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| +<html>
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| +<head>
|
| +<title>PLY (Python Lex-Yacc)</title>
|
| +</head>
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| +<body bgcolor="#ffffff">
|
| +
|
| +<h1>PLY (Python Lex-Yacc)</h1>
|
| +
|
| +<b>
|
| +David M. Beazley <br>
|
| +dave@dabeaz.com<br>
|
| +</b>
|
| +
|
| +<p>
|
| +<b>PLY Version: 3.0</b>
|
| +<p>
|
| +
|
| +<!-- INDEX -->
|
| +<div class="sectiontoc">
|
| +<ul>
|
| +<li><a href="#ply_nn1">Preface and Requirements</a>
|
| +<li><a href="#ply_nn1">Introduction</a>
|
| +<li><a href="#ply_nn2">PLY Overview</a>
|
| +<li><a href="#ply_nn3">Lex</a>
|
| +<ul>
|
| +<li><a href="#ply_nn4">Lex Example</a>
|
| +<li><a href="#ply_nn5">The tokens list</a>
|
| +<li><a href="#ply_nn6">Specification of tokens</a>
|
| +<li><a href="#ply_nn7">Token values</a>
|
| +<li><a href="#ply_nn8">Discarded tokens</a>
|
| +<li><a href="#ply_nn9">Line numbers and positional information</a>
|
| +<li><a href="#ply_nn10">Ignored characters</a>
|
| +<li><a href="#ply_nn11">Literal characters</a>
|
| +<li><a href="#ply_nn12">Error handling</a>
|
| +<li><a href="#ply_nn13">Building and using the lexer</a>
|
| +<li><a href="#ply_nn14">The @TOKEN decorator</a>
|
| +<li><a href="#ply_nn15">Optimized mode</a>
|
| +<li><a href="#ply_nn16">Debugging</a>
|
| +<li><a href="#ply_nn17">Alternative specification of lexers</a>
|
| +<li><a href="#ply_nn18">Maintaining state</a>
|
| +<li><a href="#ply_nn19">Lexer cloning</a>
|
| +<li><a href="#ply_nn20">Internal lexer state</a>
|
| +<li><a href="#ply_nn21">Conditional lexing and start conditions</a>
|
| +<li><a href="#ply_nn21">Miscellaneous Issues</a>
|
| +</ul>
|
| +<li><a href="#ply_nn22">Parsing basics</a>
|
| +<li><a href="#ply_nn23">Yacc</a>
|
| +<ul>
|
| +<li><a href="#ply_nn24">An example</a>
|
| +<li><a href="#ply_nn25">Combining Grammar Rule Functions</a>
|
| +<li><a href="#ply_nn26">Character Literals</a>
|
| +<li><a href="#ply_nn26">Empty Productions</a>
|
| +<li><a href="#ply_nn28">Changing the starting symbol</a>
|
| +<li><a href="#ply_nn27">Dealing With Ambiguous Grammars</a>
|
| +<li><a href="#ply_nn28">The parser.out file</a>
|
| +<li><a href="#ply_nn29">Syntax Error Handling</a>
|
| +<ul>
|
| +<li><a href="#ply_nn30">Recovery and resynchronization with error rules</a>
|
| +<li><a href="#ply_nn31">Panic mode recovery</a>
|
| +<li><a href="#ply_nn35">Signaling an error from a production</a>
|
| +<li><a href="#ply_nn32">General comments on error handling</a>
|
| +</ul>
|
| +<li><a href="#ply_nn33">Line Number and Position Tracking</a>
|
| +<li><a href="#ply_nn34">AST Construction</a>
|
| +<li><a href="#ply_nn35">Embedded Actions</a>
|
| +<li><a href="#ply_nn36">Miscellaneous Yacc Notes</a>
|
| +</ul>
|
| +<li><a href="#ply_nn37">Multiple Parsers and Lexers</a>
|
| +<li><a href="#ply_nn38">Using Python's Optimized Mode</a>
|
| +<li><a href="#ply_nn44">Advanced Debugging</a>
|
| +<ul>
|
| +<li><a href="#ply_nn45">Debugging the lex() and yacc() commands</a>
|
| +<li><a href="#ply_nn46">Run-time Debugging</a>
|
| +</ul>
|
| +<li><a href="#ply_nn39">Where to go from here?</a>
|
| +</ul>
|
| +</div>
|
| +<!-- INDEX -->
|
| +
|
| +
|
| +
|
| +<H2><a name="ply_nn1"></a>1. Preface and Requirements</H2>
|
| +
|
| +
|
| +<p>
|
| +This document provides an overview of lexing and parsing with PLY.
