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Diffstat (limited to 'chall/ply-2.2/doc')
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| -rw-r--r-- | chall/ply-2.2/doc/ply.html | 2874 |
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diff --git a/chall/ply-2.2/doc/makedoc.py b/chall/ply-2.2/doc/makedoc.py deleted file mode 100644 index 415a53a..0000000 --- a/chall/ply-2.2/doc/makedoc.py +++ /dev/null @@ -1,194 +0,0 @@ -#!/usr/local/bin/python - -############################################################################### -# Takes a chapter as input and adds internal links and numbering to all -# of the H1, H2, H3, H4 and H5 sections. -# -# Every heading HTML tag (H1, H2 etc) is given an autogenerated name to link -# to. However, if the name is not an autogenerated name from a previous run, -# it will be kept. If it is autogenerated, it might change on subsequent runs -# of this program. Thus if you want to create links to one of the headings, -# then change the heading link name to something that does not look like an -# autogenerated link name. -############################################################################### - -import sys -import re -import string - -############################################################################### -# Functions -############################################################################### - -# Regexs for <a name="..."></a> -alink = re.compile(r"<a *name *= *\"(.*)\"></a>", re.IGNORECASE) -heading = re.compile(r"(_nn\d)", re.IGNORECASE) - -def getheadingname(m): - autogeneratedheading = True; - if m.group(1) != None: - amatch = alink.match(m.group(1)) - if amatch: - # A non-autogenerated heading - keep it - headingname = amatch.group(1) - autogeneratedheading = heading.match(headingname) - if autogeneratedheading: - # The heading name was either non-existent or autogenerated, - # We can create a new heading / change the existing heading - headingname = "%s_nn%d" % (filenamebase, nameindex) - return headingname - -############################################################################### -# Main program -############################################################################### - -if len(sys.argv) != 2: - print "usage: makedoc.py filename" - sys.exit(1) - -filename = sys.argv[1] -filenamebase = string.split(filename,".")[0] - -section = 0 -subsection = 0 -subsubsection = 0 -subsubsubsection = 0 -nameindex = 0 - -name = "" - -# Regexs for <h1>,... <h5> sections - -h1 = re.compile(r".*?<H1>(<a.*a>)*[\d\.\s]*(.*?)</H1>", re.IGNORECASE) -h2 = re.compile(r".*?<H2>(<a.*a>)*[\d\.\s]*(.*?)</H2>", re.IGNORECASE) -h3 = re.compile(r".*?<H3>(<a.*a>)*[\d\.\s]*(.*?)</H3>", re.IGNORECASE) -h4 = re.compile(r".*?<H4>(<a.*a>)*[\d\.\s]*(.*?)</H4>", re.IGNORECASE) -h5 = re.compile(r".*?<H5>(<a.*a>)*[\d\.\s]*(.*?)</H5>", re.IGNORECASE) - -data = open(filename).read() # Read data -open(filename+".bak","w").write(data) # Make backup - -lines = data.splitlines() -result = [ ] # This is the result of postprocessing the file -index = "<!-- INDEX -->\n<div class=\"sectiontoc\">\n" # index contains the index for adding at the top of the file. Also printed to stdout. - -skip = 0 -skipspace = 0 - -for s in lines: - if s == "<!-- INDEX -->": - if not skip: - result.append("@INDEX@") - skip = 1 - else: - skip = 0 - continue; - if skip: - continue - - if not s and skipspace: - continue - - if skipspace: - result.append("") - result.append("") - skipspace = 0 - - m = h2.match(s) - if m: - prevheadingtext = m.group(2) - nameindex += 1 - section += 1 - headingname = getheadingname(m) - result.append("""<H2><a name="%s"></a>%d. %s</H2>""" % (headingname,section, prevheadingtext)) - - if subsubsubsection: - index += "</ul>\n" - if subsubsection: - index += "</ul>\n" - if subsection: - index += "</ul>\n" - if section == 1: - index += "<ul>\n" - - index += """<li><a href="#%s">%s</a>\n""" % (headingname,prevheadingtext) - subsection = 0 - subsubsection = 0 - subsubsubsection = 0 - skipspace = 1 - continue - m = h3.match(s) - if m: - prevheadingtext = m.group(2) - nameindex += 1 - subsection += 1 - headingname = getheadingname(m) - result.append("""<H3><a name="%s"></a>%d.%d %s</H3>""" % (headingname,section, subsection, prevheadingtext)) - - if subsubsubsection: - index += "</ul>\n" - if subsubsection: - index += "</ul>\n" - if subsection == 1: - index += "<ul>\n" - - index += """<li><a href="#%s">%s</a>\n""" % (headingname,prevheadingtext) - subsubsection = 0 - skipspace = 1 - continue - m = h4.match(s) - if m: - prevheadingtext = m.group(2) - nameindex += 1 - subsubsection += 1 - subsubsubsection = 0 - headingname = getheadingname(m) - result.append("""<H4><a name="%s"></a>%d.%d.%d %s</H4>""" % (headingname,section, subsection, subsubsection, prevheadingtext)) - - if subsubsubsection: - index += "</ul>\n" - if subsubsection == 1: - index += "<ul>\n" - - index += """<li><a href="#%s">%s</a>\n""" % (headingname,prevheadingtext) - skipspace = 1 - continue - m = h5.match(s) - if m: - prevheadingtext = m.group(2) - nameindex += 1 - subsubsubsection += 1 - headingname = getheadingname(m) - result.append("""<H5><a name="%s"></a>%d.%d.%d.%d %s</H5>""" % (headingname,section, subsection, subsubsection, subsubsubsection, prevheadingtext)) - - if subsubsubsection == 1: - index += "<ul>\n" - - index += """<li><a href="#%s">%s</a>\n""" % (headingname,prevheadingtext) - skipspace = 1 - continue - - result.append(s) - -if subsubsubsection: - index += "</ul>\n" - -if subsubsection: - index += "</ul>\n" - -if subsection: - index += "</ul>\n" - -if section: - index += "</ul>\n" - -index += "</div>\n<!-- INDEX -->\n" - -data = "\n".join(result) - -data = data.replace("@INDEX@",index) + "\n"; - -# Write the file back out -open(filename,"w").write(data) - - diff --git a/chall/ply-2.2/doc/ply.html b/chall/ply-2.2/doc/ply.html deleted file mode 100644 index b3219ea..0000000 --- a/chall/ply-2.2/doc/ply.html +++ /dev/null @@ -1,2874 +0,0 @@ -<html> -<head> -<title>PLY (Python Lex-Yacc)</title> -</head> -<body bgcolor="#ffffff"> - -<h1>PLY (Python Lex-Yacc)</h1> - -<b> -David M. Beazley <br> -dave@dabeaz.com<br> -</b> - -<p> -<b>PLY Version: 2.2</b> -<p> - -<!-- INDEX --> -<div class="sectiontoc"> -<ul> -<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">Duplicating lexers</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 reference</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_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">Yacc implementation notes</a> -</ul> -<li><a href="#ply_nn37">Parser and Lexer State Management</a> -<li><a href="#ply_nn38">Using Python's Optimized Mode</a> -<li><a href="#ply_nn39">Where to go from here?</a> -</ul> -</div> -<!-- INDEX --> - - - - - - -<H2><a name="ply_nn1"></a>1. 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. - -<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>2. 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>3. 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>3.1 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+' - try: - t.value = int(t.value) - except ValueError: - print "Line %d: Number %s is too large!" % (t.lineno,t.value) - t.value = 0 - return t - -# Define a rule so we can track line numbers -def t_newline(t): - r'\n+' - t.lexer.lineno += len(t.value) - -# A string containing ignored characters (spaces and tabs) -t_ignore = ' \t' - -# Error handling rule -def t_error(t): - print "Illegal character '%s'" % t.value[0] - t.lexer.skip(1) - -# Build the 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 -lex.input(data) - -# Tokenize -while 1: - tok = lex.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> - -The tokens returned by <tt>lex.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 1: - tok = lex.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>3.2 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>3.3 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+' - try: - t.value = int(t.value) - except ValueError: - print "Number %s is too large!" % t.value - t.value = 0 - 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, it is usually easier to just match an identifier and do a special name lookup in a function -like this: - -<blockquote> -<pre> -reserved = { - 'if' : 'IF', - 'then' : 'THEN', - 'else' : 'ELSE', - 'while' : 'WHILE', - ... -} - -def t_ID(t): - r'[a-zA-Z_][a-zA-Z_0-9]*' - t.type = reserved.get(t.value,'ID') # Check for reserved words - return t -</pre> -</blockquote> - -This approach greatly reduces the number of regular expression rules and is likely to make things a little faster. - -<p> -<b>Note:</b> You should avoid writing individual rules for reserved words. For example, if you write rules like this, - -<blockquote> -<pre> -t_FOR = r'for' -t_PRINT = r'print' -</pre> -</blockquote> - -those rules will be triggered for identifiers that include those words as a prefix such as "forget" or "printed". This is probably not -what you want. - -<H3><a name="ply_nn7"></a>3.4 Token values</H3> - - -When tokens are returned by lex, they have a value that is stored in the <tt>value</tt> attribute. Normally, the value is the text -that was matched. However, the value can be assigned to any Python object. For instance, when lexing identifiers, you may -want to return both the identifier name and information from some sort of symbol table. To do this, you might write a rule like this: - -<blockquote> -<pre> -def t_ID(t): - ... - # Look up symbol table information and return a tuple - t.value = (t.value, symbol_lookup(t.value)) - ... - return t -</pre> -</blockquote> - -It is important to note that storing data in other attribute names is <em>not</em> recommended. The <tt>yacc.py</tt> module only exposes the -contents of the <tt>value</tt> attribute. Thus, accessing other attributes may be unnecessarily awkward. - -<H3><a name="ply_nn8"></a>3.5 Discarded tokens</H3> - - -To discard a token, such as a comment, simply define a token rule that returns no value. For example: - -<blockquote> -<pre> -def t_COMMENT(t): - r'\#.*' - pass - # No return value. Token discarded -</pre> -</blockquote> - -Alternatively, you can include the prefix "ignore_" in the token declaration to force a token to be ignored. For example: - -<blockquote> -<pre> -t_ignore_COMMENT = r'\#.