| Author: | Paul McGuire |
|---|---|
| Address: | ptmcg@users.sourceforge.net |
| Revision: | 1.5.1 |
| Date: | October, 2008 |
| Copyright: | Copyright © 2003-2008 Paul McGuire. |
| abstract: | This document provides how-to instructions for the pyparsing library, an easy-to-use Python module for constructing and executing basic text parsers. The pyparsing module is useful for evaluating user-definable expressions, processing custom application language commands, or extracting data from formatted reports. |
|---|
To parse an incoming data string, the client code must follow these steps:
The following complete Python program will parse the greeting "Hello, World!", or any other greeting of the form "<salutation>, <addressee>!":
from pyparsing import Word, alphas greet = Word( alphas ) + "," + Word( alphas ) + "!" greeting = greet.parseString( "Hello, World!" ) print greeting
The parsed tokens are returned in the following form:
['Hello', ',', 'World', '!']
Some of the significant features added in version 1.5.0 are:
originalTextFor helper method, to simplify grammar expressions that need to preserve the original input text; should be used in place of the keepOriginalText parse action:
fullName = Word(alphas) + Word(alphas) fullName.setParseAction(keepOriginalText)
should now be written:
fullName = originalTextFor(Word(alphas) + Word(alphas))
added parameter parseAll to ParserElement.parseFile to match the arguments for ParserElement.parseString
new argument failOn for SkipTo expressions, to define literal strings or expressions that should not be included in the skipped text
Some of the significant features added in version 1.5.0 are:
The pyparsing module can be used to interpret simple command strings or algebraic expressions, or can be used to extract data from text reports with complicated format and structure ("screen or report scraping"). However, it is possible that your defined matching patterns may accept invalid inputs. Use pyparsing to extract data from strings assumed to be well-formatted.
To keep up the readability of your code, use operators such as +, |, ^, and ~ to combine expressions. You can also combine string literals with ParseExpressions - they will be automatically converted to Literal objects. For example:
integer = Word( nums ) # simple unsigned integer variable = Word( alphas, max=1 ) # single letter variable, such as x, z, m, etc. arithOp = Word( "+-*/", max=1 ) # arithmetic operators equation = variable + "=" + integer + arithOp + integer # will match "x=2+2", etc.
In the definition of equation, the string "=" will get added as a Literal("="), but in a more readable way.
The pyparsing module's default behavior is to ignore whitespace. This is the case for 99% of all parsers ever written. This allows you to write simple, clean, grammars, such as the above equation, without having to clutter it up with extraneous ws markers. The equation grammar will successfully parse all of the following statements:
x=2+2 x = 2+2 a = 10 * 4 r= 1234/ 100000
Of course, it is quite simple to extend this example to support more elaborate expressions, with nesting with parentheses, floating point numbers, scientific notation, and named constants (such as e or pi). See fourFn.py, included in the examples directory.
To modify pyparsing's default whitespace skipping, you can use one or more of the following methods:
use the static method ParserElement.setDefaultWhitespaceChars to override the normal set of whitespace chars (' tn'). For instance when defining a grammar in which newlines are significant, you should call ParserElement.setDefaultWhitespaceChars(' \t') to remove newline from the set of skippable whitespace characters. Calling this method will affect all pyparsing expressions defined afterward.
call leaveWhitespace() on individual expressions, to suppress the skipping of whitespace before trying to match the expression
use Combine to require that successive expressions must be adjacent in the input string. For instance, this expression:
real = Word(nums) + '.' + Word(nums)
will match "3.14159", but will also match "3 . 12". It will also return the matched results as ['3', '.', '14159']. By changing this expression to:
real = Combine( Word(nums) + '.' + Word(nums) )
it will not match numbers with embedded spaces, and it will return a single concatenated string '3.14159' as the parsed token.
Repetition of expressions can be indicated using the '*' operator. An expression may be multiplied by an integer value (to indicate an exact repetition count), or by a tuple containing two integers, or None and an integer, representing min and max repetitions (with None representing no min or no max, depending whether it is the first or second tuple element). See the following examples, where n is used to indicate an integer value:
Note that expr*(None,n) does not raise an exception if more than n exprs exist in the input stream; that is, expr*(None,n) does not enforce a maximum number of expr occurrences. If this behavior is desired, then write expr*(None,n) + ~expr.