|
| +Given the intrinsic complexity of parsing, I would strongly advise
|
| +that you read (or at least skim) this entire document before jumping
|
| +into a big development project with PLY.
|
| +</p>
|
| +
|
| +<p>
|
| +PLY-3.0 is compatible with both Python 2 and Python 3. Be aware that
|
| +Python 3 support is new and has not been extensively tested (although
|
| +all of the examples and unit tests pass under Python 3.0). If you are
|
| +using Python 2, you should try to use Python 2.4 or newer. Although PLY
|
| +works with versions as far back as Python 2.2, some of its optional features
|
| +require more modern library modules.
|
| +</p>
|
| +
|
| +<H2><a name="ply_nn1"></a>2. Introduction</H2>
|
| +
|
| +
|
| +PLY is a pure-Python implementation of the popular compiler
|
| +construction tools lex and yacc. The main goal of PLY is to stay
|
| +fairly faithful to the way in which traditional lex/yacc tools work.
|
| +This includes supporting LALR(1) parsing as well as providing
|
| +extensive input validation, error reporting, and diagnostics. Thus,
|
| +if you've used yacc in another programming language, it should be
|
| +relatively straightforward to use PLY.
|
| +
|
| +<p>
|
| +Early versions of PLY were developed to support an Introduction to
|
| +Compilers Course I taught in 2001 at the University of Chicago. In this course,
|
| +students built a fully functional compiler for a simple Pascal-like
|
| +language. Their compiler, implemented entirely in Python, had to
|
| +include lexical analysis, parsing, type checking, type inference,
|
| +nested scoping, and code generation for the SPARC processor.
|
| +Approximately 30 different compiler implementations were completed in
|
| +this course. Most of PLY's interface and operation has been influenced by common
|
| +usability problems encountered by students. Since 2001, PLY has
|
| +continued to be improved as feedback has been received from users.
|
| +PLY-3.0 represents a major refactoring of the original implementation
|
| +with an eye towards future enhancements.
|
| +
|
| +<p>
|
| +Since PLY was primarily developed as an instructional tool, you will
|
| +find it to be fairly picky about token and grammar rule
|
| +specification. In part, this
|
| +added formality is meant to catch common programming mistakes made by
|
| +novice users. However, advanced users will also find such features to
|
| +be useful when building complicated grammars for real programming
|
| +languages. It should also be noted that PLY does not provide much in
|
| +the way of bells and whistles (e.g., automatic construction of
|
| +abstract syntax trees, tree traversal, etc.). Nor would I consider it
|
| +to be a parsing framework. Instead, you will find a bare-bones, yet
|
| +fully capable lex/yacc implementation written entirely in Python.
|
| +
|
| +<p>
|
| +The rest of this document assumes that you are somewhat familar with
|
| +parsing theory, syntax directed translation, and the use of compiler
|
| +construction tools such as lex and yacc in other programming
|
| +languages. If you are unfamilar with these topics, you will probably
|
| +want to consult an introductory text such as "Compilers: Principles,
|
| +Techniques, and Tools", by Aho, Sethi, and Ullman. O'Reilly's "Lex
|
| +and Yacc" by John Levine may also be handy. In fact, the O'Reilly book can be
|
| +used as a reference for PLY as the concepts are virtually identical.
|
| +
|
| +<H2><a name="ply_nn2"></a>3. PLY Overview</H2>
|
| +
|
| +
|
| +PLY consists of two separate modules; <tt>lex.py</tt> and
|
| +<tt>yacc.py</tt>, both of which are found in a Python package
|
| +called <tt>ply</tt>. The <tt>lex.py</tt> module is used to break input text into a
|
| +collection of tokens specified by a collection of regular expression
|
| +rules. <tt>yacc.py</tt> is used to recognize language syntax that has
|
| +been specified in the form of a context free grammar. <tt>yacc.py</tt> uses LR parsing and generates its parsing tables
|
| +using either the LALR(1) (the default) or SLR table generation algorithms.
|
| +
|
| +<p>
|
| +The two tools are meant to work together. Specifically,
|
| +<tt>lex.py</tt> provides an external interface in the form of a
|
| +<tt>token()</tt> function that returns the next valid token on the
|
| +input stream. <tt>yacc.py</tt> calls this repeatedly to retrieve
|
| +tokens and invoke grammar rules. The output of <tt>yacc.py</tt> is
|
| +often an Abstract Syntax Tree (AST). However, this is entirely up to
|
| +the user. If desired, <tt>yacc.py</tt> can also be used to implement
|
| +simple one-pass compilers.
|
| +
|
| +<p>
|
| +Like its Unix counterpart, <tt>yacc.py</tt> provides most of the
|
| +features you expect including extensive error checking, grammar
|
| +validation, support for empty productions, error tokens, and ambiguity
|
| +resolution via precedence rules. In fact, everything that is possible in traditional yacc
|
| +should be supported in PLY.
