factor'
|
|
p[0] = p[1]
|
|
|
|
def p_factor_num(p):
|
|
'factor : NUMBER'
|
|
p[0] = p[1]
|
|
|
|
def p_factor_expr(p):
|
|
'factor : LPAREN expression RPAREN'
|
|
p[0] = p[2]
|
|
|
|
# Error rule for syntax errors
|
|
def p_error(p):
|
|
print "Syntax error in input!"
|
|
|
|
# Build the parser
|
|
yacc.yacc()
|
|
|
|
# Use this if you want to build the parser using SLR instead of LALR
|
|
# yacc.yacc(method="SLR")
|
|
|
|
while 1:
|
|
try:
|
|
s = raw_input('calc > ')
|
|
except EOFError:
|
|
break
|
|
if not s: continue
|
|
result = yacc.parse(s)
|
|
print result
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this example, each grammar rule is defined by a Python function where the docstring to that function contains the
|
|
appropriate context-free grammar specification. Each function accepts a single
|
|
argument <tt>p</tt> that is a sequence containing the values of each grammar symbol in the corresponding rule. The values of
|
|
<tt>p[i]</tt> are mapped to grammar symbols as shown here:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expression_plus(p):
|
|
'expression : expression PLUS term'
|
|
# ^ ^ ^ ^
|
|
# p[0] p[1] p[2] p[3]
|
|
|
|
p[0] = p[1] + p[3]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
For tokens, the "value" of the corresponding <tt>p[i]</tt> is the
|
|
<em>same</em> as the <tt>p.value</tt> attribute assigned
|
|
in the lexer module. For non-terminals, the value is determined by
|
|
whatever is placed in <tt>p[0]</tt> when rules are reduced. This
|
|
value can be anything at all. However, it probably most common for
|
|
the value to be a simple Python type, a tuple, or an instance. In this example, we
|
|
are relying on the fact that the <tt>NUMBER</tt> token stores an integer value in its value
|
|
field. All of the other rules simply perform various types of integer operations and store
|
|
the result.
|
|
|
|
<P>
|
|
Note: The use of negative indices have a special meaning in yacc---specially <tt>p[-1]</tt> does
|
|
not have the same value as <tt>p[3]</tt> in this example. Please see the section on "Embedded Actions" for further
|
|
details.
|
|
|
|
<p>
|
|
The first rule defined in the yacc specification determines the starting grammar
|
|
symbol (in this case, a rule for <tt>expression</tt> appears first). Whenever
|
|
the starting rule is reduced by the parser and no more input is available, parsing
|
|
stops and the final value is returned (this value will be whatever the top-most rule
|
|
placed in <tt>p[0]</tt>). Note: an alternative starting symbol can be specified using the <tt>start</tt> keyword argument to
|
|
<tt>yacc()</tt>.
|
|
|
|
<p>The <tt>p_error(p)</tt> rule is defined to catch syntax errors. See the error handling section
|
|
below for more detail.
|
|
|
|
<p>
|
|
To build the parser, call the <tt>yacc.yacc()</tt> function. This function
|
|
looks at the module and attempts to construct all of the LR parsing tables for the grammar
|
|
you have specified. The first time <tt>yacc.yacc()</tt> is invoked, you will get a message
|
|
such as this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
$ python calcparse.py
|
|
yacc: Generating LALR parsing table...
|
|
calc >
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Since table construction is relatively expensive (especially for large
|
|
grammars), the resulting parsing table is written to the current
|
|
directory in a file called <tt>parsetab.py</tt>. In addition, a
|
|
debugging file called <tt>parser.out</tt> is created. On subsequent
|
|
executions, <tt>yacc</tt> will reload the table from
|
|
<tt>parsetab.py</tt> unless it has detected a change in the underlying
|
|
grammar (in which case the tables and <tt>parsetab.py</tt> file are
|
|
regenerated). Note: The names of parser output files can be changed if necessary. See the notes that follow later.
|
|
|
|
<p>
|
|
If any errors are detected in your grammar specification, <tt>yacc.py</tt> will produce
|
|
diagnostic messages and possibly raise an exception. Some of the errors that can be detected include:
|
|
|
|
<ul>
|
|
<li>Duplicated function names (if more than one rule function have the same name in the grammar file).
|
|
<li>Shift/reduce and reduce/reduce conflicts generated by ambiguous grammars.
|
|
<li>Badly specified grammar rules.
|
|
<li>Infinite recursion (rules that can never terminate).
|
|
<li>Unused rules and tokens
|
|
<li>Undefined rules and tokens
|
|
</ul>
|
|
|
|
The next few sections now discuss a few finer points of grammar construction.
|
|
|
|
<H3><a name="ply_nn25"></a>5.2 Combining Grammar Rule Functions</H3>
|
|
|
|
|
|
When grammar rules are similar, they can be combined into a single function.
|
|
For example, consider the two rules in our earlier example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expression_plus(p):
|
|
'expression : expression PLUS term'
|
|
p[0] = p[1] + p[3]
|
|
|
|
def p_expression_minus(t):
|
|
'expression : expression MINUS term'
|
|
p[0] = p[1] - p[3]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Instead of writing two functions, you might write a single function like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expression(p):
|
|
'''expression : expression PLUS term
|
|
| expression MINUS term'''
|
|
if p[2] == '+':
|
|
p[0] = p[1] + p[3]
|
|
elif p[2] == '-':
|
|
p[0] = p[1] - p[3]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In general, the doc string for any given function can contain multiple grammar rules. So, it would
|
|
have also been legal (although possibly confusing) to write this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_binary_operators(p):
|
|
'''expression : expression PLUS term
|
|
| expression MINUS term
|
|
term : term TIMES factor
|
|
| term DIVIDE factor'''
|
|
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]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
When combining grammar rules into a single function, it is usually a good idea for all of the rules to have
|
|
a similar structure (e.g., the same number of terms). Otherwise, the corresponding action code may be more
|
|
complicated than necessary. However, it is possible to handle simple cases using len(). For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expressions(p):
|
|
'''expression : expression MINUS expression
|
|
| MINUS expression'''
|
|
if (len(p) == 4):
|
|
p[0] = p[1] - p[3]
|
|
elif (len(p) == 3):
|
|
p[0] = -p[2]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<H3><a name="ply_nn26"></a>5.3 Character Literals</H3>
|
|
|
|
|
|
If desired, a grammar may contain tokens defined as single character literals. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_binary_operators(p):
|
|
'''expression : expression '+' term
|
|
| expression '-' term
|
|
term : term '*' factor
|
|
| term '/' factor'''
|
|
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]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
A character literal must be enclosed in quotes such as <tt>'+'</tt>. In addition, if literals are used, they must be declared in the
|
|
corresponding <tt>lex</tt> file through the use of a special <tt>literals</tt> declaration.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
# Literals. Should be placed in module given to lex()
|
|
literals = ['+','-','*','/' ]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<b>Character literals are limited to a single character</b>. Thus, it is not legal to specify literals such as <tt>'<='</tt> or <tt>'=='</tt>. For this, use
|
|
the normal lexing rules (e.g., define a rule such as <tt>t_EQ = r'=='</tt>).