MatchFirst expressions are matched left-to-right, and the first match found will skip all later expressions within, so be sure to define less-specific patterns after more-specific patterns. If you are not sure which expressions are most specific, use Or expressions (defined using the ^ operator) - they will always match the longest expression, although they are more compute-intensive.
Or expressions will evaluate all of the specified subexpressions to determine which is the "best" match, that is, which matches the longest string in the input data. In case of a tie, the left-most expression in the Or list will win.
If parsing the contents of an entire file, pass it to the parseFile method using:
expr.parseFile( sourceFile )
ParseExceptions will report the location where an expected token or expression failed to match. For example, if we tried to use our "Hello, World!" parser to parse "Hello World!" (leaving out the separating comma), we would get an exception, with the message:
pyparsing.ParseException: Expected "," (6), (1,7)
In the case of complex expressions, the reported location may not be exactly where you would expect. See more information under ParseException .
Use the Group class to enclose logical groups of tokens within a sublist. This will help organize your results into more hierarchical form (the default behavior is to return matching tokens as a flat list of matching input strings).
Punctuation may be significant for matching, but is rarely of much interest in the parsed results. Use the suppress() method to keep these tokens from cluttering up your returned lists of tokens. For example, delimitedList() matches a succession of one or more expressions, separated by delimiters (commas by default), but only returns a list of the actual expressions - the delimiters are used for parsing, but are suppressed from the returned output.
Parse actions can be used to convert values from strings to other data types (ints, floats, booleans, etc.).
Results names are recommended for retrieving tokens from complex expressions. It is much easier to access a token using its field name than using a positional index, especially if the expression contains optional elements. You can also shortcut the setResultsName call:
stats = "AVE:" + realNum.setResultsName("average") + \
"MIN:" + realNum.setResultsName("min") + \
"MAX:" + realNum.setResultsName("max")
can now be written as this:
stats = "AVE:" + realNum("average") + \
"MIN:" + realNum("min") + \
"MAX:" + realNum("max")
Be careful when defining parse actions that modify global variables or data structures (as in fourFn.py), especially for low level tokens or expressions that may occur within an And expression; an early element of an And may match, but the overall expression may fail.
Performance of pyparsing may be slow for complex grammars and/or large input strings. The psyco package can be used to improve the speed of the pyparsing module with no changes to grammar or program logic - observed improvments have been in the 20-50% range.
ParserElement - abstract base class for all pyparsing classes; methods for code to use are:
parseString( sourceString, parseAll=False ) - only called once, on the overall matching pattern; returns a ParseResults object that makes the matched tokens available as a list, and optionally as a dictionary, or as an object with named attributes; if parseAll is set to True, then parseString will raise a ParseException if the grammar does not process the complete input string.
parseFile( sourceFile ) - a convenience function, that accepts an input file object or filename. The file contents are passed as a string to parseString(). parseFile also supports the parseAll argument.
scanString( sourceString ) - generator function, used to find and extract matching text in the given source string; for each matched text, returns a tuple of:
scanString allows you to scan through the input source string for random matches, instead of exhaustively defining the grammar for the entire source text (as would be required with parseString).
transformString( sourceString ) - convenience wrapper function for scanString, to process the input source string, and replace matching text with the tokens returned from parse actions defined in the grammar (see setParseAction).
searchString( sourceString ) - another convenience wrapper function for scanString, returns a list of the matching tokens returned from each call to scanString.
setName( name ) - associate a short descriptive name for this element, useful in displaying exceptions and trace information
setResultsName( string, listAllMatches=False ) - name to be given to tokens matching the element; if multiple tokens within a repetition group (such as ZeroOrMore or delimitedList) the default is to return only the last matching token - if listAllMatches is set to True, then a list of matching tokens is returned. Note: setResultsName returns a copy of the element so that a single basic element can be referenced multiple times and given different names within a complex grammar.
setParseAction( *fn ) - specify one or more functions to call after successful matching of the element; each function is defined as fn( s, loc, toks ), where:
Multiple functions can be attached to a ParserElement by specifying multiple arguments to setParseAction, or by calling setParseAction multiple times.