|
| +
|
| +<p>
|
| +The primary difference between
|
| +<tt>yacc.py</tt> and Unix <tt>yacc</tt> is that <tt>yacc.py</tt>
|
| +doesn't involve a separate code-generation process.
|
| +Instead, PLY relies on reflection (introspection)
|
| +to build its lexers and parsers. Unlike traditional lex/yacc which
|
| +require a special input file that is converted into a separate source
|
| +file, the specifications given to PLY <em>are</em> valid Python
|
| +programs. This means that there are no extra source files nor is
|
| +there a special compiler construction step (e.g., running yacc to
|
| +generate Python code for the compiler). Since the generation of the
|
| +parsing tables is relatively expensive, PLY caches the results and
|
| +saves them to a file. If no changes are detected in the input source,
|
| +the tables are read from the cache. Otherwise, they are regenerated.
|
| +
|
| +<H2><a name="ply_nn3"></a>4. Lex</H2>
|
| +
|
| +
|
| +<tt>lex.py</tt> is used to tokenize an input string. For example, suppose
|
| +you're writing a programming language and a user supplied the following input string:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +x = 3 + 42 * (s - t)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +A tokenizer splits the string into individual tokens
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +'x','=', '3', '+', '42', '*', '(', 's', '-', 't', ')'
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +Tokens are usually given names to indicate what they are. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +'ID','EQUALS','NUMBER','PLUS','NUMBER','TIMES',
|
| +'LPAREN','ID','MINUS','ID','RPAREN'
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +More specifically, the input is broken into pairs of token types and values. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +('ID','x'), ('EQUALS','='), ('NUMBER','3'),
|
| +('PLUS','+'), ('NUMBER','42), ('TIMES','*'),
|
| +('LPAREN','('), ('ID','s'), ('MINUS','-'),
|
| +('ID','t'), ('RPAREN',')'
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +The identification of tokens is typically done by writing a series of regular expression
|
| +rules. The next section shows how this is done using <tt>lex.py</tt>.
|
| +
|
| +<H3><a name="ply_nn4"></a>4.1 Lex Example</H3>
|
| +
|
| +
|
| +The following example shows how <tt>lex.py</tt> is used to write a simple tokenizer.
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +# ------------------------------------------------------------
|
| +# calclex.py
|
| +#
|
| +# tokenizer for a simple expression evaluator for
|
| +# numbers and +,-,*,/
|
| +# ------------------------------------------------------------
|
| +import ply.lex as lex
|
| +
|
| +# 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+'
|
| + t.value = int(t.value)
|
| + 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)
|
| +
|
| +# Build the lexer
|
| +lexer = lex.lex()
|
| +
|
| +</pre>
|
| +</blockquote>
|
| +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:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +
|
| +# Test it out
|
| +data = '''
|
| +3 + 4 * 10
|
| + + -20 *2
|
| +'''
|
| +
|
| +# Give the lexer some input
|
| +lexer.input(data)
|
| +
|
| +# Tokenize
|
| +while True:
|
| + tok = lexer.token()
|
| + if not tok: break # No more input
|
| + print tok
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +When executed, the example will produce the following output:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +$ python example.py
|
| +LexToken(NUMBER,3,2,1)
|
| +LexToken(PLUS,'+',2,3)
|
| +LexToken(NUMBER,4,2,5)
|
| +LexToken(TIMES,'*',2,7)
|
| +LexToken(NUMBER,10,2,10)
|
| +LexToken(PLUS,'+',3,14)
|
| +LexToken(MINUS,'-',3,16)
|
| +LexToken(NUMBER,20,3,18)
|
| +LexToken(TIMES,'*',3,20)
|
| +LexToken(NUMBER,2,3,21)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +Lexers also support the iteration protocol. So, you can write the above loop as follows:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +for tok in lexer:
|
| + print tok
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +The tokens returned by <tt>lexer.token()</tt> are instances
|
| +of <tt>LexToken</tt>. This object has
|
| +attributes <tt>tok.type</tt>, <tt>tok.value</tt>,
|
| +<tt>tok.lineno</tt>, and <tt>tok.lexpos</tt>. The following code shows an example of
|
| +accessing these attributes:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +# Tokenize
|
| +while True:
|
| + tok = lexer.token()
|
| + if not tok: break # No more input
|
| + print tok.type, tok.value, tok.line, tok.lexpos
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +The <tt>tok.type</tt> and <tt>tok.value</tt> attributes contain the
|
| +type and value of the token itself.