*' -</pre> -</blockquote> - -Be advised that if you are ignoring many different kinds of text, you may still want to use functions since these provide more precise -control over the order in which regular expressions are matched (i.e., functions are matched in order of specification whereas strings are -sorted by regular expression length). - -<H3><a name="ply_nn9"></a>3.6 Line numbers and positional information</H3> - - -<p>By default, <tt>lex.py</tt> knows nothing about line numbers. This is because <tt>lex.py</tt> doesn't know anything -about what constitutes a "line" of input (e.g., the newline character or even if the input is textual data). -To update this information, you need to write a special rule. In the example, the <tt>t_newline()</tt> rule shows how to do this. - -<blockquote> -<pre> -# Define a rule so we can track line numbers -def t_newline(t): - r'\n+' - t.lexer.lineno += len(t.value) -</pre> -</blockquote> -Within the rule, the <tt>lineno</tt> attribute of the underlying lexer <tt>t.lexer</tt> is updated. -After the line number is updated, the token is simply discarded since nothing is returned. - -<p> -<tt>lex.py</tt> does not perform and kind of automatic column tracking. However, it does record positional -information related to each token in the <tt>lexpos</tt> attribute. Using this, it is usually possible to compute -column information as a separate step. For instance, just count backwards until you reach a newline. - -<blockquote> -<pre> -# Compute column. -# input is the input text string -# token is a token instance -def find_column(input,token): - i = token.lexpos - while i > 0: - if input[i] == '\n': break - i -= 1 - column = (token.lexpos - i)+1 - return column -</pre> -</blockquote> - -Since column information is often only useful in the context of error handling, calculating the column -position can be performed when needed as opposed to doing it for each token. - -<H3><a name="ply_nn10"></a>3.7 Ignored characters</H3> - - -<p> -The special <tt>t_ignore</tt> rule is reserved by <tt>lex.py</tt> for characters -that should be completely ignored in the input stream. -Usually this is used to skip over whitespace and other non-essential characters. -Although it is possible to define a regular expression rule for whitespace in a manner -similar to <tt>t_newline()</tt>, the use of <tt>t_ignore</tt> provides substantially better -lexing performance because it is handled as a special case and is checked in a much -more efficient manner than the normal regular expression rules. - -<H3><a name="ply_nn11"></a>3.8 Literal characters</H3> - - -<p> -Literal characters can be specified by defining a variable <tt>literals</tt> in your lexing module. For example: - -<blockquote> -<pre> -literals = [ '+','-','*','/' ] -</pre> -</blockquote> - -or alternatively - -<blockquote> -<pre> -literals = "+-*/" -</pre> -</blockquote> - -A literal character is simply a single character that is returned "as is" when encountered by the lexer. Literals are checked -after all of the defined regular expression rules. Thus, if a rule starts with one of the literal characters, it will always -take precedence. -<p> -When a literal token is returned, both its <tt>type</tt> and <tt>value</tt> attributes are set to the character itself. For example, <tt>'+'</tt>. - -<H3><a name="ply_nn12"></a>3.9 Error handling</H3> - - -<p> -Finally, the <tt>t_error()</tt> -function is used to handle lexing errors that occur when illegal -characters are detected. In this case, the <tt>t.value</tt> attribute contains the -rest of the input string that has not been tokenized. In the example, the error function -was defined as follows: - -<blockquote> -<pre> -# Error handling rule -def t_error(t): - print "Illegal character '%s'" % t.value[0] - t.lexer.skip(1) -</pre> -</blockquote> - -In this case, we simply print the offending character and skip ahead one character by calling <tt>t.lexer.skip(1)</tt>. - -<H3><a name="ply_nn13"></a>3.10 Building and using the lexer</H3> - - -<p> -To build the lexer, the function <tt>lex.lex()</tt> is used. This function -uses Python reflection (or introspection) to read the the regular expression rules -out of the calling context and build the lexer. Once the lexer has been built, two functions can -be used to control the lexer. - -<ul> -<li><tt>lex.input(data)</tt>. Reset the lexer and store a new input string. -<li><tt>lex.token()</tt>. Return the next token. Returns a special <tt>LexToken</tt> instance on success or -None if the end of the input text has been reached. -</ul> - -If desired, the lexer can also be used as an object. The <tt>lex()</tt> returns a <tt>Lexer</tt> object that -can be used for this purpose. For example: - -<blockquote> -<pre> -lexer = lex.lex() -lexer.input(sometext) -while 1: - tok = lexer.token() - if not tok: break - print tok -</pre> -</blockquote> - -<p> -This latter technique should be used if you intend to use multiple lexers in your application. Simply define each -lexer in its own module and use the object returned by <tt>lex()</tt> as appropriate. - -<p> -Note: The global functions <tt>lex.input()</tt> and <tt>lex.token()</tt> are bound to the <tt>input()</tt> -and <tt>token()</tt> methods of the last lexer created by the lex module. - -<H3><a name="ply_nn14"></a>3.11 The @TOKEN decorator</H3> - - -In some applications, you may want to define build tokens from as a series of -more complex regular expression rules. For example: - -<blockquote> -<pre> -digit = r'([0-9])' -nondigit = r'([_A-Za-z])' -identifier = r'(' + nondigit + r'(' + digit + r'|' + nondigit + r')*)' - -def t_ID(t): - # want docstring to be identifier above. ????? - ... -</pre> -</blockquote> - -In this case, we want the regular expression rule for <tt>ID</tt> to be one of the variables above. However, there is no -way to directly specify this using a normal documentation string. To solve this problem, you can use the <tt>@TOKEN</tt> -decorator. For example: - -<blockquote> -<pre> -from ply.lex import TOKEN - -@TOKEN(identifier) -def t_ID(t): - ... -</pre> -</blockquote> - -This will attach <tt>identifier</tt> to the docstring for <tt>t_ID()</tt> allowing <tt>lex.py</tt> to work normally. An alternative -approach this problem is to set the docstring directly like this: - -<blockquote> -<pre> -def t_ID(t): - ... - -t_ID.__doc__ = identifier -</pre> -</blockquote> - -<b>NOTE:</b> Use of <tt>@TOKEN</tt> requires Python-2.4 or newer. If you're concerned about backwards compatibility with older -versions of Python, use the alternative approach of setting the docstring directly. - -<H3><a name="ply_nn15"></a>3.12 Optimized mode</H3> - - -For improved performance, it may be desirable to use Python's -optimized mode (e.g., running Python with the <tt>-O</tt> -option). However, doing so causes Python to ignore documentation -strings. This presents special problems for <tt>lex.py</tt>. To -handle this case, you can create your lexer using -the <tt>optimize</tt> option as follows: - -<blockquote> -<pre> -lexer = lex.lex(optimize=1) -</pre> -</blockquote> - -Next, run Python in its normal operating mode. When you do -this, <tt>lex.py</tt> will write a file called <tt>lextab.py</tt> to -the current directory. This file contains all of the regular -expression rules and tables used during lexing. On subsequent -executions, -<tt>lextab.py</tt> will simply be imported to build the lexer. This -approach substantially improves the startup time of the lexer and it -works in Python's optimized mode. - -<p> -To change the name of the lexer-generated file, use the <tt>lextab</tt> keyword argument. For example: - -<blockquote> -<pre> -lexer = lex.lex(optimize=1,lextab="footab") -</pre> -</blockquote> - -When running in optimized mode, it is important to note that lex disables most error checking. Thus, this is really only recommended -if you're sure everything is working correctly and you're ready to start releasing production code. - -<H3><a name="ply_nn16"></a>3.13 Debugging</H3> - - -For the purpose of debugging, you can run <tt>lex()</tt> in a debugging mode as follows: - -<blockquote> -<pre> -lexer = lex.lex(debug=1) -</pre> -</blockquote> - -This will result in a large amount of debugging information to be printed including all of the added rules and the master -regular expressions. - -In addition, <tt>lex.py</tt> comes with a simple main function which -will either tokenize input read from standard input or from a file specified -on the command line. To use it, simply put this in your lexer: - -<blockquote> -<pre> -if __name__ == '__main__': - lex.runmain() -</pre> -</blockquote> - -<H3><a name="ply_nn17"></a>3.14 Alternative specification of lexers</H3> - - -As shown in the example, lexers are specified all within one Python module. If you want to -put token rules in a different module from the one in which you invoke <tt>lex()</tt>, use the -<tt>module</tt> keyword argument. - -<p> -For example, you might have a dedicated module that just contains -the token rules: - -<blockquote> -<pre> -# module: tokrules.py -# This module just contains the lexing rules - -# List of token names. This is always required -tokens = ( - 'NUMBER', - 'PLUS', - 'MINUS', - 'TIMES', - 'DIVIDE', - 'LPAREN', - 'RPAREN', -) - -# Regular expression rules for simple tokens -t_PLUS = r'\+' -t_MINUS = r'-' -t_TIMES = r'\*' -t_DIVIDE = r'/' -t_LPAREN = r'\(' -t_RPAREN = r'\)' - -# A regular expression rule with some action code -def t_NUMBER(t): - r'\d+' - try: - t.value = int(t.value) - except ValueError: - print "Line %d: Number %s is too large!" % (t.lineno,t.value) - t.value = 0 - return t - -# Define a rule so we can track line numbers -def t_newline(t): - r'\n+' - t.lexer.lineno += len(t.value) - -# A string containing ignored characters (spaces and tabs) -t_ignore = ' \t' - -# Error handling rule -def t_error(t): - print "Illegal character '%s'" % t.value[0] - t.lexer.skip(1) -</pre> -</blockquote> - -Now, if you wanted to build a tokenizer from these rules from within a different module, you would do the following (shown for Python interactive mode): - -<blockquote> -<pre> ->>> import tokrules ->>> <b>lexer = lex.lex(module=tokrules)</b> ->>> lexer.input("3 + 4") ->>> lexer.token() -LexToken(NUMBER,3,1,1,0) ->>> lexer.token() -LexToken(PLUS,'+',1,2) ->>> lexer.token() -LexToken(NUMBER,4,1,4) ->>> lexer.token() -None ->>> -</pre> -</blockquote> - -The <tt>object</tt> option can be used to define lexers as a class instead of a module. For example: - -<blockquote> -<pre> -import ply.lex as lex - -class MyLexer: - # List of token names. This is always required - tokens = ( - 'NUMBER', - 'PLUS', - 'MINUS', - 'TIMES', - 'DIVIDE', - 'LPAREN', - 'RPAREN', - ) - - # Regular expression rules for simple tokens - t_PLUS = r'\+' - t_MINUS = r'-' - t_TIMES = r'\*' - t_DIVIDE = r'/' - t_LPAREN = r'\(' - t_RPAREN = r'\)' - - # A regular expression rule with some action code - # Note addition of self parameter since we're in a class - def t_NUMBER(self,t): - r'\d+' - try: - t.value = int(t.value) - except ValueError: - print "Line %d: Number %s is too large!" % (t.lineno,t.value) - t.value = 0 - return t - - # Define a rule so we can track line numbers - def t_newline(self,t): - r'\n+' - t.lexer.lineno += len(t.value) - - # A string containing ignored characters (spaces and tabs) - t_ignore = ' \t' - - # Error