|
|
|
|
<H3><a name="ply_nn26"></a>5.4 Empty Productions</H3>
|
|
|
|
|
|
<tt>yacc.py</tt> can handle empty productions by defining a rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_empty(p):
|
|
'empty :'
|
|
pass
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Now to use the empty production, simply use 'empty' as a symbol. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_optitem(p):
|
|
'optitem : item'
|
|
' | empty'
|
|
...
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Note: You can write empty rules anywhere by simply specifying an empty right hand side. However, I personally find that
|
|
writing an "empty" rule and using "empty" to denote an empty production is easier to read.
|
|
|
|
<H3><a name="ply_nn28"></a>5.5 Changing the starting symbol</H3>
|
|
|
|
|
|
Normally, the first rule found in a yacc specification defines the starting grammar rule (top level rule). To change this, simply
|
|
supply a <tt>start</tt> specifier in your file. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
start = 'foo'
|
|
|
|
def p_bar(p):
|
|
'bar : A B'
|
|
|
|
# This is the starting rule due to the start specifier above
|
|
def p_foo(p):
|
|
'foo : bar X'
|
|
...
|
|
</pre>
|
|
</blockquote>
|
|
|
|
The use of a <tt>start</tt> specifier may be useful during debugging since you can use it to have yacc build a subset of
|
|
a larger grammar. For this purpose, it is also possible to specify a starting symbol as an argument to <tt>yacc()</tt>. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.yacc(start='foo')
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<H3><a name="ply_nn27"></a>5.6 Dealing With Ambiguous Grammars</H3>
|
|
|
|
|
|
The expression grammar given in the earlier example has been written in a special format to eliminate ambiguity.
|
|
However, in many situations, it is extremely difficult or awkward to write grammars in this format. A
|
|
much more natural way to express the grammar is in a more compact form like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
expression : expression PLUS expression
|
|
| expression MINUS expression
|
|
| expression TIMES expression
|
|
| expression DIVIDE expression
|
|
| LPAREN expression RPAREN
|
|
| NUMBER
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Unfortunately, this grammar specification is ambiguous. For example, if you are parsing the string
|
|
"3 * 4 + 5", there is no way to tell how the operators are supposed to be grouped.
|
|
For example, does the expression mean "(3 * 4) + 5" or is it "3 * (4+5)"?
|
|
|
|
<p>
|
|
When an ambiguous grammar is given to <tt>yacc.py</tt> it will print messages about "shift/reduce conflicts"
|
|
or a "reduce/reduce conflicts". A shift/reduce conflict is caused when the parser generator can't decide
|
|
whether or not to reduce a rule or shift a symbol on the parsing stack. For example, consider
|
|
the string "3 * 4 + 5" and the internal parsing stack:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
Step Symbol Stack Input Tokens Action
|
|
---- --------------------- --------------------- -------------------------------
|
|
1 $ 3 * 4 + 5$ Shift 3
|
|
2 $ 3 * 4 + 5$ Reduce : expression : NUMBER
|
|
3 $ expr * 4 + 5$ Shift *
|
|
4 $ expr * 4 + 5$ Shift 4
|
|
5 $ expr * 4 + 5$ Reduce: expression : NUMBER
|
|
6 $ expr * expr + 5$ SHIFT/REDUCE CONFLICT ????
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, when the parser reaches step 6, it has two options. One is to reduce the
|
|
rule <tt>expr : expr * expr</tt> on the stack. The other option is to shift the
|
|
token <tt>+</tt> on the stack. Both options are perfectly legal from the rules
|
|
of the context-free-grammar.
|
|
|
|
<p>
|
|
By default, all shift/reduce conflicts are resolved in favor of shifting. Therefore, in the above
|
|
example, the parser will always shift the <tt>+</tt> instead of reducing. Although this
|
|
strategy works in many cases (including the ambiguous if-then-else), it is not enough for arithmetic
|
|
expressions. In fact, in the above example, the decision to shift <tt>+</tt> is completely wrong---we should have
|
|
reduced <tt>expr * expr</tt> since multiplication has higher mathematical precedence than addition.
|
|
|
|
<p>To resolve ambiguity, especially in expression grammars, <tt>yacc.py</tt> allows individual
|
|
tokens to be assigned a precedence level and associativity. This is done by adding a variable
|
|
<tt>precedence</tt> to the grammar file like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
precedence = (
|
|
('left', 'PLUS', 'MINUS'),
|
|
('left', 'TIMES', 'DIVIDE'),
|
|
)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This declaration specifies that <tt>PLUS</tt>/<tt>MINUS</tt> have
|
|
the same precedence level and are left-associative and that
|
|
<tt>TIMES</tt>/<tt>DIVIDE</tt> have the same precedence and are left-associative.
|
|
Within the <tt>precedence</tt> declaration, tokens are ordered from lowest to highest precedence. Thus,
|
|
this declaration specifies that <tt>TIMES</tt>/<tt>DIVIDE</tt> have higher
|
|
precedence than <tt>PLUS</tt>/<tt>MINUS</tt> (since they appear later in the
|
|
precedence specification).