|
| +<tt>tok.line</tt> and <tt>tok.lexpos</tt> contain information about
|
| +the location of the token. <tt>tok.lexpos</tt> is the index of the
|
| +token relative to the start of the input text.
|
| +
|
| +<H3><a name="ply_nn5"></a>4.2 The tokens list</H3>
|
| +
|
| +
|
| +All lexers must provide a list <tt>tokens</tt> that defines all of the possible token
|
| +names that can be produced by the lexer. This list is always required
|
| +and is used to perform a variety of validation checks. The tokens list is also used by the
|
| +<tt>yacc.py</tt> module to identify terminals.
|
| +
|
| +<p>
|
| +In the example, the following code specified the token names:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +tokens = (
|
| + 'NUMBER',
|
| + 'PLUS',
|
| + 'MINUS',
|
| + 'TIMES',
|
| + 'DIVIDE',
|
| + 'LPAREN',
|
| + 'RPAREN',
|
| +)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +<H3><a name="ply_nn6"></a>4.3 Specification of tokens</H3>
|
| +
|
| +
|
| +Each token is specified by writing a regular expression rule. Each of these rules are
|
| +are defined by making declarations with a special prefix <tt>t_</tt> to indicate that it
|
| +defines a token. For simple tokens, the regular expression can
|
| +be specified as strings such as this (note: Python raw strings are used since they are the
|
| +most convenient way to write regular expression strings):
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +t_PLUS = r'\+'
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +In this case, the name following the <tt>t_</tt> must exactly match one of the
|
| +names supplied in <tt>tokens</tt>. If some kind of action needs to be performed,
|
| +a token rule can be specified as a function. For example, this rule matches numbers and
|
| +converts the string into a Python integer.
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +def t_NUMBER(t):
|
| + r'\d+'
|
| + t.value = int(t.value)
|
| + return t
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +When a function is used, the regular expression rule is specified in the function documentation string.
|
| +The function always takes a single argument which is an instance of
|
| +<tt>LexToken</tt>. This object has attributes of <tt>t.type</tt> which is the token type (as a string),
|
| +<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
|
| +is the position of the token relative to the beginning of the input text.
|
| +By default, <tt>t.type</tt> is set to the name following the <tt>t_</tt> prefix. The action
|
| +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, you should write a single rule to 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',
|
| + ...
|
| +}
|
| +
|
| +tokens = ['LPAREN','RPAREN',...,'ID'] + list(reserved.values())
|
| +
|
| +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>4.4 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. If you
|
| +need to store multiple values on a token, assign a tuple, dictionary, or instance to <tt>value</tt>.
|
| +
|
| +<H3><a name="ply_nn8"></a>4.5 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>4.6 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):
|
| + last_cr = input.rfind('\n',0,token.lexpos)
|
| + if last_cr < 0:
|
| + last_cr = 0
|
| + column = (token.lexpos - last_cr) + 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>4.7 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>4.8 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>4.9 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>4.10 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 methods can
|
| +be used to control the lexer.
|
| +
|
| +<ul>
|
| +<li><tt>lexer.input(data)</tt>. Reset the lexer and store a new input string.
|
| +<li><tt>lexer.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>
|
| +
|
| +The preferred way to use PLY is to invoke the above methods directly on the lexer object returned by the
|
| +<tt>lex()</tt> function. The legacy interface to PLY involves module-level functions <tt>lex.input()</tt> and <tt>lex.token()</tt>.
|
| +For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +lex.lex()
|
| +lex.input(sometext)
|
| +while 1:
|
| + tok = lex.token()
|
| + if not tok: break
|
| + print tok
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +<p>
|
| +In this example, the module-level 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. This interface may go away at some point so
|
| +it's probably best not to use it.
|
| +
|
| +<H3><a name="ply_nn14"></a>4.11 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>4.12 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>4.13 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>
|
| +
|
| +<p>
|
| +This will produce various sorts of debugging information including all of the added rules,
|
| +the master regular expressions used by the lexer, and tokens generating during lexing.
|
| +</p>
|
| +
|
| +<p>
|
| +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:
|
| +</p>
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +if __name__ == '__main__':
|
| + lex.runmain()
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +Please refer to the "Debugging" section near the end for some more advanced details
|
| +of debugging.
|
| +
|
| +<H3><a name="ply_nn17"></a>4.14 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+'
|
| + t.value = int(t.value)
|
| + 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>module</tt> option can also be used to define lexers from instances of a class. 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+'
|
| + t.value = int(t.value)
|
| + 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(module=self, **kwargs)</b>
|
| +
|
| + # Test it output
|
| + def test(self,data):
|
| + self.lexer.input(data)
|
| + while True:
|
| + 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>
|
| +
|
| +
|
| +When building a lexer from class, <em>you should construct the lexer from
|
| +an instance of the class</em>, not the class object itself. This is because
|
| +PLY only works properly if the lexer actions are defined by bound-methods.