Each parse action function can return a modified toks list, to perform conversion, or string modifications. For brevity, fn may also be a lambda - here is an example of using a parse action to convert matched integer tokens from strings to integers:
intNumber = Word(nums).setParseAction( lambda s,l,t: [ int(t[0]) ] )
If fn does not modify the toks list, it does not need to return anything at all.
setBreak( breakFlag=True ) - if breakFlag is True, calls pdb.set_break() as this expression is about to be parsed
copy() - returns a copy of a ParserElement; can be used to use the same parse expression in different places in a grammar, with different parse actions attached to each
leaveWhitespace() - change default behavior of skipping whitespace before starting matching (mostly used internally to the pyparsing module, rarely used by client code)
setWhitespaceChars( chars ) - define the set of chars to be ignored as whitespace before trying to match a specific ParserElement, in place of the default set of whitespace (space, tab, newline, and return)
setDefaultWhitespaceChars( chars ) - class-level method to override the default set of whitespace chars for all subsequently created ParserElements (including copies); useful when defining grammars that treat one or more of the default whitespace characters as significant (such as a line-sensitive grammar, to omit newline from the list of ignorable whitespace)
suppress() - convenience function to suppress the output of the given element, instead of wrapping it with a Suppress object.
ignore( expr ) - function to specify parse expression to be ignored while matching defined patterns; can be called repeatedly to specify multiple expressions; useful to specify patterns of comment syntax, for example
setDebug( dbgFlag=True ) - function to enable/disable tracing output when trying to match this element
validate() - function to verify that the defined grammar does not contain infinitely recursive constructs
Word - one or more contiguous characters; construct with a string containing the set of allowed initial characters, and an optional second string of allowed body characters; for instance, a common Word construct is to match a code identifier - in C, a valid identifier must start with an alphabetic character or an underscore ('_'), followed by a body that can also include numeric digits. That is, a, i, MAX_LENGTH, _a1, b_109_, and plan9FromOuterSpace are all valid identifiers; 9b7z, $a, .section, and 0debug are not. To define an identifier using a Word, use either of the following:
- Word( alphas+"_", alphanums+"_" )
- Word( srange("[a-zA-Z_]"), srange("[a-zA-Z0-9_]") )
If only one string given, it specifies that the same character set defined for the initial character is used for the word body; for instance, to define an identifier that can only be composed of capital letters and underscores, use:
- Word( "ABCDEFGHIJKLMNOPQRSTUVWXYZ_" )
- Word( srange("[A-Z_]") )
A Word may also be constructed with any of the following optional parameters:
If exact is specified, it will override any values for min or max.
CharsNotIn - similar to Word, but matches characters not in the given constructor string (accepts only one string for both initial and body characters); also supports min, max, and exact optional parameters.
Regex - a powerful construct, that accepts a regular expression to be matched at the current parse position; accepts an optional flags parameter, corresponding to the flags parameter in the re.compile method; if the expression includes named sub-fields, they will be represented in the returned ParseResults
QuotedString - supports the definition of custom quoted string formats, in addition to pyparsing's built-in dblQuotedString and sglQuotedString. QuotedString allows you to specify the following parameters:
SkipTo - skips ahead in the input string, accepting any characters up to the specified pattern; may be constructed with the following optional parameters:
And - construct with a list of ParserElements, all of which must match for And to match; can also be created using the '+' operator; multiple expressions can be Anded together using the '*' operator as in:
ipAddress = Word(nums) + ('.'+Word(nums))*3
A tuple can be used as the multiplier, indicating a min/max:
usPhoneNumber = Word(nums) + ('-'+Word(nums))*(1,2)
A special form of And is created if the '-' operator is used instead of the '+' operator. In the ipAddress example above, if no trailing '.' and Word(nums) are found after matching the initial Word(nums), then pyparsing will back up in the grammar and try other alternatives to ipAddress. However, if ipAddress is defined as:
strictIpAddress = Word(nums) - ('.'+Word(nums))*3
then no backing up is done. If the first Word(nums) of strictIpAddress is matched, then any mismatch after that will raise a ParseSyntaxException, which will halt the parsing process immediately. By careful use of the '-' operator, grammars can provide meaningful error messages close to the location where the incoming text does not match the specified grammar.