|
|
|
|
<p>
|
|
The precedence specification works by associating a numerical precedence level value and associativity direction to
|
|
the listed tokens. For example, in the above example you get:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
PLUS : level = 1, assoc = 'left'
|
|
MINUS : level = 1, assoc = 'left'
|
|
TIMES : level = 2, assoc = 'left'
|
|
DIVIDE : level = 2, assoc = 'left'
|
|
</pre>
|
|
</blockquote>
|
|
|
|
These values are then used to attach a numerical precedence value and associativity direction
|
|
to each grammar rule. <em>This is always determined by looking at the precedence of the right-most terminal symbol.</em>
|
|
For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
expression : expression PLUS expression # level = 1, left
|
|
| expression MINUS expression # level = 1, left
|
|
| expression TIMES expression # level = 2, left
|
|
| expression DIVIDE expression # level = 2, left
|
|
| LPAREN expression RPAREN # level = None (not specified)
|
|
| NUMBER # level = None (not specified)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
When shift/reduce conflicts are encountered, the parser generator resolves the conflict by
|
|
looking at the precedence rules and associativity specifiers.
|
|
|
|
<p>
|
|
<ol>
|
|
<li>If the current token has higher precedence, it is shifted.
|
|
<li>If the grammar rule on the stack has higher precedence, the rule is reduced.
|
|
<li>If the current token and the grammar rule have the same precedence, the
|
|
rule is reduced for left associativity, whereas the token is shifted for right associativity.
|
|
<li>If nothing is known about the precedence, shift/reduce conflicts are resolved in
|
|
favor of shifting (the default).
|
|
</ol>
|
|
|
|
For example, if "expression PLUS expression" has been parsed and the next token
|
|
is "TIMES", the action is going to be a shift because "TIMES" has a higher precedence level than "PLUS". On the other
|
|
hand, if "expression TIMES expression" has been parsed and the next token is "PLUS", the action
|
|
is going to be reduce because "PLUS" has a lower precedence than "TIMES."
|
|
|
|
<p>
|
|
When shift/reduce conflicts are resolved using the first three techniques (with the help of
|
|
precedence rules), <tt>yacc.py</tt> will report no errors or conflicts in the grammar.
|
|
|
|
<p>
|
|
One problem with the precedence specifier technique is that it is sometimes necessary to
|
|
change the precedence of an operator in certain contents. For example, consider a unary-minus operator
|
|
in "3 + 4 * -5". Normally, unary minus has a very high precedence--being evaluated before the multiply.
|
|
However, in our precedence specifier, MINUS has a lower precedence than TIMES. To deal with this,
|
|
precedence rules can be given for fictitious tokens like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
precedence = (
|
|
('left', 'PLUS', 'MINUS'),
|
|
('left', 'TIMES', 'DIVIDE'),
|
|
('right', 'UMINUS'), # Unary minus operator
|
|
)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Now, in the grammar file, we can write our unary minus rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expr_uminus(p):
|
|
'expression : MINUS expression %prec UMINUS'
|
|
p[0] = -p[2]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, <tt>%prec UMINUS</tt> overrides the default rule precedence--setting it to that
|
|
of UMINUS in the precedence specifier.
|
|
|
|
<p>
|
|
At first, the use of UMINUS in this example may appear very confusing.
|
|
UMINUS is not an input token or a grammer rule. Instead, you should
|
|
think of it as the name of a special marker in the precedence table. When you use the <tt>%prec</tt> qualifier, you're simply
|
|
telling yacc that you want the precedence of the expression to be the same as for this special marker instead of the usual precedence.
|
|
|
|
<p>
|
|
It is also possible to specify non-associativity in the <tt>precedence</tt> table. This would
|
|
be used when you <em>don't</em> want operations to chain together. For example, suppose
|
|
you wanted to support comparison operators like <tt><</tt> and <tt>></tt> but you didn't want to allow
|
|
combinations like <tt>a < b < c</tt>. To do this, simply specify a rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
precedence = (
|
|
('nonassoc', 'LESSTHAN', 'GREATERTHAN'), # Nonassociative operators
|
|
('left', 'PLUS', 'MINUS'),
|
|
('left', 'TIMES', 'DIVIDE'),
|
|
('right', 'UMINUS'), # Unary minus operator
|
|
)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
If you do this, the occurrence of input text such as <tt> a < b < c</tt> will result in a syntax error. However, simple
|
|
expressions such as <tt>a < b</tt> will still be fine.
|
|
|
|
<p>
|
|
Reduce/reduce conflicts are caused when there are multiple grammar
|
|
rules that can be applied to a given set of symbols. This kind of
|
|
conflict is almost always bad and is always resolved by picking the
|
|
rule that appears first in the grammar file. Reduce/reduce conflicts
|
|
are almost always caused when different sets of grammar rules somehow
|
|
generate the same set of symbols. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
assignment : ID EQUALS NUMBER
|
|
| ID EQUALS expression
|
|
|
|
expression : expression PLUS expression
|
|
| expression MINUS expression
|
|
| expression TIMES expression
|
|
| expression DIVIDE expression
|
|
| LPAREN expression RPAREN
|
|
| NUMBER
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, a reduce/reduce conflict exists between these two rules:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
assignment : ID EQUALS NUMBER
|
|
expression : NUMBER
|
|
</pre>
|
|
</blockquote>
|
|
|
|
For example, if you wrote "a = 5", the parser can't figure out if this
|
|
is supposed to be reduced as <tt>assignment : ID EQUALS NUMBER</tt> or
|
|
whether it's supposed to reduce the 5 as an expression and then reduce
|
|
the rule <tt>assignment : ID EQUALS expression</tt>.
|
|
|
|
<p>
|
|
It should be noted that reduce/reduce conflicts are notoriously difficult to spot
|
|
simply looking at the input grammer. To locate these, it is usually easier to look at the
|
|
<tt>parser.out</tt> debugging file with an appropriately high level of caffeination.