handling rule - def t_error(self,t): - print "Illegal character '%s'" % t.value[0] - t.lexer.skip(1) - - <b># Build the lexer - def build(self,**kwargs): - self.lexer = lex.lex(object=self, **kwargs)</b> - - # Test it output - def test(self,data): - self.lexer.input(data) - while 1: - tok = lexer.token() - if not tok: break - print tok - -# Build the lexer and try it out -m = MyLexer() -m.build() # Build the lexer -m.test("3 + 4") # Test it -</pre> -</blockquote> - -For reasons that are subtle, you should <em>NOT</em> invoke <tt>lex.lex()</tt> inside the <tt>__init__()</tt> method of your class. If you -do, it may cause bizarre behavior if someone tries to duplicate a lexer object. Keep reading. - -<H3><a name="ply_nn18"></a>3.15 Maintaining state</H3> - - -In your lexer, you may want to maintain a variety of state information. This might include mode settings, symbol tables, and other details. There are a few -different ways to handle this situation. First, you could just keep some global variables: - -<blockquote> -<pre> -num_count = 0 -def t_NUMBER(t): - r'\d+' - global num_count - num_count += 1 - try: - t.value = int(t.value) - except ValueError: - print "Line %d: Number %s is too large!" % (t.lineno,t.value) - t.value = 0 - return t -</pre> -</blockquote> - -Alternatively, you can store this information inside the Lexer object created by <tt>lex()</tt>. To this, you can use the <tt>lexer</tt> attribute -of tokens passed to the various rules. For example: - -<blockquote> -<pre> -def t_NUMBER(t): - r'\d+' - t.lexer.num_count += 1 # Note use of lexer attribute - try: - t.value = int(t.value) - except ValueError: - print "Line %d: Number %s is too large!" % (t.lineno,t.value) - t.value = 0 - return t - -lexer = lex.lex() -lexer.num_count = 0 # Set the initial count -</pre> -</blockquote> - -This latter approach has the advantage of storing information inside -the lexer itself---something that may be useful if multiple instances -of the same lexer have been created. However, it may also feel kind -of "hacky" to the purists. Just to put their mind at some ease, all -internal attributes of the lexer (with the exception of <tt>lineno</tt>) have names that are prefixed -by <tt>lex</tt> (e.g., <tt>lexdata</tt>,<tt>lexpos</tt>, etc.). Thus, -it should be perfectly safe to store attributes in the lexer that -don't have names starting with that prefix. - -<p> -A third approach is to define the lexer as a class as shown in the previous example: - -<blockquote> -<pre> -class MyLexer: - ... - def t_NUMBER(self,t): - r'\d+' - self.num_count += 1 - try: - t.value = int(t.value) - except ValueError: - print "Line %d: Number %s is too large!" % (t.lineno,t.value) - t.value = 0 - return t - - def build(self, **kwargs): - self.lexer = lex.lex(object=self,**kwargs) - - def __init__(self): - self.num_count = 0 - -# Create a lexer -m = MyLexer() -lexer = lex.lex(object=m) -</pre> -</blockquote> - -The class approach may be the easiest to manage if your application is going to be creating multiple instances of the same lexer and -you need to manage a lot of state. - -<H3><a name="ply_nn19"></a>3.16 Duplicating lexers</H3> - - -<b>NOTE: I am thinking about deprecating this feature. Post comments on <a href="http://groups.google.com/group/ply-hack">ply-hack@googlegroups.com</a> or send me a private email at dave@dabeaz.com.</b> - -<p> -If necessary, a lexer object can be quickly duplicated by invoking its <tt>clone()</tt> method. For example: - -<blockquote> -<pre> -lexer = lex.lex() -... -newlexer = lexer.clone() -</pre> -</blockquote> - -When a lexer is cloned, the copy is identical to the original lexer, -including any input text. However, once created, different text can be -fed to the clone which can be used independently. This capability may -be useful in situations when you are writing a parser/compiler that -involves recursive or reentrant processing. For instance, if you -needed to scan ahead in the input for some reason, you could create a -clone and use it to look ahead. - -<p> -The advantage of using <tt>clone()</tt> instead of reinvoking <tt>lex()</tt> is -that it is significantly faster. Namely, it is not necessary to re-examine all of the -token rules, build a regular expression, and construct internal tables. All of this -information can simply be reused in the new lexer. - -<p> -Special considerations need to be made when cloning a lexer that is defined as a class. Previous sections -showed an example of a class <tt>MyLexer</tt>. If you have the following code: - -<blockquote> -<pre> -m = MyLexer() -a = lex.lex(object=m) # Create a lexer - -b = a.clone() # Clone the lexer -</pre> -</blockquote> - -Then both <tt>a</tt> and <tt>b</tt> are going to be bound to the same -object <tt>m</tt>. If the object <tt>m</tt> contains internal state -related to lexing, this sharing may lead to quite a bit of confusion. To fix this, -the <tt>clone()</tt> method accepts an optional argument that can be used to supply a new object. This -can be used to clone the lexer and bind it to a new instance. For example: - -<blockquote> -<pre> -m = MyLexer() # Create a lexer -a = lex.lex(object=m) - -# Create a clone -n = MyLexer() # New instance of MyLexer -b = a.clone(n) # New lexer bound to n -</pre> -</blockquote> - -It may make sense to encapsulate all of this inside a method: - -<blockquote> -<pre> -class MyLexer: - ... - def clone(self): - c = MyLexer() # Create a new instance of myself - # Copy attributes from self to c as appropriate - ... - # Clone the lexer - c.lexer = self.lexer.clone(c) - return c -</pre> -</blockquote> - -The fact that a new instance of <tt>MyLexer</tt> may be created while cloning a lexer is the reason why you should never -invoke <tt>lex.lex()</tt> inside <tt>__init__()</tt>. If you do, the lexer will be rebuilt from scratch and you lose -all of the performance benefits of using <tt>clone()</tt> in the first place. - -<H3><a name="ply_nn20"></a>3.17 Internal lexer state</H3> - - -A Lexer object <tt>lexer</tt> has a number of internal attributes that may be useful in certain -situations. - -<p> -<tt>lexer.lexpos</tt> -<blockquote> -This attribute is an integer that contains the current position within the input text. If you modify -the value, it will change the result of the next call to <tt>token()</tt>. Within token rule functions, this points -to the first character <em>after</em> the matched text. If the value is modified within a rule, the next returned token will be -matched at the new position. -</blockquote> - -<p> -<tt>lexer.lineno</tt> -<blockquote> -The current value of the line number attribute stored in the lexer. This can be modified as needed to -change the line number. -</blockquote> - -<p> -<tt>lexer.lexdata</tt> -<blockquote> -The current input text stored in the lexer. This is the string passed with the <tt>input()</tt> method. It -would probably be a bad idea to modify this unless you really know what you're doing. -</blockquote> - -<P> -<tt>lexer.lexmatch</tt> -<blockquote> -This is the raw <tt>Match</tt> object returned by the Python <tt>re.match()</tt> function (used internally by PLY) for the -current token. If you have written a regular expression that contains named groups, you can use this to retrieve those values. -</blockquote> - -<H3><a name="ply_nn21"></a>3.18 Conditional lexing and start conditions</H3> - - -In advanced parsing applications, it may be useful to have different -lexing states. For instance, you may want the occurrence of a certain -token or syntactic construct to trigger a different kind of lexing. -PLY supports a feature that allows the underlying lexer to be put into -a series of different states. Each state can have its own tokens, -lexing rules, and so forth. The implementation is based largely on -the "start condition" feature of GNU flex. Details of this can be found -at <a -href="http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html">http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html.</a>. - -<p> -To define a new lexing state, it must first be declared. This is done by including a "states" declaration in your -lex file. For example: - -<blockquote> -<pre> -states = ( - ('foo','exclusive'), - ('bar','inclusive'), -) -</pre> -</blockquote> - -This declaration declares two states, <tt>'foo'</tt> -and <tt>'bar'</tt>. States may be of two types; <tt>'exclusive'</tt> -and <tt>'inclusive'</tt>. An exclusive state completely overrides the -default behavior of the lexer. That is, lex will only return tokens -and apply rules defined specifically for that state. An inclusive -state adds additional tokens and rules to the default set of rules. -Thus, lex will return both the tokens defined by default in addition -to those defined for the inclusive state. - -<p> -Once a state has been declared, tokens and rules are declared by including the -state name in token/rule declaration. For example: - -<blockquote> -<pre> -t_foo_NUMBER = r'\d+' # Token 'NUMBER' in state 'foo' -t_bar_ID = r'[a-zA-Z_][a-zA-Z0-9_]*' # Token 'ID' in state 'bar' - -def t_foo_newline(t): - r'\n' - t.lexer.lineno += 1 -</pre> -</blockquote> - -A token can be declared in multiple states by including multiple state names in the declaration. For example: - -<blockquote> -<pre> -t_foo_bar_NUMBER = r'\d+' # Defines token 'NUMBER' in both state 'foo' and 'bar' -</pre> -</blockquote> - -Alternative, a token can be declared in all states using the 'ANY' in the name. - -<blockquote> -<pre> -t_ANY_NUMBER = r'\d+' # Defines a token 'NUMBER' in all states -</pre> -</blockquote> - -If no state name is supplied, as is normally the case, the token is associated with a special state <tt>'INITIAL'</tt>. For example, -these two declarations are identical: - -<blockquote> -<pre> -t_NUMBER = r'\d+' -t_INITIAL_NUMBER = r'\d+' -</pre> -</blockquote> - -<p> -States are also associated with the special <tt>t_ignore</tt> and <tt>t_error()</tt> declarations. For example, if a state treats -these differently, you can declare: - -<blockquote> -<pre> -t_foo_ignore = " \t\n" # Ignored characters for state 'foo' - -def t_bar_error(t): # Special error handler for state 'bar' - pass -</pre> -</blockquote> - -By default, lexing operates in the <tt>'INITIAL'</tt> state. This state includes all of the normally defined tokens. -For users who aren't using different states, this fact is completely transparent. If, during lexing or parsing, you want to change -the lexing state, use the <tt>begin()</tt> method. For example: - -<blockquote> -<pre> -def t_begin_foo(t): - r'start_foo' - t.lexer.begin('foo') # Starts 'foo' state -</pre> -</blockquote> - -To get out of a state, you use <tt>begin()</tt> to switch back to the initial state. For example: - -<blockquote> -<pre> -def t_foo_end(t): - r'end_foo' - t.lexer.begin('INITIAL') # Back to the