Or - construct with a list of ParserElements, any of which must match for Or to match; if more than one expression matches, the expression that makes the longest match will be used; can also be created using the '^' operator
MatchFirst - construct with a list of ParserElements, any of which must match for MatchFirst to match; matching is done left-to-right, taking the first expression that matches; can also be created using the '|' operator
Each - similar to And, in that all of the provided expressions must match; however, Each permits matching to be done in any order; can also be created using the '&' operator
Optional - construct with a ParserElement, but this element is not required to match; can be constructed with an optional default argument, containing a default string or object to be supplied if the given optional parse element is not found in the input string; parse action will only be called if a match is found, or if a default is specified
ZeroOrMore - similar to Optional, but can be repeated
OneOrMore - similar to ZeroOrMore, but at least one match must be present
FollowedBy - a lookahead expression, requires matching of the given expressions, but does not advance the parsing position within the input string
NotAny - a negative lookahead expression, prevents matching of named expressions, does not advance the parsing position within the input string; can also be created using the unary '~' operator
ParseResults - class used to contain and manage the lists of tokens created from parsing the input using the user-defined parse expression. ParseResults can be accessed in a number of ways:
ParseResults can also be converted to an ordinary list of strings by calling asList(). Note that this will strip the results of any field names that have been defined for any embedded parse elements. (The pprint module is especially good at printing out the nested contents given by asList().)
Finally, ParseResults can be converted to an XML string by calling asXML(). Where possible, results will be tagged using the results names defined for the respective ParseExpressions. asXML() takes two optional arguments:
ParseException - exception returned when a grammar parse fails; ParseExceptions have attributes loc, msg, line, lineno, and column; to view the text line and location where the reported ParseException occurs, use:
except ParseException, err:
print err.line
print " "*(err.column-1) + "^"
print err
RecursiveGrammarException - exception returned by validate() if the grammar contains a recursive infinite loop, such as:
badGrammar = Forward()
goodToken = Literal("A")
badGrammar << Optional(goodToken) + badGrammar
ParseFatalException - exception that parse actions can raise to stop parsing immediately. Should be used when a semantic error is found in the input text, such as a mismatched XML tag.
ParseSyntaxException - subclass of ParseFatalException raised when a syntax error is found, based on the use of the '-' operator when defining a sequence of expressions in an And expression.
You can also get some insights into the parsing logic using diagnostic parse actions, and setDebug(), or test the matching of expression fragments by testing them using scanString().
delimitedList( expr, delim=',') - convenience function for matching one or more occurrences of expr, separated by delim. By default, the delimiters are suppressed, so the returned results contain only the separate list elements. Can optionally specify combine=True, indicating that the expressions and delimiters should be returned as one combined value (useful for scoped variables, such as "a.b.c", or "a::b::c", or paths such as "a/b/c").
countedArray( expr ) - convenience function for a pattern where an list of instances of the given expression are preceded by an integer giving the count of elements in the list. Returns an expression that parses the leading integer, reads exactly that many expressions, and returns the array of expressions in the parse results - the leading integer is suppressed from the results (although it is easily reconstructed by using len on the returned array).
oneOf( string, caseless=False ) - convenience function for quickly declaring an alternative set of Literal tokens, by splitting the given string on whitespace boundaries. The tokens are sorted so that longer matches are attempted first; this ensures that a short token does not mask a longer one that starts with the same characters. If caseless=True, will create an alternative set of CaselessLiteral tokens.
dictOf( key, value ) - convenience function for quickly declaring a dictionary pattern of Dict( ZeroOrMore( Group( key + value ) ) ).
nestedExpr(opener, closer, content, ignoreExpr) - helper method for defining nested lists enclosed in opening and closing delimiters ("(" and ")" are the default).
makeHTMLTags( tagName ) and makeXMLTags( tagName ) - convenience functions to create definitions of opening and closing tag expressions. Returns a pair of expressions, for the corresponding <tag> and </tag> strings. Includes support for attributes in the opening tag, such as <tag attr1="abc"> - attributes are returned as keyed tokens in the returned ParseResults. makeHTMLTags is less restrictive than makeXMLTags, especially with respect to case sensitivity.
operatorPrecedence(baseOperand, operatorList) - convenience function to define a grammar for parsing expressions with a hierarchical precedence of operators. To use the operatorPrecedence helper:
matchPreviousLiteral and matchPreviousExpr - function to define and expression that matches the same content as was parsed in a previous parse expression. For instance:
first = Word(nums) matchExpr = first + ":" + matchPreviousLiteral(first)
will match "1:1", but not "1:2". Since this matches at the literal level, this will also match the leading "1:1" in "1:10".