|
|
|
|
<H3><a name="ply_nn28"></a>5.7 The parser.out file</H3>
|
|
|
|
|
|
Tracking down shift/reduce and reduce/reduce conflicts is one of the finer pleasures of using an LR
|
|
parsing algorithm. To assist in debugging, <tt>yacc.py</tt> creates a debugging file called
|
|
'parser.out' when it generates the parsing table. The contents of this file look like the following:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
Unused terminals:
|
|
|
|
|
|
Grammar
|
|
|
|
Rule 1 expression -> expression PLUS expression
|
|
Rule 2 expression -> expression MINUS expression
|
|
Rule 3 expression -> expression TIMES expression
|
|
Rule 4 expression -> expression DIVIDE expression
|
|
Rule 5 expression -> NUMBER
|
|
Rule 6 expression -> LPAREN expression RPAREN
|
|
|
|
Terminals, with rules where they appear
|
|
|
|
TIMES : 3
|
|
error :
|
|
MINUS : 2
|
|
RPAREN : 6
|
|
LPAREN : 6
|
|
DIVIDE : 4
|
|
PLUS : 1
|
|
NUMBER : 5
|
|
|
|
Nonterminals, with rules where they appear
|
|
|
|
expression : 1 1 2 2 3 3 4 4 6 0
|
|
|
|
|
|
Parsing method: LALR
|
|
|
|
|
|
state 0
|
|
|
|
S' -> . expression
|
|
expression -> . expression PLUS expression
|
|
expression -> . expression MINUS expression
|
|
expression -> . expression TIMES expression
|
|
expression -> . expression DIVIDE expression
|
|
expression -> . NUMBER
|
|
expression -> . LPAREN expression RPAREN
|
|
|
|
NUMBER shift and go to state 3
|
|
LPAREN shift and go to state 2
|
|
|
|
|
|
state 1
|
|
|
|
S' -> expression .
|
|
expression -> expression . PLUS expression
|
|
expression -> expression . MINUS expression
|
|
expression -> expression . TIMES expression
|
|
expression -> expression . DIVIDE expression
|
|
|
|
PLUS shift and go to state 6
|
|
MINUS shift and go to state 5
|
|
TIMES shift and go to state 4
|
|
DIVIDE shift and go to state 7
|
|
|
|
|
|
state 2
|
|
|
|
expression -> LPAREN . expression RPAREN
|
|
expression -> . expression PLUS expression
|
|
expression -> . expression MINUS expression
|
|
expression -> . expression TIMES expression
|
|
expression -> . expression DIVIDE expression
|
|
expression -> . NUMBER
|
|
expression -> . LPAREN expression RPAREN
|
|
|
|
NUMBER shift and go to state 3
|
|
LPAREN shift and go to state 2
|
|
|
|
|
|
state 3
|
|
|
|
expression -> NUMBER .
|
|
|
|
$ reduce using rule 5
|
|
PLUS reduce using rule 5
|
|
MINUS reduce using rule 5
|
|
TIMES reduce using rule 5
|
|
DIVIDE reduce using rule 5
|
|
RPAREN reduce using rule 5
|
|
|
|
|
|
state 4
|
|
|
|
expression -> expression TIMES . expression
|
|
expression -> . expression PLUS expression
|
|
expression -> . expression MINUS expression
|
|
expression -> . expression TIMES expression
|
|
expression -> . expression DIVIDE expression
|
|
expression -> . NUMBER
|
|
expression -> . LPAREN expression RPAREN
|
|
|
|
NUMBER shift and go to state 3
|
|
LPAREN shift and go to state 2
|
|
|
|
|
|
state 5
|
|
|
|
expression -> expression MINUS . expression
|
|
expression -> . expression PLUS expression
|
|
expression -> . expression MINUS expression
|
|
expression -> . expression TIMES expression
|
|
expression -> . expression DIVIDE expression
|
|
expression -> . NUMBER
|
|
expression -> . LPAREN expression RPAREN
|
|
|
|
NUMBER shift and go to state 3
|
|
LPAREN shift and go to state 2
|
|
|
|
|
|
state 6
|
|
|
|
expression -> expression PLUS . expression
|
|
expression -> . expression PLUS expression
|
|
expression -> . expression MINUS expression
|
|
expression -> . expression TIMES expression
|
|
expression -> . expression DIVIDE expression
|
|
expression -> . NUMBER
|
|
expression -> . LPAREN expression RPAREN
|
|
|
|
NUMBER shift and go to state 3
|
|
LPAREN shift and go to state 2
|
|
|
|
|
|
state 7
|
|
|
|
expression -> expression DIVIDE . expression
|
|
expression -> . expression PLUS expression
|
|
expression -> . expression MINUS expression
|
|
expression -> . expression TIMES expression
|
|
expression -> . expression DIVIDE expression
|
|
expression -> . NUMBER
|
|
expression -> . LPAREN expression RPAREN
|
|
|
|
NUMBER shift and go to state 3
|
|
LPAREN shift and go to state 2
|
|
|
|
|
|
state 8
|
|
|
|
expression -> LPAREN expression . RPAREN
|
|
expression -> expression . PLUS expression
|
|
expression -> expression . MINUS expression
|
|
expression -> expression . TIMES expression
|
|
expression -> expression . DIVIDE expression
|
|
|
|
RPAREN shift and go to state 13
|
|
PLUS shift and go to state 6
|
|
MINUS shift and go to state 5
|
|
TIMES shift and go to state 4
|
|
DIVIDE shift and go to state 7
|
|
|
|
|
|
state 9
|
|
|
|
expression -> expression TIMES expression .
|
|
expression -> expression . PLUS expression
|
|
expression -> expression . MINUS expression
|
|
expression -> expression . TIMES expression
|
|
expression -> expression . DIVIDE expression
|
|
|
|
$ reduce using rule 3
|
|
PLUS reduce using rule 3
|
|
MINUS reduce using rule 3
|
|
TIMES reduce using rule 3
|
|
DIVIDE reduce using rule 3
|
|
RPAREN reduce using rule 3
|
|
|
|
! PLUS [ shift and go to state 6 ]
|
|
! MINUS [ shift and go to state 5 ]
|
|
! TIMES [ shift and go to state 4 ]
|
|
! DIVIDE [ shift and go to state 7 ]
|
|
|
|
state 10
|
|
|
|
expression -> expression MINUS expression .