|
| +
|
| +<p>
|
| +When using the <tt>module</tt> option to <tt>lex()</tt>, PLY collects symbols
|
| +from the underlying object using the <tt>dir()</tt> function. There is no
|
| +direct access to the <tt>__dict__</tt> attribute of the object supplied as a
|
| +module value.
|
| +
|
| +<P>
|
| +Finally, if you want to keep things nicely encapsulated, but don't want to use a
|
| +full-fledged class definition, lexers can be defined using closures. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +import ply.lex as lex
|
| +
|
| +# List of token names. This is always required
|
| +tokens = (
|
| + 'NUMBER',
|
| + 'PLUS',
|
| + 'MINUS',
|
| + 'TIMES',
|
| + 'DIVIDE',
|
| + 'LPAREN',
|
| + 'RPAREN',
|
| +)
|
| +
|
| +def MyLexer():
|
| + # 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+'
|
| + t.value = int(t.value)
|
| + 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)
|
| +
|
| + # Build the lexer from my environment and return it
|
| + return lex.lex()
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +
|
| +<H3><a name="ply_nn18"></a>4.15 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. As an example, suppose that you wanted to keep
|
| +track of how many NUMBER tokens had been encountered.
|
| +
|
| +<p>
|
| +One way to do this is to keep a set of global variables in the module
|
| +where you created the lexer. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +num_count = 0
|
| +def t_NUMBER(t):
|
| + r'\d+'
|
| + global num_count
|
| + num_count += 1
|
| + t.value = int(t.value)
|
| + return t
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +If you don't like the use of a global variable, another place to store
|
| +information is 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
|
| + t.value = int(t.value)
|
| + return t
|
| +
|
| +lexer = lex.lex()
|
| +lexer.num_count = 0 # Set the initial count
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +This latter approach has the advantage of being simple and working
|
| +correctly in applications where multiple instantiations of a given
|
| +lexer exist in the same application. However, this might also feel
|
| +like a gross violation of encapsulation to OO purists.
|
| +Just to put your 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 is perfectly safe to store attributes in the lexer that
|
| +don't have names starting with that prefix or a name that conlicts with one of the
|
| +predefined methods (e.g., <tt>input()</tt>, <tt>token()</tt>, etc.).
|
| +
|
| +<p>
|
| +If you don't like assigning values on the lexer object, you can define your lexer as a class as
|
| +shown in the previous section:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +class MyLexer:
|
| + ...
|
| + def t_NUMBER(self,t):
|
| + r'\d+'
|
| + self.num_count += 1
|
| + t.value = int(t.value)
|
| + return t
|
| +
|
| + def build(self, **kwargs):
|
| + self.lexer = lex.lex(object=self,**kwargs)
|
| +
|
| + def __init__(self):
|
| + self.num_count = 0
|
| +</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.
|
| +
|
| +<p>
|
| +State can also be managed through closures. For example, in Python 3:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +def MyLexer():
|
| + num_count = 0
|
| + ...
|
| + def t_NUMBER(t):
|
| + r'\d+'
|
| + nonlocal num_count
|
| + num_count += 1
|
| + t.value = int(t.value)
|
| + return t
|
| + ...
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +<H3><a name="ply_nn19"></a>4.16 Lexer cloning</H3>
|
| +
|
| +
|
| +<p>
|
| +If necessary, a lexer object can be 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 exactly identical to the original lexer
|
| +including any input text and internal state. However, the clone allows a
|
| +different set of input text to be supplied which may be processed separately.
|
| +This 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. Or, if you were implementing some kind of preprocessor,
|
| +cloned lexers could be used to handle different input files.
|
| +
|
| +<p>
|
| +Creating a clone is different than calling <tt>lex.lex()</tt> in that
|
| +PLY doesn't regenerate any of the internal tables or regular expressions. So,
|
| +
|
| +<p>
|
| +Special considerations need to be made when cloning lexers that also
|
| +maintain their own internal state using classes or closures. Namely,
|
| +you need to be aware that the newly created lexers will share all of
|
| +this state with the original lexer. For example, if you defined a
|
| +lexer as a class and did this:
|
| +
|
| +<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> and any changes to <tt>m</tt> will be reflected in both lexers. It's
|
| +important to emphasize that <tt>clone()</tt> is only meant to create a new lexer
|
| +that reuses the regular expressions and environment of another lexer. If you
|
| +need to make a totally new copy of a lexer, then call <tt>lex()</tt> again.