initial state -</pre> -</blockquote> - -The management of states can also be done with a stack. For example: - -<blockquote> -<pre> -def t_begin_foo(t): - r'start_foo' - t.lexer.push_state('foo') # Starts 'foo' state - -def t_foo_end(t): - r'end_foo' - t.lexer.pop_state() # Back to the previous state -</pre> -</blockquote> - -<p> -The use of a stack would be useful in situations where there are many ways of entering a new lexing state and you merely want to go back -to the previous state afterwards. - -<P> -An example might help clarify. Suppose you were writing a parser and you wanted to grab sections of arbitrary C code enclosed by -curly braces. That is, whenever you encounter a starting brace '{', you want to read all of the enclosed code up to the ending brace '}' -and return it as a string. Doing this with a normal regular expression rule is nearly (if not actually) impossible. This is because braces can -be nested and can be included in comments and strings. Thus, simply matching up to the first matching '}' character isn't good enough. Here is how -you might use lexer states to do this: - -<blockquote> -<pre> -# Declare the state -states = ( - ('ccode','exclusive'), -) - -# Match the first {. Enter ccode state. -def t_ccode(t): - r'\{' - t.lexer.code_start = t.lexer.lexpos # Record the starting position - t.lexer.level = 1 # Initial brace level - t.lexer.begin('ccode') # Enter 'ccode' state - -# Rules for the ccode state -def t_ccode_lbrace(t): - r'\{' - t.lexer.level +=1 - -def t_ccode_rbrace(t): - r'\}' - t.lexer.level -=1 - - # If closing brace, return the code fragment - if t.lexer.level == 0: - t.value = t.lexer.lexdata[t.lexer.code_start:t.lexer.lexpos+1] - t.type = "CCODE" - t.lexer.lineno += t.value.count('\n') - t.lexer.begin('INITIAL') - return t - -# C or C++ comment (ignore) -def t_ccode_comment(t): - r'(/\*(.|\n)*?*/)|(//.*)' - pass - -# C string -def t_ccode_string(t): - r'\"([^\\\n]|(\\.))*?\"' - -# C character literal -def t_ccode_char(t): - r'\'([^\\\n]|(\\.))*?\'' - -# Any sequence of non-whitespace characters (not braces, strings) -def t_ccode_nonspace(t): - r'[^\s\{\}\'\"]+' - -# Ignored characters (whitespace) -t_ccode_ignore = " \t\n" - -# For bad characters, we just skip over it -def t_ccode_error(t): - t.lexer.skip(1) -</pre> -</blockquote> - -In this example, the occurrence of the first '{' causes the lexer to record the starting position and enter a new state <tt>'ccode'</tt>. A collection of rules then match -various parts of the input that follow (comments, strings, etc.). All of these rules merely discard the token (by not returning a value). -However, if the closing right brace is encountered, the rule <tt>t_ccode_rbrace</tt> collects all of the code (using the earlier recorded starting -position), stores it, and returns a token 'CCODE' containing all of that text. When returning the token, the lexing state is restored back to its -initial state. - -<H3><a name="ply_nn21"></a>3.19 Miscellaneous Issues</H3> - - -<P> -<li>The lexer requires input to be supplied as a single input string. Since most machines have more than enough memory, this -rarely presents a performance concern. However, it means that the lexer currently can't be used with streaming data -such as open files or sockets. This limitation is primarily a side-effect of using the <tt>re</tt> module. - -<p> -<li>The lexer should work properly with both Unicode strings given as token and pattern matching rules as -well as for input text. - -<p> -<li>If you need to supply optional flags to the re.compile() function, use the reflags option to lex. For example: - -<blockquote> -<pre> -lex.lex(reflags=re.UNICODE) -</pre> -</blockquote> - -<p> -<li>Since the lexer is written entirely in Python, its performance is -largely determined by that of the Python <tt>re</tt> module. Although -the lexer has been written to be as efficient as possible, it's not -blazingly fast when used on very large input files. If -performance is concern, you might consider upgrading to the most -recent version of Python, creating a hand-written lexer, or offloading -the lexer into a C extension module. - -<p> -If you are going to create a hand-written lexer and you plan to use it with <tt>yacc.py</tt>, -it only needs to conform to the following requirements: - -<ul> -<li>It must provide a <tt>token()</tt> method that returns the next token or <tt>None</tt> if no more -tokens are available. -<li>The <tt>token()</tt> method must return an object <tt>tok</tt> that has <tt>type</tt> and <tt>value</tt> attributes. -</ul> - -<H2><a name="ply_nn22"></a>4. Parsing basics</H2> - - -<tt>yacc.py</tt> is used to parse language syntax. Before showing an -example, there are a few important bits of background that must be -mentioned. First, <em>syntax</em> is usually specified in terms of a BNF grammar. -For example, if you wanted to parse -simple arithmetic expressions, you might first write an unambiguous -grammar specification like this: - -<blockquote> -<pre> -expression : expression + term - | expression - term - | term - -term : term * factor - | term / factor - | factor - -factor : NUMBER - | ( expression ) -</pre> -</blockquote> - -In the grammar, symbols such as <tt>NUMBER</tt>, <tt>+</tt>, <tt>-</tt>, <tt>*</tt>, and <tt>/</tt> are known -as <em>terminals</em> and correspond to raw input tokens. Identifiers such as <tt>term</tt> and <tt>factor</tt> refer to more -complex rules, typically comprised of a collection of tokens. These identifiers are known as <em>non-terminals</em>. -<P> -The semantic behavior of a language is often specified using a -technique known as syntax directed translation. In syntax directed -translation, attributes are attached to each symbol in a given grammar -rule along with an action. Whenever a particular grammar rule is -recognized, the action describes what to do. For example, given the -expression grammar above, you might write the specification for a -simple calculator like this: - -<blockquote> -<pre> -Grammar Action --------------------------------- -------------------------------------------- -expression0 : expression1 + term expression0.val = expression1.val + term.val - | expression1 - term expression0.val = expression1.val - term.val - | term expression0.val = term.val - -term0 : term1 * factor term0.val = term1.val * factor.val - | term1 / factor term0.val = term1.val / factor.val - | factor term0.val = factor.val - -factor : NUMBER factor.val = int(NUMBER.lexval) - | ( expression ) factor.val = expression.val -</pre> -</blockquote> - -A good way to think about syntax directed translation is to simply think of each symbol in the grammar as some -kind of object. The semantics of the language are then expressed as a collection of methods/operations on these -objects. - -<p> -Yacc uses a parsing technique known as LR-parsing or shift-reduce parsing. LR parsing is a -bottom up technique that tries to recognize the right-hand-side of various grammar rules. -Whenever a valid right-hand-side is found in the input, the appropriate action code is triggered and the -grammar symbols are replaced by the grammar symbol on the left-hand-side. - -<p> -LR parsing is commonly implemented by shifting grammar symbols onto a stack and looking at the stack and the next -input token for patterns. The details of the algorithm can be found in a compiler text, but the -following example illustrates the steps that are performed if you wanted to parse the expression -<tt>3 + 5 * (10 - 20)</tt> using the grammar defined above: - -<blockquote> -<pre> -Step Symbol Stack Input Tokens Action ----- --------------------- --------------------- ------------------------------- -1 $ 3 + 5 * ( 10 - 20 )$ Shift 3 -2 $ 3 + 5 * ( 10 - 20 )$ Reduce factor : NUMBER -3 $ factor + 5 * ( 10 - 20 )$ Reduce term : factor -4 $ term + 5 * ( 10 - 20 )$ Reduce expr : term -5 $ expr + 5 * ( 10 - 20 )$ Shift + -6 $ expr + 5 * ( 10 - 20 )$ Shift 5 -7 $ expr + 5 * ( 10 - 20 )$ Reduce factor : NUMBER -8 $ expr + factor * ( 10 - 20 )$ Reduce term : factor -9 $ expr + term * ( 10 - 20 )$ Shift * -10 $ expr + term * ( 10 - 20 )$ Shift ( -11 $ expr + term * ( 10 - 20 )$ Shift 10 -12 $ expr + term * ( 10 - 20 )$ Reduce factor : NUMBER -13 $ expr + term * ( factor - 20 )$ Reduce term : factor -14 $ expr + term * ( term - 20 )$ Reduce expr : term -15 $ expr + term * ( expr - 20 )$ Shift - -16 $ expr + term * ( expr - 20 )$ Shift 20 -17 $ expr + term * ( expr - 20 )$ Reduce factor : NUMBER -18 $ expr + term * ( expr - factor )$ Reduce term : factor -19 $ expr + term * ( expr - term )$ Reduce expr : expr - term -20 $ expr + term * ( expr )$ Shift ) -21 $ expr + term * ( expr ) $ Reduce factor : (expr) -22 $ expr + term * factor $ Reduce term : term * factor -23 $ expr + term $ Reduce expr : expr + term -24 $ expr $ Reduce expr -25 $ $ Success! -</pre> -</blockquote> - -When parsing the expression, an underlying state machine and the current input token determine what to do next. -If the next token looks like part of a valid grammar rule (based on other items on the stack), it is generally shifted -onto the stack. If the top of the stack contains a valid right-hand-side of a grammar rule, it is -usually "reduced" and the symbols replaced with the symbol on the left-hand-side. When this reduction occurs, the -appropriate action is triggered (if defined). If the input token can't be shifted and the top of stack doesn't match -any grammar rules, a syntax error has occurred and the parser must take some kind of recovery step (or bail out). - -<p> -It is important to note that the underlying implementation is built around a large finite-state machine that is encoded -in a collection of tables. The construction of these tables is quite complicated and beyond the scope of this discussion. -However, subtle details of this process explain why, in the example above, the parser chooses to shift a token -onto the stack in step 9 rather than reducing the rule <tt>expr : expr + term</tt>. - -<H2><a name="ply_nn23"></a>5. Yacc reference</H2> - - -This section describes how to use write parsers in PLY. - -<H3><a name="ply_nn24"></a>5.1 An example</H3> - - -Suppose you wanted to make a grammar for simple arithmetic expressions as previously described. Here is -how you would do it with <tt>yacc.py</tt>: - -<blockquote> -<pre> -# Yacc example - -import ply.yacc as yacc - -# Get the token map from the lexer. This is required. -from calclex import tokens - -def p_expression_plus(p): - 'expression : expression PLUS term' - p[0] = p[1] + p[3] - -def p_expression_minus(p): - 'expression : expression MINUS term' - p[0] = p[1] - p[3] - -def p_expression_term(p): - 'expression : term' - p[0] = p[1] - -def p_term_times(p): - 'term : term TIMES factor' - p[0] = p[1] * p[3] - -def p_term_div(p): - 'term : term DIVIDE