|
|
expression -> expression . PLUS expression
|
|
expression -> expression . MINUS expression
|
|
expression -> expression . TIMES expression
|
|
expression -> expression . DIVIDE expression
|
|
|
|
$ reduce using rule 2
|
|
PLUS reduce using rule 2
|
|
MINUS reduce using rule 2
|
|
RPAREN reduce using rule 2
|
|
TIMES shift and go to state 4
|
|
DIVIDE shift and go to state 7
|
|
|
|
! TIMES [ reduce using rule 2 ]
|
|
! DIVIDE [ reduce using rule 2 ]
|
|
! PLUS [ shift and go to state 6 ]
|
|
! MINUS [ shift and go to state 5 ]
|
|
|
|
state 11
|
|
|
|
expression -> expression PLUS expression .
|
|
expression -> expression . PLUS expression
|
|
expression -> expression . MINUS expression
|
|
expression -> expression . TIMES expression
|
|
expression -> expression . DIVIDE expression
|
|
|
|
$ reduce using rule 1
|
|
PLUS reduce using rule 1
|
|
MINUS reduce using rule 1
|
|
RPAREN reduce using rule 1
|
|
TIMES shift and go to state 4
|
|
DIVIDE shift and go to state 7
|
|
|
|
! TIMES [ reduce using rule 1 ]
|
|
! DIVIDE [ reduce using rule 1 ]
|
|
! PLUS [ shift and go to state 6 ]
|
|
! MINUS [ shift and go to state 5 ]
|
|
|
|
state 12
|
|
|
|
expression -> expression DIVIDE expression .
|
|
expression -> expression . PLUS expression
|
|
expression -> expression . MINUS expression
|
|
expression -> expression . TIMES expression
|
|
expression -> expression . DIVIDE expression
|
|
|
|
$ reduce using rule 4
|
|
PLUS reduce using rule 4
|
|
MINUS reduce using rule 4
|
|
TIMES reduce using rule 4
|
|
DIVIDE reduce using rule 4
|
|
RPAREN reduce using rule 4
|
|
|
|
! PLUS [ shift and go to state 6 ]
|
|
! MINUS [ shift and go to state 5 ]
|
|
! TIMES [ shift and go to state 4 ]
|
|
! DIVIDE [ shift and go to state 7 ]
|
|
|
|
state 13
|
|
|
|
expression -> LPAREN expression RPAREN .
|
|
|
|
$ reduce using rule 6
|
|
PLUS reduce using rule 6
|
|
MINUS reduce using rule 6
|
|
TIMES reduce using rule 6
|
|
DIVIDE reduce using rule 6
|
|
RPAREN reduce using rule 6
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In the file, each state of the grammar is described. Within each state the "." indicates the current
|
|
location of the parse within any applicable grammar rules. In addition, the actions for each valid
|
|
input token are listed. When a shift/reduce or reduce/reduce conflict arises, rules <em>not</em> selected
|
|
are prefixed with an !. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
! TIMES [ reduce using rule 2 ]
|
|
! DIVIDE [ reduce using rule 2 ]
|
|
! PLUS [ shift and go to state 6 ]
|
|
! MINUS [ shift and go to state 5 ]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
By looking at these rules (and with a little practice), you can usually track down the source
|
|
of most parsing conflicts. It should also be stressed that not all shift-reduce conflicts are
|
|
bad. However, the only way to be sure that they are resolved correctly is to look at <tt>parser.out</tt>.
|
|
|
|
<H3><a name="ply_nn29"></a>5.8 Syntax Error Handling</H3>
|
|
|
|
|
|
When a syntax error occurs during parsing, the error is immediately
|
|
detected (i.e., the parser does not read any more tokens beyond the
|
|
source of the error). Error recovery in LR parsers is a delicate
|
|
topic that involves ancient rituals and black-magic. The recovery mechanism
|
|
provided by <tt>yacc.py</tt> is comparable to Unix yacc so you may want
|
|
consult a book like O'Reilly's "Lex and Yacc" for some of the finer details.
|
|
|
|
<p>
|
|
When a syntax error occurs, <tt>yacc.py</tt> performs the following steps:
|
|
|
|
<ol>
|
|
<li>On the first occurrence of an error, the user-defined <tt>p_error()</tt> function
|
|
is called with the offending token as an argument. Afterwards, the parser enters
|
|
an "error-recovery" mode in which it will not make future calls to <tt>p_error()</tt> until it
|
|
has successfully shifted at least 3 tokens onto the parsing stack.
|
|
|
|
<p>
|
|
<li>If no recovery action is taken in <tt>p_error()</tt>, the offending lookahead token is replaced
|
|
with a special <tt>error</tt> token.
|
|
|
|
<p>
|
|
<li>If the offending lookahead token is already set to <tt>error</tt>, the top item of the parsing stack is
|
|
deleted.
|
|
|
|
<p>
|
|
<li>If the entire parsing stack is unwound, the parser enters a restart state and attempts to start
|
|
parsing from its initial state.
|
|
|
|
<p>
|
|
<li>If a grammar rule accepts <tt>error</tt> as a token, it will be
|
|
shifted onto the parsing stack.
|
|
|
|
<p>
|
|
<li>If the top item of the parsing stack is <tt>error</tt>, lookahead tokens will be discarded until the
|
|
parser can successfully shift a new symbol or reduce a rule involving <tt>error</tt>.
|
|
</ol>
|
|
|
|
<H4><a name="ply_nn30"></a>5.8.1 Recovery and resynchronization with error rules</H4>
|
|
|
|
|
|
The most well-behaved approach for handling syntax errors is to write grammar rules that include the <tt>error</tt>
|
|
token. For example, suppose your language had a grammar rule for a print statement like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_statement_print(p):
|
|
'statement : PRINT expr SEMI'
|
|
...
|
|
</pre>
|
|
</blockquote>
|
|
|
|
To account for the possibility of a bad expression, you might write an additional grammar rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_statement_print_error(p):
|
|
'statement : PRINT error SEMI'
|
|
print "Syntax error in print statement. Bad expression"
|
|
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, the <tt>error</tt> token will match any sequence of
|
|
tokens that might appear up to the first semicolon that is
|
|
encountered. Once the semicolon is reached, the rule will be
|
|
invoked and the <tt>error</tt> token will go away.
|
|
|
|
<p>
|
|
This type of recovery is sometimes known as parser resynchronization.
|
|
The <tt>error</tt> token acts as a wildcard for any bad input text and
|
|
the token immediately following <tt>error</tt> acts as a
|
|
synchronization token.