|
| +
|
| +<H3><a name="ply_nn20"></a>4.17 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. PLY only specifies that the attribute
|
| +exists---it never sets, updates, or performs any processing with it. If you want to track line numbers,
|
| +you will need to add code yourself (see the section on line numbers and positional information).
|
| +</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.
|
| +Note: This attribute is only updated when tokens are defined and processed by functions.
|
| +</blockquote>
|
| +
|
| +<H3><a name="ply_nn21"></a>4.18 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>4.19 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>5. 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
|
| +grammar rules comprised of a collection of terminals and other rules. 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
|
| +view each symbol in the grammar as a kind of object. Associated
|
| +with each symbol is a value representing its "state" (for example, the
|
| +<tt>val</tt> attribute above). Semantic
|
| +actions are then expressed as a collection of functions or methods
|
| +that operate on the symbols and associated values.
|
| +
|
| +<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 that
|
| +match one of the grammar rules.
|
| +The details of the algorithm can be found in a compiler textbook, 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. In the example,
|
| +the special symbol <tt>$</tt> represents the end of input.
|
| +
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +Step Symbol Stack Input Tokens Action
|
| +---- --------------------- --------------------- -------------------------------
|
| +1 3 + 5 * ( 10 - 20 )$ 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 happens 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). A parse is only successful if the parser reaches a state where
|
| +the symbol stack is empty and there are no more input tokens.
|
| +
|
| +<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 non-trivial 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>6. Yacc</H2>
|
| +
|
| +
|
| +The <tt>ply.yacc</tt> module implements the parsing component of PLY.
|
| +The name "yacc" stands for "Yet Another Compiler Compiler" and is
|
| +borrowed from the Unix tool of the same name.
|
| +
|
| +<H3><a name="ply_nn24"></a>6.1 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
|
| +parser = yacc.yacc()
|
| +
|
| +while True:
|
| + try:
|
| + s = raw_input('calc > ')
|
| + except EOFError:
|
| + break
|
| + if not s: continue
|
| + result = parser.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. The statements that make up the
|
| +function body implement the semantic actions of the rule. 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>
|
| +
|
| +<p>
|
| +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 propagate the result.
|
| +</p>
|
| +
|
| +<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>
|
| +
|
| +<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
|
| +Generating LALR tables
|
| +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 <a href="reference.html">PLY Reference</a> for details.
|
| +
|
| +<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 discuss grammar specification in more detail.
|
| +
|
| +<p>
|
| +The final part of the example shows how to actually run the parser
|
| +created by
|
| +<tt>yacc()</tt>. To run the parser, you simply have to call
|
| +the <tt>parse()</tt> with a string of input text. This will run all
|
| +of the grammar rules and return the result of the entire parse. This
|
| +result return is the value assigned to <tt>p[0]</tt> in the starting
|
| +grammar rule.
|
| +
|
| +<H3><a name="ply_nn25"></a>6.2 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>
|
| +
|
| +If parsing performance is a concern, you should resist the urge to put
|
| +too much conditional processing into a single grammar rule as shown in
|
| +these examples. When you add checks to see which grammar rule is
|
| +being handled, you are actually duplicating the work that the parser
|
| +has already performed (i.e., the parser already knows exactly what rule it
|
| +matched). You can eliminate this overhead by using a
|
| +separate <tt>p_rule()</tt> function for each grammar rule.
|
| +
|
| +<H3><a name="ply_nn26"></a>6.3 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>6.4 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
|
| +and more clearly states your intentions.
|
| +
|
| +<H3><a name="ply_nn28"></a>6.5 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>6.6 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 "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 (for example, the case of
|
| +"if-then" versus "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 than the rule on the stack, 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 (although it will print
|
| +some information in the <tt>parser.out</tt> debugging file).
|
| +
|
| +<p>
|
| +One problem with the precedence specifier technique is that it is
|
| +sometimes necessary to change the precedence of an operator in certain
|
| +contexts. For example, consider a unary-minus operator in "3 + 4 *
|
| +-5". Mathematically, the unary minus is normally given 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 so-called "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. When a
|
| +reduce/reduce conflict occurs, <tt>yacc()</tt> will try to help by
|
| +printing a warning message such as this:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +WARNING: 1 reduce/reduce conflict
|
| +WARNING: reduce/reduce conflict in state 15 resolved using rule (assignment -> ID EQUALS NUMBER)
|
| +WARNING: rejected rule (expression -> NUMBER)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +This message identifies the two rules that are in conflict. However,
|
| +it may not tell you how the parser arrived at such a state. To try
|
| +and figure it out, you'll probably have to look at your grammar and
|
| +the contents of the
|
| +<tt>parser.out</tt> debugging file with an appropriately high level of
|
| +caffeination.