In contrast:
first = Word(nums) matchExpr = first + ":" + matchPreviousExpr(first)
will not match the leading "1:1" in "1:10"; the expressions are evaluated first, and then compared, so "1" is compared with "10".
nestedExpr(opener, closer, content=None, ignoreExpr=quotedString) - method for defining nested lists enclosed in opening and closing delimiters.
If an expression is not provided for the content argument, the nested expression will capture all whitespace-delimited content between delimiters as a list of separate values.
Use the ignoreExpr argument to define expressions that may contain opening or closing characters that should not be treated as opening or closing characters for nesting, such as quotedString or a comment expression. Specify multiple expressions using an Or or MatchFirst. The default is quotedString, but if no expressions are to be ignored, then pass None for this argument.
indentedBlock( statementExpr, indentationStackVar, indent=True) - function to define an indented block of statements, similar to indentation-based blocking in Python source code:
originalTextFor( expr ) - helper function to preserve the originally parsed text, regardless of any token processing or conversion done by the contained expression. For instance, the following expression:
fullName = Word(alphas) + Word(alphas)
will return the parse of "John Smith" as ['John', 'Smith']. In some applications, the actual name as it was given in the input string is what is desired. To do this, use originalTextFor:
fullName = originalTextFor(Word(alphas) + Word(alphas))
lineno( loc, string ) - function to give the line number of the location within the string; the first line is line 1, newlines start new rows
col( loc, string ) - function to give the column number of the location within the string; the first column is column 1, newlines reset the column number to 1
line( loc, string ) - function to retrieve the line of text representing lineno( loc, string ); useful when printing out diagnostic messages for exceptions
srange( rangeSpec ) - function to define a string of characters, given a string of the form used by regexp string ranges, such as "[0-9]" for all numeric digits, "[A-Z_]" for uppercase characters plus underscore, and so on (note that rangeSpec does not include support for generic regular expressions, just string range specs)
getTokensEndLoc() - function to call from within a parse action to get the ending location for the matched tokens
traceParseAction(fn) - decorator function to debug parse actions. Lists each call, called arguments, and return value or exception
removeQuotes - removes the first and last characters of a quoted string; useful to remove the delimiting quotes from quoted strings
replaceWith(replString) - returns a parse action that simply returns the replString; useful when using transformString, or converting HTML entities, as in:
nbsp = Literal(" ").setParseAction( replaceWith("<BLANK>") )
keepOriginalText- (deprecated, use originalTextFor instead) restores any internal whitespace or suppressed text within the tokens for a matched parse expression. This is especially useful when defining expressions for scanString or transformString applications.
withAttribute( *args, **kwargs ) - helper to create a validating parse action to be used with start tags created with makeXMLTags or makeHTMLTags. Use withAttribute to qualify a starting tag with a required attribute value, to avoid false matches on common tags such as <TD> or <DIV>.
withAttribute can be called with:
An attribute can be specified to have the special value withAttribute.ANY_VALUE, which will match any value - use this to ensure that an attribute is present but any attribute value is acceptable.
downcaseTokens - converts all matched tokens to lowercase
upcaseTokens - converts all matched tokens to uppercase
matchOnlyAtCol( columnNumber ) - a parse action that verifies that an expression was matched at a particular column, raising a ParseException if matching at a different column number; useful when parsing tabular data
alphas - same as string.letters
nums - same as string.digits
alphanums - a string containing alphas + nums
alphas8bit - a string containing alphabetic 8-bit characters:
ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþ
printables - same as string.printable, minus the space (' ') character
empty - a global Empty(); will always match
sglQuotedString - a string of characters enclosed in 's; may include whitespace, but not newlines
dblQuotedString - a string of characters enclosed in "s; may include whitespace, but not newlines
quotedString - sglQuotedString | dblQuotedString
cStyleComment - a comment block delimited by '/*' and '*/' sequences; can span multiple lines, but does not support nesting of comments
htmlComment - a comment block delimited by '<!--' and '-->' sequences; can span multiple lines, but does not support nesting of comments
commaSeparatedList - similar to delimitedList, except that the list expressions can be any text value, or a quoted string; quoted strings can safely include commas without incorrectly breaking the string into two tokens
restOfLine - all remaining printable characters up to but not including the next newline