|
|
|
|
<p>
|
|
It is important to note that the <tt>error</tt> token usually does not appear as the last token
|
|
on the right in an error rule. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_statement_print_error(p):
|
|
'statement : PRINT error'
|
|
print "Syntax error in print statement. Bad expression"
|
|
</pre>
|
|
</blockquote>
|
|
|
|
This is because the first bad token encountered will cause the rule to
|
|
be reduced--which may make it difficult to recover if more bad tokens
|
|
immediately follow.
|
|
|
|
<H4><a name="ply_nn31"></a>5.8.2 Panic mode recovery</H4>
|
|
|
|
|
|
An alternative error recovery scheme is to enter a panic mode recovery in which tokens are
|
|
discarded to a point where the parser might be able to recover in some sensible manner.
|
|
|
|
<p>
|
|
Panic mode recovery is implemented entirely in the <tt>p_error()</tt> function. For example, this
|
|
function starts discarding tokens until it reaches a closing '}'. Then, it restarts the
|
|
parser in its initial state.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_error(p):
|
|
print "Whoa. You are seriously hosed."
|
|
# Read ahead looking for a closing '}'
|
|
while 1:
|
|
tok = yacc.token() # Get the next token
|
|
if not tok or tok.type == 'RBRACE': break
|
|
yacc.restart()
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
This function simply discards the bad token and tells the parser that the error was ok.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_error(p):
|
|
print "Syntax error at token", p.type
|
|
# Just discard the token and tell the parser it's okay.
|
|
yacc.errok()
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<P>
|
|
Within the <tt>p_error()</tt> function, three functions are available to control the behavior
|
|
of the parser:
|
|
<p>
|
|
<ul>
|
|
<li><tt>yacc.errok()</tt>. This resets the parser state so it doesn't think it's in error-recovery
|
|
mode. This will prevent an <tt>error</tt> token from being generated and will reset the internal
|
|
error counters so that the next syntax error will call <tt>p_error()</tt> again.
|
|
|
|
<p>
|
|
<li><tt>yacc.token()</tt>. This returns the next token on the input stream.
|
|
|
|
<p>
|
|
<li><tt>yacc.restart()</tt>. This discards the entire parsing stack and resets the parser
|
|
to its initial state.
|
|
</ul>
|
|
|
|
Note: these functions are only available when invoking <tt>p_error()</tt> and are not available
|
|
at any other time.
|
|
|
|
<p>
|
|
To supply the next lookahead token to the parser, <tt>p_error()</tt> can return a token. This might be
|
|
useful if trying to synchronize on special characters. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_error(p):
|
|
# Read ahead looking for a terminating ";"
|
|
while 1:
|
|
tok = yacc.token() # Get the next token
|
|
if not tok or tok.type == 'SEMI': break
|
|
yacc.errok()
|
|
|
|
# Return SEMI to the parser as the next lookahead token
|
|
return tok
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<H4><a name="ply_nn32"></a>5.8.3 General comments on error handling</H4>
|
|
|
|
|
|
For normal types of languages, error recovery with error rules and resynchronization characters is probably the most reliable
|
|
technique. This is because you can instrument the grammar to catch errors at selected places where it is relatively easy
|
|
to recover and continue parsing. Panic mode recovery is really only useful in certain specialized applications where you might want
|
|
to discard huge portions of the input text to find a valid restart point.
|
|
|
|
<H3><a name="ply_nn33"></a>5.9 Line Number and Position Tracking</H3>
|
|
|
|
Position tracking is often a tricky problem when writing compilers. By default, PLY tracks the line number and position of
|
|
all tokens. This information is available using the following functions:
|
|
|
|
<ul>
|
|
<li><tt>p.lineno(num)</tt>. Return the line number for symbol <em>num</em>
|
|
<li><tt>p.lexpos(num)</tt>. Return the lexing position for symbol <em>num</em>
|
|
</ul>
|
|
|
|
For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expression(p):
|
|
'expression : expression PLUS expression'
|
|
line = p.lineno(2) # line number of the PLUS token
|
|
index = p.lexpos(2) # Position of the PLUS token
|
|
</pre>
|
|
</blockquote>
|
|
|
|
As an optional feature, <tt>yacc.py</tt> can automatically track line numbers and positions for all of the grammar symbols
|
|
as well. However, this
|
|
extra tracking requires extra processing and can significantly slow down parsing. Therefore, it must be enabled by passing the
|
|
<tt>tracking=True</tt> option to <tt>yacc.parse()</tt>. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.parse(data,tracking=True)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Once enabled, the <tt>lineno()</tt> and <tt>lexpos()</tt> methods work for all grammar symbols. In addition, two
|
|
additional methods can be used:
|
|
|
|
<ul>
|
|
<li><tt>p.linespan(num)</tt>. Return a tuple (startline,endline) with the starting and ending line number for symbol <em>num</em>.
|
|
<li><tt>p.lexspan(num)</tt>. Return a tuple (start,end) with the starting and ending positions for symbol <em>num</em>.
|
|
</ul>
|
|
|
|
For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expression(p):
|
|
'expression : expression PLUS expression'
|
|
p.lineno(1) # Line number of the left expression
|
|
p.lineno(2) # line number of the PLUS operator
|
|
p.lineno(3) # line number of the right expression
|
|
...
|
|
start,end = p.linespan(3) # Start,end lines of the right expression
|
|
starti,endi = p.lexspan(3) # Start,end positions of right expression
|
|
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Note: The <tt>lexspan()</tt> function only returns the range of values up to the start of the last grammar symbol.
|
|
|
|
<p>
|
|
Although it may be convenient for PLY to track position information on
|
|
all grammar symbols, this is often unnecessary. For example, if you
|
|
are merely using line number information in an error message, you can
|
|
often just key off of a specific token in the grammar rule. For
|
|
example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_bad_func(p):
|
|
'funccall : fname LPAREN error RPAREN'
|
|
# Line number reported from LPAREN token
|
|
print "Bad function call at line", p.lineno(2)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
Similarly, you may get better parsing performance if you only propagate line number
|
|
information where it's needed. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_fname(p):
|
|
'fname : ID'
|
|
p[0] = (p[1],p.lineno(1))
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Finally, it should be noted that PLY does not store position information after a rule has been
|
|
processed. If it is important for you to retain this information in an abstract syntax tree, you
|
|
must make your own copy.