|
| +
|
| +<H3><a name="ply_nn28"></a>6.7 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>
|
| +
|
| +The different states that appear in this file are a representation of
|
| +every possible sequence of valid input tokens allowed by the grammar.
|
| +When receiving input tokens, the parser is building up a stack and
|
| +looking for matching rules. Each state keeps track of the grammar
|
| +rules that might be in the process of being matched at that point. Within each
|
| +rule, the "." character indicates the current location of the parse
|
| +within that rule. 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>6.8 Syntax Error Handling</H3>
|
| +
|
| +
|
| +If you are creating a parser for production use, the handling of
|
| +syntax errors is important. As a general rule, you don't want a
|
| +parser to simply throw up its hands and stop at the first sign of
|
| +trouble. Instead, you want it to report the error, recover if possible, and
|
| +continue parsing so that all of the errors in the input get reported
|
| +to the user at once. This is the standard behavior found in compilers
|
| +for languages such as C, C++, and Java.
|
| +
|
| +In PLY, 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). However, at this point, the parser enters a
|
| +recovery mode that can be used to try and continue further parsing.
|
| +As a general rule, 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. However, if the syntax error is due to
|
| +reaching the end-of-file, <tt>p_error()</tt> is called with an argument of <tt>None</tt>.
|
| +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>6.8.1 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>6.8.2 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_nn35"></a>6.8.3 Signaling an error from a production</H4>
|
| +
|
| +
|
| +If necessary, a production rule can manually force the parser to enter error recovery. This
|
| +is done by raising the <tt>SyntaxError</tt> exception like this:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +def p_production(p):
|
| + 'production : some production ...'
|
| + raise SyntaxError
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +The effect of raising <tt>SyntaxError</tt> is the same as if the last symbol shifted onto the
|
| +parsing stack was actually a syntax error. Thus, when you do this, the last symbol shifted is popped off
|
| +of the parsing stack and the current lookahead token is set to an <tt>error</tt> token. The parser
|
| +then enters error-recovery mode where it tries to reduce rules that can accept <tt>error</tt> tokens.
|
| +The steps that follow from this point are exactly the same as if a syntax error were detected and
|
| +<tt>p_error()</tt> were called.
|
| +
|
| +<P>
|
| +One important aspect of manually setting an error is that the <tt>p_error()</tt> function will <b>NOT</b> be
|
| +called in this case. If you need to issue an error message, make sure you do it in the production that
|
| +raises <tt>SyntaxError</tt>.
|
| +
|
| +<P>
|
| +Note: This feature of PLY is meant to mimic the behavior of the YYERROR macro in yacc.
|
| +
|
| +
|
| +<H4><a name="ply_nn32"></a>6.8.4 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>6.9 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
|
| +selectively propagate line number information where it's needed using
|
| +the <tt>p.set_lineno()</tt> method. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +def p_fname(p):
|
| + 'fname : ID'
|
| + p[0] = p[1]
|
| + p.set_lineno(0,p.lineno(1))
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +PLY doesn't retain line number information from rules that have already been
|
| +parsed. If you are building an abstract syntax tree and need to have line numbers,
|
| +you should make sure that the line numbers appear in the tree itself.
|
| +
|
| +<H3><a name="ply_nn34"></a>6.10 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.
|
| +
|
| +<p>A minimal way to construct a tree is to simply create and
|
| +propagate a tuple or list in each grammar rule function. There
|
| +are many possible ways to do this, but one example would be something
|
| +like this:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +def p_expression_binop(p):
|
| + '''expression : expression PLUS expression
|
| + | expression MINUS expression
|
| + | expression TIMES expression
|
| + | expression DIVIDE expression'''
|
| +
|
| + p[0] = ('binary-expression',p[2],p[1],p[3])
|
| +
|
| +def p_expression_group(p):
|
| + 'expression : LPAREN expression RPAREN'
|
| + p[0] = ('group-expression',p[2])
|
| +
|
| +def p_expression_number(p):
|
| + 'expression : NUMBER'
|
| + p[0] = ('number-expression',p[1])
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +<p>
|
| +Another approach is to create a set of data structure for different
|
| +kinds of 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>
|
| +
|
| +The advantage to this approach is that it may make it easier to attach more complicated
|
| +semantics, type checking, code generation, and other features to the node classes.
|
| +
|
| +<p>
|
| +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>6.11 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, write an 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>6.12 Miscellaneous Yacc 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>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>7. Multiple Parsers and Lexers</H2>
|
| +
|
| +
|
| +In advanced parsing applications, you may want to have multiple
|
| +parsers and lexers.