factor' - p[0] = p[1] / p[3] - -def p_term_factor(p): - 'term : factor' - p[0] = p[1] - -def p_factor_num(p): - 'factor : NUMBER' - p[0] = p[1] - -def p_factor_expr(p): - 'factor : LPAREN expression RPAREN' - p[0] = p[2] - -# Error rule for syntax errors -def p_error(p): - print "Syntax error in input!" - -# Build the parser -yacc.yacc() - -# Use this if you want to build the parser using SLR instead of LALR -# yacc.yacc(method="SLR") - -while 1: - try: - s = raw_input('calc > ') - except EOFError: - break - if not s: continue - result = yacc.parse(s) - print result -</pre> -</blockquote> - -In this example, each grammar rule is defined by a Python function where the docstring to that function contains the -appropriate context-free grammar specification. Each function accepts a single -argument <tt>p</tt> that is a sequence containing the values of each grammar symbol in the corresponding rule. The values of -<tt>p[i]</tt> are mapped to grammar symbols as shown here: - -<blockquote> -<pre> -def p_expression_plus(p): - 'expression : expression PLUS term' - # ^ ^ ^ ^ - # p[0] p[1] p[2] p[3] - - p[0] = p[1] + p[3] -</pre> -</blockquote> - -For tokens, the "value" of the corresponding <tt>p[i]</tt> is the -<em>same</em> as the <tt>p.value</tt> attribute assigned -in the lexer module. For non-terminals, the value is determined by -whatever is placed in <tt>p[0]</tt> when rules are reduced. This -value can be anything at all. However, it probably most common for -the value to be a simple Python type, a tuple, or an instance. In this example, we -are relying on the fact that the <tt>NUMBER</tt> token stores an integer value in its value -field. All of the other rules simply perform various types of integer operations and store -the result. - -<P> -Note: The use of negative indices have a special meaning in yacc---specially <tt>p[-1]</tt> does -not have the same value as <tt>p[3]</tt> in this example. Please see the section on "Embedded Actions" for further -details. - -<p> -The first rule defined in the yacc specification determines the starting grammar -symbol (in this case, a rule for <tt>expression</tt> appears first). Whenever -the starting rule is reduced by the parser and no more input is available, parsing -stops and the final value is returned (this value will be whatever the top-most rule -placed in <tt>p[0]</tt>). Note: an alternative starting symbol can be specified using the <tt>start</tt> keyword argument to -<tt>yacc()</tt>. - -<p>The <tt>p_error(p)</tt> rule is defined to catch syntax errors. See the error handling section -below for more detail. - -<p> -To build the parser, call the <tt>yacc.yacc()</tt> function. This function -looks at the module and attempts to construct all of the LR parsing tables for the grammar -you have specified. The first time <tt>yacc.yacc()</tt> is invoked, you will get a message -such as this: - -<blockquote> -<pre> -$ python calcparse.py -yacc: Generating LALR parsing table... -calc > -</pre> -</blockquote> - -Since table construction is relatively expensive (especially for large -grammars), the resulting parsing table is written to the current -directory in a file called <tt>parsetab.py</tt>. In addition, a -debugging file called <tt>parser.out</tt> is created. On subsequent -executions, <tt>yacc</tt> will reload the table from -<tt>parsetab.py</tt> unless it has detected a change in the underlying -grammar (in which case the tables and <tt>parsetab.py</tt> file are -regenerated). Note: The names of parser output files can be changed if necessary. See the notes that follow later. - -<p> -If any errors are detected in your grammar specification, <tt>yacc.py</tt> will produce -diagnostic messages and possibly raise an exception. Some of the errors that can be detected include: - -<ul> -<li>Duplicated function names (if more than one rule function have the same name in the grammar file). -<li>Shift/reduce and reduce/reduce conflicts generated by ambiguous grammars. -<li>Badly specified grammar rules. -<li>Infinite recursion (rules that can never terminate). -<li>Unused rules and tokens -<li>Undefined rules and tokens -</ul> - -The next few sections now discuss a few finer points of grammar construction. - -<H3><a name="ply_nn25"></a>5.2 Combining Grammar Rule Functions</H3> - - -When grammar rules are similar, they can be combined into a single function. -For example, consider the two rules in our earlier example: - -<blockquote> -<pre> -def p_expression_plus(p): - 'expression : expression PLUS term' - p[0] = p[1] + p[3] - -def p_expression_minus(t): - 'expression : expression MINUS term' - p[0] = p[1] - p[3] -</pre> -</blockquote> - -Instead of writing two functions, you might write a single function like this: - -<blockquote> -<pre> -def p_expression(p): - '''expression : expression PLUS term - | expression MINUS term''' - if p[2] == '+': - p[0] = p[1] + p[3] - elif p[2] == '-': - p[0] = p[1] - p[3] -</pre> -</blockquote> - -In general, the doc string for any given function can contain multiple grammar rules. So, it would -have also been legal (although possibly confusing) to write this: - -<blockquote> -<pre> -def p_binary_operators(p): - '''expression : expression PLUS term - | expression MINUS term - term : term TIMES factor - | term DIVIDE factor''' - if p[2] == '+': - p[0] = p[1] + p[3] - elif p[2] == '-': - p[0] = p[1] - p[3] - elif p[2] == '*': - p[0] = p[1] * p[3] - elif p[2] == '/': - p[0] = p[1] / p[3] -</pre> -</blockquote> - -When combining grammar rules into a single function, it is usually a good idea for all of the rules to have -a similar structure (e.g., the same number of terms). Otherwise, the corresponding action code may be more -complicated than necessary. However, it is possible to handle simple cases using len(). For example: - -<blockquote> -<pre> -def p_expressions(p): - '''expression : expression MINUS expression - | MINUS expression''' - if (len(p) == 4): - p[0] = p[1] - p[3] - elif (len(p) == 3): - p[0] = -p[2] -</pre> -</blockquote> - -<H3><a name="ply_nn26"></a>5.3 Character Literals</H3> - - -If desired, a grammar may contain tokens defined as single character literals. For example: - -<blockquote> -<pre> -def p_binary_operators(p): - '''expression : expression '+' term - | expression '-' term - term : term '*' factor - | term '/' factor''' - if p[2] == '+': - p[0] = p[1] + p[3] - elif p[2] == '-': - p[0] = p[1] - p[3] - elif p[2] == '*': - p[0] = p[1] * p[3] - elif p[2] == '/': - p[0] = p[1] / p[3] -</pre> -</blockquote> - -A character literal must be enclosed in quotes such as <tt>'+'</tt>. In addition, if literals are used, they must be declared in the -corresponding <tt>lex</tt> file through the use of a special <tt>literals</tt> declaration. - -<blockquote> -<pre> -# Literals. Should be placed in module given to lex() -literals = ['+','-','*','/' ] -</pre> -</blockquote> - -<b>Character literals are limited to a single character</b>. Thus, it is not legal to specify literals such as <tt>'<='</tt> or <tt>'=='</tt>. For this, use -the normal lexing rules (e.g., define a rule such as <tt>t_EQ = r'=='</tt>). - -<H3><a name="ply_nn26"></a>5.4 Empty Productions</H3> - - -<tt>yacc.py</tt> can handle empty productions by defining a rule like this: - -<blockquote> -<pre> -def p_empty(p): - 'empty :' - pass -</pre> -</blockquote> - -Now to use the empty production, simply use 'empty' as a symbol. For example: - -<blockquote> -<pre> -def p_optitem(p): - 'optitem : item' - ' | empty' - ... -</pre> -</blockquote> - -Note: You can write empty rules anywhere by simply specifying an empty right hand side. However, I personally find that -writing an "empty" rule and using "empty" to denote an empty production is easier to read. - -<H3><a name="ply_nn28"></a>5.5 Changing the starting symbol</H3> - - -Normally, the first rule found in a yacc specification defines the starting grammar rule (top level rule). To change this, simply -supply a <tt>start</tt> specifier in your file. For example: - -<blockquote> -<pre> -start = 'foo' - -def p_bar(p): - 'bar : A B' - -# This is the starting rule due to the start specifier above -def p_foo(p): - 'foo : bar X' -... -</pre> -</blockquote> - -The use of a <tt>start</tt> specifier may be useful during debugging since you can use it to have yacc build a subset of -a larger grammar. For this purpose, it is also possible to specify a starting symbol as an argument to <tt>yacc()</tt>. For example: - -<blockquote> -<pre> -yacc.yacc(start='foo') -</pre> -</blockquote> - -<H3><a name="ply_nn27"></a>5.6 Dealing With Ambiguous Grammars</H3> - - -The expression grammar given in the earlier example has been written in a special format to eliminate ambiguity. -However, in many situations, it is extremely difficult or awkward to write grammars in this format. A -much more natural way to express the grammar is in a more compact form like this: - -<blockquote> -<pre> -expression : expression PLUS expression - | expression MINUS expression - | expression TIMES expression - | expression DIVIDE expression - | LPAREN expression RPAREN - | NUMBER -</pre> -</blockquote> - -Unfortunately, this grammar specification is ambiguous. For example, if you are parsing the string -"3 * 4 + 5", there is no way to tell how the operators are supposed to be grouped. -For example, does the expression mean "(3 * 4) + 5" or is it "3 * (4+5)"? - -<p> -When an ambiguous grammar is given to <tt>yacc.py</tt> it will print messages about "shift/reduce conflicts" -or a "reduce/reduce conflicts". A shift/reduce conflict is caused when the parser generator can't decide -whether or not to reduce a rule or shift a symbol on the parsing stack. For example, consider -the string "3 * 4 + 5" and the internal parsing stack: - -<blockquote> -<pre> -Step Symbol Stack Input Tokens Action ----- --------------------- --------------------- ------------------------------- -1 $ 3 * 4 + 5$ Shift 3 -2 $ 3 * 4 + 5$ Reduce : expression : NUMBER -3 $ expr * 4 + 5$ Shift * -4 $ expr * 4 + 5$ Shift 4 -5 $ expr * 4 + 5$ Reduce: expression : NUMBER -6 $ expr * expr + 5$ SHIFT/REDUCE CONFLICT ???? -</pre> -</blockquote> - -In this case, when the parser reaches step 6, it has two options. One is to reduce the -rule <tt>expr : expr * expr</tt> on the stack. The other option is to shift the -token <tt>+</tt> on the stack. Both options are perfectly legal from the rules -of the context-free-grammar. - -<p> -By default, all shift/reduce conflicts are resolved in favor of shifting. Therefore, in the above -example, the parser will always shift the <tt>+</tt> instead of reducing. Although this -strategy works in many cases (including the ambiguous if-then-else), it is not enough for arithmetic -expressions. In fact, in the above example, the decision to shift <tt>+</tt> is completely wrong---we should have -reduced <tt>expr * expr</tt> since multiplication has higher mathematical precedence than addition. - -<p>To resolve ambiguity, especially in expression grammars, <tt>yacc.py</tt> allows individual -tokens to be assigned a precedence level and associativity. This is done by adding a variable -<tt>precedence</tt> to the grammar file like this: - -<blockquote> -<pre> -precedence = ( - ('left', 'PLUS', 'MINUS'), - ('left', 'TIMES', 'DIVIDE'), -) -</pre> -</blockquote> - -This declaration specifies that <tt>PLUS</tt>/<tt>MINUS</tt> have -the same precedence level and are left-associative and that -<tt>TIMES</tt>/<tt>DIVIDE</tt> have the same precedence and are left-associative. -Within the <tt>precedence</tt> declaration, tokens are ordered from lowest to highest precedence. Thus, -this declaration specifies that <tt>TIMES</tt>/<tt>DIVIDE</tt> have higher -precedence than <tt>PLUS</tt>/<tt>MINUS</tt> (since they appear later in the -precedence specification). - -<p> -The precedence specification works by associating a numerical precedence level value and associativity direction to -the listed tokens. For example, in the above example you get: - -<blockquote> -<pre> -PLUS : level = 1, assoc = 'left' -MINUS : level = 1, assoc = 'left' -TIMES : level = 2, assoc = 'left' -DIVIDE : level = 2, assoc = 'left' -</pre> -</blockquote> - -These values are then used to attach a numerical precedence value and associativity direction -to each grammar rule. <em>This is always determined by looking at the precedence of the right-most terminal symbol.