|
|
|
|
<H3><a name="ply_nn34"></a>5.10 AST Construction</H3>
|
|
|
|
|
|
<tt>yacc.py</tt> provides no special functions for constructing an abstract syntax tree. However, such
|
|
construction is easy enough to do on your own. Simply create a data structure for abstract syntax tree nodes
|
|
and assign nodes to <tt>p[0]</tt> in each rule.
|
|
|
|
For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
class Expr: pass
|
|
|
|
class BinOp(Expr):
|
|
def __init__(self,left,op,right):
|
|
self.type = "binop"
|
|
self.left = left
|
|
self.right = right
|
|
self.op = op
|
|
|
|
class Number(Expr):
|
|
def __init__(self,value):
|
|
self.type = "number"
|
|
self.value = value
|
|
|
|
def p_expression_binop(p):
|
|
'''expression : expression PLUS expression
|
|
| expression MINUS expression
|
|
| expression TIMES expression
|
|
| expression DIVIDE expression'''
|
|
|
|
p[0] = BinOp(p[1],p[2],p[3])
|
|
|
|
def p_expression_group(p):
|
|
'expression : LPAREN expression RPAREN'
|
|
p[0] = p[2]
|
|
|
|
def p_expression_number(p):
|
|
'expression : NUMBER'
|
|
p[0] = Number(p[1])
|
|
</pre>
|
|
</blockquote>
|
|
|
|
To simplify tree traversal, it may make sense to pick a very generic tree structure for your parse tree nodes.
|
|
For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
class Node:
|
|
def __init__(self,type,children=None,leaf=None):
|
|
self.type = type
|
|
if children:
|
|
self.children = children
|
|
else:
|
|
self.children = [ ]
|
|
self.leaf = leaf
|
|
|
|
def p_expression_binop(p):
|
|
'''expression : expression PLUS expression
|
|
| expression MINUS expression
|
|
| expression TIMES expression
|
|
| expression DIVIDE expression'''
|
|
|
|
p[0] = Node("binop", [p[1],p[3]], p[2])
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<H3><a name="ply_nn35"></a>5.11 Embedded Actions</H3>
|
|
|
|
|
|
The parsing technique used by yacc only allows actions to be executed at the end of a rule. For example,
|
|
suppose you have a rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_foo(p):
|
|
"foo : A B C D"
|
|
print "Parsed a foo", p[1],p[2],p[3],p[4]
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
In this case, the supplied action code only executes after all of the
|
|
symbols <tt>A</tt>, <tt>B</tt>, <tt>C</tt>, and <tt>D</tt> have been
|
|
parsed. Sometimes, however, it is useful to execute small code
|
|
fragments during intermediate stages of parsing. For example, suppose
|
|
you wanted to perform some action immediately after <tt>A</tt> has
|
|
been parsed. To do this, you can write a empty rule like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_foo(p):
|
|
"foo : A seen_A B C D"
|
|
print "Parsed a foo", p[1],p[3],p[4],p[5]
|
|
print "seen_A returned", p[2]
|
|
|
|
def p_seen_A(p):
|
|
"seen_A :"
|
|
print "Saw an A = ", p[-1] # Access grammar symbol to left
|
|
p[0] = some_value # Assign value to seen_A
|
|
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
In this example, the empty <tt>seen_A</tt> rule executes immediately
|
|
after <tt>A</tt> is shifted onto the parsing stack. Within this
|
|
rule, <tt>p[-1]</tt> refers to the symbol on the stack that appears
|
|
immediately to the left of the <tt>seen_A</tt> symbol. In this case,
|
|
it would be the value of <tt>A</tt> in the <tt>foo</tt> rule
|
|
immediately above. Like other rules, a value can be returned from an
|
|
embedded action by simply assigning it to <tt>p[0]</tt>
|
|
|
|
<p>
|
|
The use of embedded actions can sometimes introduce extra shift/reduce conflicts. For example,
|
|
this grammar has no conflicts:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_foo(p):
|
|
"""foo : abcd
|
|
| abcx"""
|
|
|
|
def p_abcd(p):
|
|
"abcd : A B C D"
|
|
|
|
def p_abcx(p):
|
|
"abcx : A B C X"
|
|
</pre>
|
|
</blockquote>
|
|
|
|
However, if you insert an embedded action into one of the rules like this,
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_foo(p):
|
|
"""foo : abcd
|
|
| abcx"""
|
|
|
|
def p_abcd(p):
|
|
"abcd : A B C D"
|
|
|
|
def p_abcx(p):
|
|
"abcx : A B seen_AB C X"
|
|
|
|
def p_seen_AB(p):
|
|
"seen_AB :"
|
|
</pre>
|
|
</blockquote>
|
|
|
|
an extra shift-reduce conflict will be introduced. This conflict is caused by the fact that the same symbol <tt>C</tt> appears next in
|
|
both the <tt>abcd</tt> and <tt>abcx</tt> rules. The parser can either shift the symbol (<tt>abcd</tt> rule) or reduce the empty rule <tt>seen_AB</tt> (<tt>abcx</tt> rule).
|
|
|
|
<p>
|
|
A common use of embedded rules is to control other aspects of parsing
|
|
such as scoping of local variables. For example, if you were parsing C code, you might
|
|
write code like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_statements_block(p):
|
|
"statements: LBRACE new_scope statements RBRACE"""
|
|
# Action code
|
|
...
|
|
pop_scope() # Return to previous scope
|
|
|
|
def p_new_scope(p):
|
|
"new_scope :"
|
|
# Create a new scope for local variables
|
|
s = new_scope()
|
|
push_scope(s)
|
|
...