|
| +
|
| +<p>
|
| +As a general rules this isn't a problem. However, to make it work,
|
| +you need to carefully make sure everything gets hooked up correctly.
|
| +First, make sure you save the objects returned by <tt>lex()</tt> and
|
| +<tt>yacc()</tt>. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +lexer = lex.lex() # Return lexer object
|
| +parser = yacc.yacc() # Return parser object
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +Next, when parsing, make sure you give the <tt>parse()</tt> function a reference to the lexer it
|
| +should be using. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +parser.parse(text,lexer=lexer)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +If you forget to do this, the parser will use the last lexer
|
| +created--which is not always what you want.
|
| +
|
| +<p>
|
| +Within lexer and parser rule functions, these objects are also
|
| +available. In the lexer, the "lexer" attribute of a token refers to
|
| +the lexer object that triggered the rule. 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>8. 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. One of the purposes of
|
| +optimized mode is to substantially decrease the startup time of
|
| +your compiler (by assuming that everything is already properly
|
| +specified and works).
|
| +
|
| +<H2><a name="ply_nn44"></a>9. Advanced Debugging</H2>
|
| +
|
| +
|
| +<p>
|
| +Debugging a compiler is typically not an easy task. PLY provides some
|
| +advanced diagonistic capabilities through the use of Python's
|
| +<tt>logging</tt> module. The next two sections describe this:
|
| +
|
| +<H3><a name="ply_nn45"></a>9.1 Debugging the lex() and yacc() commands</H3>
|
| +
|
| +
|
| +<p>
|
| +Both the <tt>lex()</tt> and <tt>yacc()</tt> commands have a debugging
|
| +mode that can be enabled using the <tt>debug</tt> flag. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +lex.lex(debug=True)
|
| +yacc.yacc(debug=True)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +Normally, the output produced by debugging is routed to either
|
| +standard error or, in the case of <tt>yacc()</tt>, to a file
|
| +<tt>parser.out</tt>. This output can be more carefully controlled
|
| +by supplying a logging object. Here is an example that adds
|
| +information about where different debugging messages are coming from:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +# Set up a logging object
|
| +import logging
|
| +logging.basicConfig(
|
| + level = logging.DEBUG,
|
| + filename = "parselog.txt",
|
| + filemode = "w",
|
| + format = "%(filename)10s:%(lineno)4d:%(message)s"
|
| +)
|
| +log = logging.getLogger()
|
| +
|
| +lex.lex(debug=True,debuglog=log)
|
| +yacc.yacc(debug=True,debuglog=log)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +If you supply a custom logger, the amount of debugging
|
| +information produced can be controlled by setting the logging level.
|
| +Typically, debugging messages are either issued at the <tt>DEBUG</tt>,
|
| +<tt>INFO</tt>, or <tt>WARNING</tt> levels.
|
| +
|
| +<p>
|
| +PLY's error messages and warnings are also produced using the logging
|
| +interface. This can be controlled by passing a logging object
|
| +using the <tt>errorlog</tt> parameter.
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +lex.lex(errorlog=log)
|
| +yacc.yacc(errorlog=log)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +If you want to completely silence warnings, you can either pass in a
|
| +logging object with an appropriate filter level or use the <tt>NullLogger</tt>
|
| +object defined in either <tt>lex</tt> or <tt>yacc</tt>. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +yacc.yacc(errorlog=yacc.NullLogger())
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +<H3><a name="ply_nn46"></a>9.2 Run-time Debugging</H3>
|
| +
|
| +
|
| +<p>
|
| +To enable run-time debugging of a parser, use the <tt>debug</tt> option to parse. This
|
| +option can either be an integer (which simply turns debugging on or off) or an instance
|
| +of a logger object. For example:
|
| +
|
| +<blockquote>
|
| +<pre>
|
| +log = logging.getLogger()
|
| +parser.parse(input,debug=log)
|
| +</pre>
|
| +</blockquote>
|
| +
|
| +If a logging object is passed, you can use its filtering level to control how much
|
| +output gets generated. The <tt>INFO</tt> level is used to produce information
|
| +about rule reductions. The <tt>DEBUG</tt> level will show information about the
|
| +parsing stack, token shifts, and other details. The <tt>ERROR</tt> level shows information
|
| +related to parsing errors.
|
| +
|
| +<p>
|
| +For very complicated problems, you should pass in a logging object that
|
| +redirects to a file where you can more easily inspect the output after
|
| +execution.
|
| +
|
| +<H2><a name="ply_nn39"></a>10. 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>
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
| +
|
|
|
| Property changes on: tools/nixysa/third_party/ply-3.1/doc/ply.html
|
| ___________________________________________________________________
|
| Added: svn:eol-style
|
| + LF
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|
|
|
|
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