</em> -For example: - -<blockquote> -<pre> -expression : expression PLUS expression # level = 1, left - | expression MINUS expression # level = 1, left - | expression TIMES expression # level = 2, left - | expression DIVIDE expression # level = 2, left - | LPAREN expression RPAREN # level = None (not specified) - | NUMBER # level = None (not specified) -</pre> -</blockquote> - -When shift/reduce conflicts are encountered, the parser generator resolves the conflict by -looking at the precedence rules and associativity specifiers. - -<p> -<ol> -<li>If the current token has higher precedence, it is shifted. -<li>If the grammar rule on the stack has higher precedence, the rule is reduced. -<li>If the current token and the grammar rule have the same precedence, the -rule is reduced for left associativity, whereas the token is shifted for right associativity. -<li>If nothing is known about the precedence, shift/reduce conflicts are resolved in -favor of shifting (the default). -</ol> - -For example, if "expression PLUS expression" has been parsed and the next token -is "TIMES", the action is going to be a shift because "TIMES" has a higher precedence level than "PLUS". On the other -hand, if "expression TIMES expression" has been parsed and the next token is "PLUS", the action -is going to be reduce because "PLUS" has a lower precedence than "TIMES." - -<p> -When shift/reduce conflicts are resolved using the first three techniques (with the help of -precedence rules), <tt>yacc.py</tt> will report no errors or conflicts in the grammar. - -<p> -One problem with the precedence specifier technique is that it is sometimes necessary to -change the precedence of an operator in certain contents. For example, consider a unary-minus operator -in "3 + 4 * -5". Normally, unary minus has a very high precedence--being evaluated before the multiply. -However, in our precedence specifier, MINUS has a lower precedence than TIMES. To deal with this, -precedence rules can be given for fictitious tokens like this: - -<blockquote> -<pre> -precedence = ( - ('left', 'PLUS', 'MINUS'), - ('left', 'TIMES', 'DIVIDE'), - ('right', 'UMINUS'), # Unary minus operator -) -</pre> -</blockquote> - -Now, in the grammar file, we can write our unary minus rule like this: - -<blockquote> -<pre> -def p_expr_uminus(p): - 'expression : MINUS expression %prec UMINUS' - p[0] = -p[2] -</pre> -</blockquote> - -In this case, <tt>%prec UMINUS</tt> overrides the default rule precedence--setting it to that -of UMINUS in the precedence specifier. - -<p> -At first, the use of UMINUS in this example may appear very confusing. -UMINUS is not an input token or a grammer rule. Instead, you should -think of it as the name of a special marker in the precedence table. When you use the <tt>%prec</tt> qualifier, you're simply -telling yacc that you want the precedence of the expression to be the same as for this special marker instead of the usual precedence. - -<p> -It is also possible to specify non-associativity in the <tt>precedence</tt> table. This would -be used when you <em>don't</em> want operations to chain together. For example, suppose -you wanted to support comparison operators like <tt><</tt> and <tt>></tt> but you didn't want to allow -combinations like <tt>a < b < c</tt>. To do this, simply specify a rule like this: - -<blockquote> -<pre> -precedence = ( - ('nonassoc', 'LESSTHAN', 'GREATERTHAN'), # Nonassociative operators - ('left', 'PLUS', 'MINUS'), - ('left', 'TIMES', 'DIVIDE'), - ('right', 'UMINUS'), # Unary minus operator -) -</pre> -</blockquote> - -<p> -If you do this, the occurrence of input text such as <tt> a < b < c</tt> will result in a syntax error. However, simple -expressions such as <tt>a < b</tt> will still be fine. - -<p> -Reduce/reduce conflicts are caused when there are multiple grammar -rules that can be applied to a given set of symbols. This kind of -conflict is almost always bad and is always resolved by picking the -rule that appears first in the grammar file. Reduce/reduce conflicts -are almost always caused when different sets of grammar rules somehow -generate the same set of symbols. For example: - -<blockquote> -<pre> -assignment : ID EQUALS NUMBER - | ID EQUALS expression - -expression : expression PLUS expression - | expression MINUS expression - | expression TIMES expression - | expression DIVIDE expression - | LPAREN expression RPAREN - | NUMBER -</pre> -</blockquote> - -In this case, a reduce/reduce conflict exists between these two rules: - -<blockquote> -<pre> -assignment : ID EQUALS NUMBER -expression : NUMBER -</pre> -</blockquote> - -For example, if you wrote "a = 5", the parser can't figure out if this -is supposed to be reduced as <tt>assignment : ID EQUALS NUMBER</tt> or -whether it's supposed to reduce the 5 as an expression and then reduce -the rule <tt>assignment : ID EQUALS expression</tt>. - -<p> -It should be noted that reduce/reduce conflicts are notoriously difficult to spot -simply looking at the input grammer. To locate these, it is usually easier to look at the -<tt>parser.out</tt> debugging file with an appropriately high level of caffeination. - -<H3><a name="ply_nn28"></a>5.7 The parser.out file</H3> - - -Tracking down shift/reduce and reduce/reduce conflicts is one of the finer pleasures of using an LR -parsing algorithm. To assist in debugging, <tt>yacc.py</tt> creates a debugging file called -'parser.out' when it generates the parsing table. The contents of this file look like the following: - -<blockquote> -<pre> -Unused terminals: - - -Grammar - -Rule 1 expression -> expression PLUS expression -Rule 2 expression -> expression MINUS expression -Rule 3 expression -> expression TIMES expression -Rule 4 expression -> expression DIVIDE expression -Rule 5 expression -> NUMBER -Rule 6 expression -> LPAREN expression RPAREN - -Terminals, with rules where they appear - -TIMES : 3 -error : -MINUS : 2 -RPAREN : 6 -LPAREN : 6 -DIVIDE : 4 -PLUS : 1 -NUMBER : 5 - -Nonterminals, with rules where they appear - -expression : 1 1 2 2 3 3 4 4 6 0 - - -Parsing method: LALR - - -state 0 - - S' -> . expression - expression -> . expression PLUS expression - expression -> . expression MINUS expression - expression -> . expression TIMES expression - expression -> . expression DIVIDE expression - expression -> . NUMBER - expression -> . LPAREN expression RPAREN - - NUMBER shift and go to state 3 - LPAREN shift and go to state 2 - - -state 1 - - S' -> expression . - expression -> expression . PLUS expression - expression -> expression . MINUS expression - expression -> expression . TIMES expression - expression -> expression . DIVIDE expression - - PLUS shift and go to state 6 - MINUS shift and go to state 5 - TIMES shift and go to state 4 - DIVIDE shift and go to state 7 - - -state 2 - - expression -> LPAREN . expression RPAREN - expression -> . expression PLUS expression - expression -> . expression MINUS expression - expression -> . expression TIMES expression - expression -> . expression DIVIDE expression - expression -> . NUMBER - expression -> . LPAREN expression RPAREN - - NUMBER shift and go to state 3 - LPAREN shift and go to state 2 - - -state 3 - - expression -> NUMBER . - - $ reduce using rule 5 - PLUS reduce using rule 5 - MINUS reduce using rule 5 - TIMES reduce using rule 5 - DIVIDE reduce using rule 5 - RPAREN reduce using rule 5 - - -state 4 - - expression -> expression TIMES . expression - expression -> . expression PLUS expression - expression -> . expression MINUS expression - expression -> . expression TIMES expression - expression -> . expression DIVIDE expression - expression -> . NUMBER - expression -> . LPAREN expression RPAREN - - NUMBER shift and go to state 3 - LPAREN shift and go to state 2 - - -state 5 - - expression -> expression MINUS . expression - expression -> . expression PLUS expression - expression -> . expression MINUS expression - expression -> . expression TIMES expression - expression -> . expression DIVIDE expression - expression -> . NUMBER - expression -> . LPAREN expression RPAREN - - NUMBER shift and go to state 3 - LPAREN shift and go to state 2 - - -state 6 - - expression -> expression PLUS . expression - expression -> . expression PLUS expression - expression -> . expression MINUS expression - expression -> . expression TIMES expression - expression -> . expression DIVIDE expression - expression -> . NUMBER - expression -> . LPAREN expression RPAREN - - NUMBER shift and go to state 3 - LPAREN shift and go to state 2 - - -state 7 - - expression -> expression DIVIDE . expression - expression -> . expression PLUS expression - expression -> . expression MINUS expression - expression -> . expression TIMES expression - expression -> . expression DIVIDE expression - expression -> . NUMBER - expression -> . LPAREN expression RPAREN - - NUMBER shift and go to state 3 - LPAREN shift and go to state 2 - - -state 8 - - expression -> LPAREN expression . RPAREN - expression -> expression . PLUS expression - expression -> expression . MINUS expression - expression -> expression . TIMES expression - expression -> expression . DIVIDE expression - - RPAREN shift and go to state 13 - PLUS shift and go to state 6 - MINUS shift and go to state 5 - TIMES shift and go to state 4 - DIVIDE shift and go to state 7 - - -state 9 - - expression -> expression TIMES expression . - expression -> expression . PLUS expression - expression -> expression . MINUS expression - expression -> expression . TIMES expression - expression -> expression . DIVIDE expression - - $ reduce using rule 3 - PLUS reduce using rule 3 - MINUS reduce using rule 3 - TIMES reduce using rule 3 - DIVIDE reduce using rule 3 - RPAREN reduce using rule 3 - - ! PLUS [ shift and go to state 6 ] - ! MINUS [ shift and go to state 5 ] - ! TIMES [ shift and go to state 4 ] - ! DIVIDE [ shift and go to state 7 ] - -state 10 - - expression -> expression MINUS expression . - expression -> expression . PLUS expression - expression -> expression . MINUS expression - expression -> expression . TIMES expression - expression -> expression . DIVIDE expression - - $ reduce using rule 2 - PLUS reduce using rule 2 - MINUS reduce using rule 2 - RPAREN reduce using rule 2 - TIMES shift and go to state 4 - DIVIDE shift and go to state 7 - - ! TIMES [ reduce using rule 2 ] - ! DIVIDE [ reduce using rule 2 ] - ! PLUS [ shift and go to state 6 ] - ! MINUS [ shift and go to state 5 ] - -state 11 - - expression -> expression PLUS expression . - expression -> expression . PLUS expression - expression -> expression . MINUS expression - expression -> expression . TIMES expression - expression -> expression . DIVIDE expression - - $ reduce using rule 1 - PLUS reduce using rule 1 - MINUS reduce using rule 1 - RPAREN reduce using rule 1 - TIMES shift and go to state 4 - DIVIDE shift and go to state 7 - - ! TIMES [ reduce using rule 1 ] - ! DIVIDE [ reduce using rule 1 ] - ! PLUS [ shift and go to state 6 ] - ! MINUS [ shift and go to state 5 ] - -state 12 - - expression -> expression DIVIDE expression . - expression -> expression . PLUS expression - expression -> expression . MINUS expression - expression -> expression . TIMES expression - expression -> expression . DIVIDE expression - - $ reduce using rule 4 - PLUS reduce using rule 4 - MINUS reduce using rule 4 - TIMES reduce using rule 4 - DIVIDE reduce using rule 4 - RPAREN reduce using rule 4 - - ! PLUS [ shift and go to state 6 ] - ! MINUS [ shift and go to state 5 ] - ! TIMES [ shift and go to state 4 ] - ! DIVIDE [ shift and go to state 7 ] - -state 13 - - expression -> LPAREN expression RPAREN . - - $ reduce using rule 6 - PLUS reduce using rule 6 - MINUS reduce using rule 6 - TIMES reduce using rule 6 - DIVIDE reduce using rule 6 - RPAREN reduce using rule 6 -</pre> -</blockquote> - -In the file, each state of the grammar is described. Within each state the "." indicates the current -location of the parse within any applicable grammar rules. In addition, the actions for each valid -input token are listed. When a shift/reduce or reduce/reduce conflict arises, rules <em>not</em> selected -are prefixed with an !. For example: - -<blockquote> -<pre> - ! TIMES [ reduce using rule 2 ] - ! DIVIDE [ reduce using rule 2 ] - ! PLUS [ shift and go to state 6 ] - ! MINUS [ shift and go to state 5 ] -</pre> -</blockquote> - -By looking at these rules (and with a little practice), you can usually track down the source -of most parsing conflicts. It should also be stressed that not all shift-reduce conflicts are -bad. However, the only way to be sure that they are resolved correctly is to look at <tt>parser.out</tt>. - -<H3><a name="ply_nn29"></a>5.8 Syntax Error Handling</H3> - - -When a syntax error occurs during parsing, the error is immediately -detected (i.e., the parser does not read any more tokens beyond the -source of the error). Error recovery in LR parsers is a delicate -topic that involves ancient rituals and black-magic. The recovery mechanism -provided by <tt>yacc.py</tt> is comparable to Unix yacc so you may want -consult a book like O'Reilly's "Lex and Yacc" for some of the finer details. - -<p> -When a syntax error occurs, <tt>yacc.py</tt> performs the following steps: - -<ol> -<li>On the first occurrence of an error, the user-defined <tt>p_error()</tt> function -is called with the offending token as an argument. Afterwards, the parser enters -an "error-recovery" mode in which it will not make future calls to <tt>p_error()</tt> until it -has successfully shifted at least 3 tokens onto the parsing stack. - -<p> -<li>If no recovery action is taken in <tt>p_error()</tt>, the offending lookahead token is replaced -with a special <tt>error</tt> token. - -<p> -<li>If the offending lookahead token is already set to <tt>error</tt>, the top item of the parsing stack is -deleted. - -<p> -<li>If the entire parsing stack is unwound, the parser enters a restart state and attempts to start -parsing from its initial state. - -<p> -<li>If a grammar rule accepts <tt>error</tt> as a token, it will be -shifted onto the parsing stack. - -<p> -<li>If the top item of the parsing stack is <tt>error</tt>, lookahead tokens will be discarded until the -parser can successfully shift a new symbol or reduce a rule involving <tt>error</tt>. -</ol> - -<H4><a name="ply_nn30"></a>5.8.1 Recovery and resynchronization with error rules</H4> - - -The most well-behaved approach for handling syntax errors is to write grammar rules that include the <tt>error</tt> -token. For example, suppose your language had a grammar rule for a print statement like this: - -<blockquote> -<pre> -def p_statement_print(p): - 'statement : PRINT expr SEMI' - ... -</pre> -</blockquote> - -To account for the possibility of a bad expression, you might write an additional grammar rule like this: - -<blockquote> -<pre> -def p_statement_print_error(p): - 'statement : PRINT error SEMI' - print "Syntax error in print statement. Bad expression" - -</pre> -</blockquote> - -In this case, the <tt>error</tt> token will match any sequence of -tokens that might appear up to the first semicolon that is -encountered. Once the semicolon is reached, the rule will be -invoked and the <tt>error</tt> token will go away. - -<p> -This type of recovery is sometimes known as parser resynchronization. -The <tt>error</tt> token acts as a wildcard for any bad input text and -the token immediately following <tt>error</tt> acts as a -synchronization token. - -<p> -It is important to note that the <tt>error</tt> token usually does not appear as the last token -on the right in an error rule. For example: - -<blockquote> -<pre> -def p_statement_print_error(p): - 'statement : PRINT error' - print "Syntax error in print statement. Bad expression" -</pre> -</blockquote> - -This is because the first bad token encountered will cause the rule to -be reduced--which may make it difficult to recover if more bad tokens -immediately follow. - -<H4><a name="ply_nn31"></a>5.8.2 Panic mode recovery</H4> - - -An alternative error recovery scheme is to enter a panic mode recovery in which tokens are -discarded to a point where the parser might be able to recover in some sensible manner. - -<p> -Panic mode recovery is implemented entirely in the <tt>p_error()</tt> function. For example, this -function starts discarding tokens until it reaches a closing '}'. Then, it restarts the -parser in its initial state. - -<blockquote> -<pre> -def p_error(p): - print "Whoa. You are seriously hosed." - # Read ahead looking for a closing '}' - while 1: - tok = yacc.token() # Get the next token - if not tok or tok.type == 'RBRACE': break - yacc.restart() -</pre> -</blockquote> - -<p> -This function simply discards the bad token and tells the parser that the error was ok. - -<blockquote> -<pre> -def p_error(p): - print "Syntax error at token", p.type - # Just discard the token and tell the parser it's okay. - yacc.errok() -</pre> -</blockquote> - -<P> -Within the <tt>p_error()</tt> function, three functions are available to control the behavior -of the parser: -<p> -<ul> -<li><tt>yacc.errok()</tt>. This resets the parser state so it doesn't think it's in error-recovery -mode. This will prevent an <tt>error</tt> token from being generated and will reset the internal -error counters so that the next syntax error will call <tt>p_error()</tt> again. - -<p> -<li><tt>yacc.token()</tt>. This returns the next token on the input stream. - -<p> -<li><tt>yacc.restart()</tt>. This discards the entire parsing stack and resets the parser -to its initial state. -</ul> - -Note: these functions are only available when invoking <tt>p_error()</tt> and are not available -at any other time. - -<p> -To supply the next lookahead token to the parser, <tt>p_error()</tt> can return a token. This might be -useful if trying to synchronize on special characters. For example: - -<blockquote> -<pre> -def p_error(p): - # Read ahead looking for a terminating ";" - while 1: - tok = yacc.token() # Get the next token - if not tok or tok.type == 'SEMI': break - yacc.errok() - - # Return SEMI to the parser as the next lookahead token - return tok -</pre> -</blockquote> - -<H4><a name="ply_nn32"></a>5.8.3 General comments on error handling</H4> - - -For normal types of languages, error recovery with error rules and resynchronization characters is probably the most reliable -technique. This is because you can instrument the grammar to catch errors at selected places where it is relatively easy -to recover and continue parsing. Panic mode recovery is really only useful in certain specialized applications where you might want -to discard huge portions of the input text to find a valid restart point. - -<H3><a name="ply_nn33"></a>5.9 Line Number and Position Tracking</H3> - - -<tt>yacc.py</tt> automatically tracks line numbers and positions for all of the grammar symbols and tokens it processes. To retrieve the line -numbers, two functions are used in grammar rules: - -<ul> -<li><tt>p.lineno(num)</tt>. Return the starting line number for symbol <em>num</em> -<li><tt>p.linespan(num)</tt>. Return a tuple (startline,endline) with the starting and ending line number 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 - -</pre> -</blockquote> - -Since line numbers are managed internally by the parser, there is usually no need to modify the line -numbers. However, if you want to save the line numbers in a parse-tree node, you will need to make your own -private copy. - -<p> -To get positional information about where tokens were lexed, the following two functions are used: - -<ul> -<li><tt>p.lexpos(num)</tt>. Return the starting lexing position 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.lexpos(1) # Lexing position of the left expression - p.lexpos(2) # Lexing position of the PLUS operator - p.lexpos(3) # Lexing position of the right expression - ... - start,end = p.lexspan(3) # Start,end positions of the right expression -</pre> -</blockquote> - -Note: The <tt>lexspan()</tt> function only returns the range of values up the start of the last grammar symbol. - -<H3><a name="ply_nn34"></a>5.10 AST