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In this case, the embedded action <tt>new_scope</tt> executes immediately after a <tt>LBRACE</tt> (<tt>{</tt>) symbol is parsed. This might
|
|
adjust internal symbol tables and other aspects of the parser. Upon completion of the rule <tt>statements_block</tt>, code might undo the operations performed in the embedded action (e.g., <tt>pop_scope()</tt>).
|
|
|
|
<H3><a name="ply_nn36"></a>5.12 Yacc implementation notes</H3>
|
|
|
|
|
|
<ul>
|
|
<li>The default parsing method is LALR. To use SLR instead, run yacc() as follows:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.yacc(method="SLR")
|
|
</pre>
|
|
</blockquote>
|
|
Note: LALR table generation takes approximately twice as long as SLR table generation. There is no
|
|
difference in actual parsing performance---the same code is used in both cases. LALR is preferred when working
|
|
with more complicated grammars since it is more powerful.
|
|
|
|
<p>
|
|
|
|
<li>By default, <tt>yacc.py</tt> relies on <tt>lex.py</tt> for tokenizing. However, an alternative tokenizer
|
|
can be supplied as follows:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.parse(lexer=x)
|
|
</pre>
|
|
</blockquote>
|
|
in this case, <tt>x</tt> must be a Lexer object that minimally has a <tt>x.token()</tt> method for retrieving the next
|
|
token. If an input string is given to <tt>yacc.parse()</tt>, the lexer must also have an <tt>x.input()</tt> method.
|
|
|
|
<p>
|
|
<li>By default, the yacc generates tables in debugging mode (which produces the parser.out file and other output).
|
|
To disable this, use
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.yacc(debug=0)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
<li>To change the name of the <tt>parsetab.py</tt> file, use:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.yacc(tabmodule="foo")
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
<li>To change the directory in which the <tt>parsetab.py</tt> file (and other output files) are written, use:
|
|
<blockquote>
|
|
<pre>
|
|
yacc.yacc(tabmodule="foo",outputdir="somedirectory")
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
<li>To prevent yacc from generating any kind of parser table file, use:
|
|
<blockquote>
|
|
<pre>
|
|
yacc.yacc(write_tables=0)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Note: If you disable table generation, yacc() will regenerate the parsing tables
|
|
each time it runs (which may take awhile depending on how large your grammar is).
|
|
|
|
<P>
|
|
<li>To print copious amounts of debugging during parsing, use:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.parse(debug=1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
<li>To redirect the debugging output to a filename of your choosing, use:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
yacc.parse(debug=1, debugfile="debugging.out")
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
<li>The <tt>yacc.yacc()</tt> function really returns a parser object. If you want to support multiple
|
|
parsers in the same application, do this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
p = yacc.yacc()
|
|
...
|
|
p.parse()
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Note: The function <tt>yacc.parse()</tt> is bound to the last parser that was generated.
|
|
|
|
<p>
|
|
<li>Since the generation of the LALR tables is relatively expensive, previously generated tables are
|
|
cached and reused if possible. The decision to regenerate the tables is determined by taking an MD5
|
|
checksum of all grammar rules and precedence rules. Only in the event of a mismatch are the tables regenerated.
|
|
|
|
<p>
|
|
It should be noted that table generation is reasonably efficient, even for grammars that involve around a 100 rules
|
|
and several hundred states. For more complex languages such as C, table generation may take 30-60 seconds on a slow
|
|
machine. Please be patient.
|
|
|
|
<p>
|
|
<li>Since LR parsing is driven by tables, the performance of the parser is largely independent of the
|
|
size of the grammar. The biggest bottlenecks will be the lexer and the complexity of the code in your grammar rules.
|
|
</ul>
|
|
|
|
<H2><a name="ply_nn37"></a>6. Parser and Lexer State Management</H2>
|
|
|
|
|
|
In advanced parsing applications, you may want to have multiple
|
|
parsers and lexers. Furthermore, the parser may want to control the
|
|
behavior of the lexer in some way.
|
|
|
|
<p>
|
|
To do this, it is important to note that both the lexer and parser are
|
|
actually implemented as objects. These objects are returned by the
|
|
<tt>lex()</tt> and <tt>yacc()</tt> functions respectively. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lexer = lex.lex() # Return lexer object
|
|
parser = yacc.yacc() # Return parser object
|
|
</pre>
|
|
</blockquote>
|
|
|
|
To attach the lexer and parser together, make sure you use the <tt>lexer</tt> argumemnt to parse. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
parser.parse(text,lexer=lexer)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
Within lexer and parser rules, these objects are also available. In the lexer,
|
|
the "lexer" attribute of a token refers to the lexer object in use. For example:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def t_NUMBER(t):
|
|
r'\d+'
|
|
...
|
|
print t.lexer # Show lexer object
|
|
</pre>
|
|
</blockquote>
|
|
|
|
In the parser, the "lexer" and "parser" attributes refer to the lexer
|
|
and parser objects respectively.
|
|
|
|
<blockquote>
|
|
<pre>
|
|
def p_expr_plus(p):
|
|
'expr : expr PLUS expr'
|
|
...
|
|
print p.parser # Show parser object
|
|
print p.lexer # Show lexer object
|
|
</pre>
|
|
</blockquote>
|
|
|
|
If necessary, arbitrary attributes can be attached to the lexer or parser object.
|
|
For example, if you wanted to have different parsing modes, you could attach a mode
|
|
attribute to the parser object and look at it later.
|
|
|
|
<H2><a name="ply_nn38"></a>7. Using Python's Optimized Mode</H2>
|
|
|
|
|
|
Because PLY uses information from doc-strings, parsing and lexing
|
|
information must be gathered while running the Python interpreter in
|
|
normal mode (i.e., not with the -O or -OO options). However, if you
|
|
specify optimized mode like this:
|
|
|
|
<blockquote>
|
|
<pre>
|
|
lex.lex(optimize=1)
|
|
yacc.yacc(optimize=1)
|
|
</pre>
|
|
</blockquote>
|
|
|
|
then PLY can later be used when Python runs in optimized mode. To make this work,
|
|
make sure you first run Python in normal mode. Once the lexing and parsing tables
|
|
have been generated the first time, run Python in optimized mode. PLY will use
|
|
the tables without the need for doc strings.
|
|
|
|
<p>
|
|
Beware: running PLY in optimized mode disables a lot of error
|
|
checking. You should only do this when your project has stabilized
|
|
and you don't need to do any debugging.
|
|
|
|
<H2><a name="ply_nn39"></a>8. Where to go from here?</H2>
|
|
|
|
|
|
The <tt>examples</tt> directory of the PLY distribution contains several simple examples. Please consult a
|
|
compilers textbook for the theory and underlying implementation details or LR parsing.
|
|
|
|
</body>
|
|
</html>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|