Construction</H3> - - -<tt>yacc.py</tt> provides no special functions for constructing an abstract syntax tree. However, such -construction is easy enough to do on your own. Simply create a data structure for abstract syntax tree nodes -and assign nodes to <tt>p[0]</tt> in each rule. - -For example: - -<blockquote> -<pre> -class Expr: pass - -class BinOp(Expr): - def __init__(self,left,op,right): - self.type = "binop" - self.left = left - self.right = right - self.op = op - -class Number(Expr): - def __init__(self,value): - self.type = "number" - self.value = value - -def p_expression_binop(p): - '''expression : expression PLUS expression - | expression MINUS expression - | expression TIMES expression - | expression DIVIDE expression''' - - p[0] = BinOp(p[1],p[2],p[3]) - -def p_expression_group(p): - 'expression : LPAREN expression RPAREN' - p[0] = p[2] - -def p_expression_number(p): - 'expression : NUMBER' - p[0] = Number(p[1]) -</pre> -</blockquote> - -To simplify tree traversal, it may make sense to pick a very generic tree structure for your parse tree nodes. -For example: - -<blockquote> -<pre> -class Node: - def __init__(self,type,children=None,leaf=None): - self.type = type - if children: - self.children = children - else: - self.children = [ ] - self.leaf = leaf - -def p_expression_binop(p): - '''expression : expression PLUS expression - | expression MINUS expression - | expression TIMES expression - | expression DIVIDE expression''' - - p[0] = Node("binop", [p[1],p[3]], p[2]) -</pre> -</blockquote> - -<H3><a name="ply_nn35"></a>5.11 Embedded Actions</H3> - - -The parsing technique used by yacc only allows actions to be executed at the end of a rule. For example, -suppose you have a rule like this: - -<blockquote> -<pre> -def p_foo(p): - "foo : A B C D" - print "Parsed a foo", p[1],p[2],p[3],p[4] -</pre> -</blockquote> - -<p> -In this case, the supplied action code only executes after all of the -symbols <tt>A</tt>, <tt>B</tt>, <tt>C</tt>, and <tt>D</tt> have been -parsed. Sometimes, however, it is useful to execute small code -fragments during intermediate stages of parsing. For example, suppose -you wanted to perform some action immediately after <tt>A</tt> has -been parsed. To do this, you can write a empty rule like this: - -<blockquote> -<pre> -def p_foo(p): - "foo : A seen_A B C D" - print "Parsed a foo", p[1],p[3],p[4],p[5] - print "seen_A returned", p[2] - -def p_seen_A(p): - "seen_A :" - print "Saw an A = ", p[-1] # Access grammar symbol to left - p[0] = some_value # Assign value to seen_A - -</pre> -</blockquote> - -<p> -In this example, the empty <tt>seen_A</tt> rule executes immediately -after <tt>A</tt> is shifted onto the parsing stack. Within this -rule, <tt>p[-1]</tt> refers to the symbol on the stack that appears -immediately to the left of the <tt>seen_A</tt> symbol. In this case, -it would be the value of <tt>A</tt> in the <tt>foo</tt> rule -immediately above. Like other rules, a value can be returned from an -embedded action by simply assigning it to <tt>p[0]</tt> - -<p> -The use of embedded actions can sometimes introduce extra shift/reduce conflicts. For example, -this grammar has no conflicts: - -<blockquote> -<pre> -def p_foo(p): - """foo : abcd - | abcx""" - -def p_abcd(p): - "abcd : A B C D" - -def p_abcx(p): - "abcx : A B C X" -</pre> -</blockquote> - -However, if you insert an embedded action into one of the rules like this, - -<blockquote> -<pre> -def p_foo(p): - """foo : abcd - | abcx""" - -def p_abcd(p): - "abcd : A B C D" - -def p_abcx(p): - "abcx : A B seen_AB C X" - -def p_seen_AB(p): - "seen_AB :" -</pre> -</blockquote> - -an extra shift-reduce conflict will be introduced. This conflict is caused by the fact that the same symbol <tt>C</tt> appears next in -both the <tt>abcd</tt> and <tt>abcx</tt> rules. The parser can either shift the symbol (<tt>abcd</tt> rule) or reduce the empty rule <tt>seen_AB</tt> (<tt>abcx</tt> rule). - -<p> -A common use of embedded rules is to control other aspects of parsing -such as scoping of local variables. For example, if you were parsing C code, you might -write code like this: - -<blockquote> -<pre> -def p_statements_block(p): - "statements: LBRACE new_scope statements RBRACE""" - # Action code - ... - pop_scope() # Return to previous scope - -def p_new_scope(p): - "new_scope :" - # Create a new scope for local variables - s = new_scope() - push_scope(s) - ... -</pre> -</blockquote> - -In this case, the embedded action <tt>new_scope</tt> executes immediately after a <tt>LBRACE</tt> (<tt>{</tt>) symbol is parsed. This might -adjust internal symbol tables and other aspects of the parser. Upon completion of the rule <tt>statements_block</tt>, code might undo the operations performed in the embedded action (e.g., <tt>pop_scope()</tt>). - -<H3><a name="ply_nn36"></a>5.12 Yacc implementation notes</H3> - - -<ul> -<li>The default parsing method is LALR. To use SLR instead, run yacc() as follows: - -<blockquote> -<pre> -yacc.yacc(method="SLR") -</pre> -</blockquote> -Note: LALR table generation takes approximately twice as long as SLR table generation. There is no -difference in actual parsing performance---the same code is used in both cases. LALR is preferred when working -with more complicated grammars since it is more powerful. - -<p> - -<li>By default, <tt>yacc.py</tt> relies on <tt>lex.py</tt> for tokenizing. However, an alternative tokenizer -can be supplied as follows: - -<blockquote> -<pre> -yacc.parse(lexer=x) -</pre> -</blockquote> -in this case, <tt>x</tt> must be a Lexer object that minimally has a <tt>x.token()</tt> method for retrieving the next -token. If an input string is given to <tt>yacc.parse()</tt>, the lexer must also have an <tt>x.input()</tt> method. - -<p> -<li>By default, the yacc generates tables in debugging mode (which produces the parser.out file and other output). -To disable this, use - -<blockquote> -<pre> -yacc.yacc(debug=0) -</pre> -</blockquote> - -<p> -<li>To change the name of the <tt>parsetab.py</tt> file, use: - -<blockquote> -<pre> -yacc.yacc(tabmodule="foo") -</pre> -</blockquote> - -<p> -<li>To change the directory in which the <tt>parsetab.py</tt> file (and other output files) are written, use: -<blockquote> -<pre> -yacc.yacc(tabmodule="foo",outputdir="somedirectory") -</pre> -</blockquote> - -<p> -<li>To prevent yacc from generating any kind of parser table file, use: -<blockquote> -<pre> -yacc.yacc(write_tables=0) -</pre> -</blockquote> - -Note: If you disable table generation, yacc() will regenerate the parsing tables -each time it runs (which may take awhile depending on how large your grammar is). - -<P> -<li>To print copious amounts of debugging during parsing, use: - -<blockquote> -<pre> -yacc.parse(debug=1) -</pre> -</blockquote> - -<p> -<li>To redirect the debugging output to a filename of your choosing, use: - -<blockquote> -<pre> -yacc.parse(debug=1, debugfile="debugging.out") -</pre> -</blockquote> - -<p> -<li>The <tt>yacc.yacc()</tt> function really returns a parser object. If you want to support multiple -parsers in the same application, do this: - -<blockquote> -<pre> -p = yacc.yacc() -... -p.parse() -</pre> -</blockquote> - -Note: The function <tt>yacc.parse()</tt> is bound to the last parser that was generated. - -<p> -<li>Since the generation of the LALR tables is relatively expensive, previously generated tables are -cached and reused if possible. The decision to regenerate the tables is determined by taking an MD5 -checksum of all grammar rules and precedence rules. Only in the event of a mismatch are the tables regenerated. - -<p> -It should be noted that table generation is reasonably efficient, even for grammars that involve around a 100 rules -and several hundred states. For more complex languages such as C, table generation may take 30-60 seconds on a slow -machine. Please be patient. - -<p> -<li>Since LR parsing is driven by tables, the performance of the parser is largely independent of the -size of the grammar. The biggest bottlenecks will be the lexer and the complexity of the code in your grammar rules. -</ul> - -<H2><a name="ply_nn37"></a>6. Parser and Lexer State Management</H2> - - -In advanced parsing applications, you may want to have multiple -parsers and lexers. Furthermore, the parser may want to control the -behavior of the lexer in some way. - -<p> -To do this, it is important to note that both the lexer and parser are -actually implemented as objects. These objects are returned by the -<tt>lex()</tt> and <tt>yacc()</tt> functions respectively. For example: - -<blockquote> -<pre> -lexer = lex.lex() # Return lexer object -parser = yacc.yacc() # Return parser object -</pre> -</blockquote> - -To attach the lexer and parser together, make sure you use the <tt>lexer</tt> argumemnt to parse. For example: - -<blockquote> -<pre> -parser.parse(text,lexer=lexer) -</pre> -</blockquote> - -Within lexer and parser rules, these objects are also available. In the lexer, -the "lexer" attribute of a token refers to the lexer object in use. For example: - -<blockquote> -<pre> -def t_NUMBER(t): - r'\d+' - ... - print t.lexer # Show lexer object -</pre> -</blockquote> - -In the parser, the "lexer" and "parser" attributes refer to the lexer -and parser objects respectively. - -<blockquote> -<pre> -def p_expr_plus(p): - 'expr : expr PLUS expr' - ... - print p.parser # Show parser object - print p.lexer # Show lexer object -</pre> -</blockquote> - -If necessary, arbitrary attributes can be attached to the lexer or parser object. -For example, if you wanted to have different parsing modes, you could attach a mode -attribute to the parser object and look at it later. - -<H2><a name="ply_nn38"></a>7. Using Python's Optimized Mode</H2> - - -Because PLY uses information from doc-strings, parsing and lexing -information must be gathered while running the Python interpreter in -normal mode (i.e., not with the -O or -OO options). However, if you -specify optimized mode like this: - -<blockquote> -<pre> -lex.lex(optimize=1) -yacc.yacc(optimize=1) -</pre> -</blockquote> - -then PLY can later be used when Python runs in optimized mode. To make this work, -make sure you first run Python in normal mode. Once the lexing and parsing tables -have been generated the first time, run Python in optimized mode. PLY will use -the tables without the need for doc strings. - -<p> -Beware: running PLY in optimized mode disables a lot of error -checking. You should only do this when your project has stabilized -and you don't need to do any debugging. - -<H2><a name="ply_nn39"></a>8. Where to go from here?</H2> - - -The <tt>examples</tt> directory of the PLY distribution contains several simple examples. Please consult a -compilers textbook for the theory and underlying implementation details or LR parsing. - -</body> -</html> - - - - - - - |