1   Using the pyparsing module

author:Paul McGuire
address:ptmcg@users.sourceforge.net
revision:3.0.0
date:August, 2020
copyright:Copyright © 2003-2020 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.

Note: While this content is still valid, there are more detailed descriptions and extensive examples at the online doc server, and in the online help for the various pyparsing classes and methods (viewable using the Python interpreter’s built-in help() function). You will also find many example scripts in the examples directory of the pyparsing GitHub repo.

1.1   Steps to follow

To parse an incoming data string, the client code must follow these steps:

  1. First define the tokens and patterns to be matched, and assign this to a program variable. Optional results names or parse actions can also be defined at this time.
  2. Call parseString() or scanString() on this variable, passing in the string to be parsed. During the matching process, whitespace between tokens is skipped by default (although this can be changed). When token matches occur, any defined parse action methods are called.
  3. Process the parsed results, returned as a ParseResults object. The ParseResults object can be accessed as if it were a list of strings. Matching results may also be accessed as named attributes of the returned results, if names are defined in the definition of the token pattern, using setResultsName().

1.1.1   Hello, World!

The following complete Python program will parse the greeting “Hello, World!”, or any other greeting of the form “<salutation>, <addressee>!”:

import pyparsing as pp

greet = pp.Word(pp.alphas) + "," + pp.Word(pp.alphas) + "!"
for greeting_str in [
            "Hello, World!",
            "Bonjour, Monde!",
            "Hola, Mundo!",
            "Hallo, Welt!",
        ]:
    greeting = greet.parseString(greeting_str)
    print(greeting)

The parsed tokens are returned in the following form:

['Hello', ',', 'World', '!']
['Bonjour', ',', 'Monde', '!']
['Hola', ',', 'Mundo', '!']
['Hallo', ',', 'Welt', '!']

1.1.2   Usage notes

  • 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 = Char(alphas)          # single letter variable, such as x, z, m, etc.
    arithOp  = oneOf("+ - * /")      # 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, and simpleArith.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 (' \t\n'). 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 * or [] notation. An expression may be multiplied by an integer value (to indicate an exact repetition count), or indexed with a tuple, representing min and max repetitions (with ... 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:

    • expr*3 is equivalent to expr + expr + expr
    • expr[2, 3] is equivalent to expr + expr + Optional(expr)
    • expr[n, ...] or expr[n,] is equivalent to expr*n + ZeroOrMore(expr) (read as “at least n instances of expr”)
    • expr[... ,n] is equivalent to expr*(0, n) (read as “0 to n instances of expr”)
    • expr[...] and expr[0, ...] are equivalent to ZeroOrMore(expr)
    • expr[1, ...] is equivalent to OneOrMore(expr)

    Note that expr[..., n] does not raise an exception if more than n exprs exist in the input stream; that is, expr[..., n] does not enforce a maximum number of expr occurrences. If this behavior is desired, then write expr[..., 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 more simply and cleanly 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.

1.2   Classes

1.2.1   Classes in the pyparsing module

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:

    • matched tokens (packaged as a ParseResults object)
    • start location of the matched text in the given source string
    • end location in the given source string

    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

  • runTests(testsString) - useful development and testing method on expressions, to pass a multiline string of sample strings to test against the expression. Comment lines (beginning with #) can be inserted and they will be included in the test output:

    digits = Word(nums).setName("numeric digits")
    real_num = Combine(digits + '.' + digits)
    real_num.runTests("""\
        # valid number
        3.14159
    
        # no integer part
        .00001
    
        # no decimal
        101
    
        # no decimal value
        101.
        """)
    

    will print:

    # valid number
    3.14159
    ['3.14159']
    
    # no integer part
    .00001
    ^
    FAIL: Expected numeric digits, found '.'  (at char 0), (line:1, col:1)
    
    # no decimal
    101
       ^
    FAIL: Expected ".", found end of text  (at char 3), (line:1, col:4)
    
    # no decimal value
    101.
        ^
    FAIL: Expected numeric digits, found end of text  (at char 4), (line:1, col:5)
    
  • 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 all the matching tokens is returned.

    expr.setResultsName("key") can also be written expr("key") (a results name with a trailing ‘*’ character will be interpreted as setting listAllMatches to True).

    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:

    • s is the original parse string
    • loc is the location in the string where matching started
    • toks is the list of the matched tokens, packaged as a ParseResults object

    Parse actions can have any of the following signatures:

    fn(s, loc, tokens)
    fn(loc, tokens)
    fn(tokens)
    fn()
    

    Multiple functions can be attached to a ParserElement by specifying multiple arguments to setParseAction, or by calling addParseAction. Calls to setParseAction will replace any previously defined parse actions. setParseAction(None) will clear any previously defined parse action.

    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 modifies the toks list in-place, it does not need to return and pyparsing will use the modified toks list.

  • addParseAction - similar to setParseAction, but instead of replacing any previously defined parse actions, will append the given action or actions to the existing defined parse actions.

  • addCondition - a simplified form of addParseAction if the purpose of the parse action is to simply do some validation, and raise an exception if the validation fails. Takes a method that takes the same arguments, but simply returns True or False. If False is returned, an exception will be raised.

  • 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; a short-form expr() is equivalent to expr.copy()

  • 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

  • parseWithTabs() - function to override default behavior of converting tabs to spaces before parsing the input string; rarely used, except when specifying whitespace-significant grammars using the White class.
  • enablePackrat() - a class-level static method to enable a memoizing performance enhancement, known as “packrat parsing”. packrat parsing is disabled by default, since it may conflict with some user programs that use parse actions. To activate the packrat feature, your program must call the class method ParserElement.enablePackrat(). For best results, call enablePackrat() immediately after importing pyparsing.

1.2.2   Basic ParserElement subclasses

  • Literal - construct with a string to be matched exactly
  • CaselessLiteral - construct with a string to be matched, but without case checking; results are always returned as the defining literal, NOT as they are found in the input string
  • Keyword - similar to Literal, but must be immediately followed by whitespace, punctuation, or other non-keyword characters; prevents accidental matching of a non-keyword that happens to begin with a defined keyword
  • CaselessKeyword - similar to Keyword, but with caseless matching behavior
  • 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:

    • min - indicating a minimum length of matching characters
    • max - indicating a maximum length of matching characters
    • exact - indicating an exact length of matching characters

    If exact is specified, it will override any values for min or max.

    Sometimes you want to define a word using all characters in a range except for one or two of them; you can do this with the new excludeChars argument. This is helpful if you want to define a word with all printables except for a single delimiter character, such as ‘.’. Previously, you would have to create a custom string to pass to Word. With this change, you can just create Word(printables, excludeChars='.').

  • Char - a convenience form of Word that will match just a single character from a string of matching characters:

    single_digit = Char(nums)
    
  • 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:

    • quoteChar - string of one or more characters defining the quote delimiting string
    • escChar - character to escape quotes, typically backslash (default=None)
    • escQuote - special quote sequence to escape an embedded quote string (such as SQL’s “” to escape an embedded “) (default=None)
    • multiline - boolean indicating whether quotes can span multiple lines (default=False)
    • unquoteResults - boolean indicating whether the matched text should be unquoted (default=True)
    • endQuoteChar - string of one or more characters defining the end of the quote delimited string (default=None => same as quoteChar)
  • SkipTo - skips ahead in the input string, accepting any characters up to the specified pattern; may be constructed with the following optional parameters:

    • include - if set to true, also consumes the match expression (default is false)
    • ignore - allows the user to specify patterns to not be matched, to prevent false matches
    • failOn - if a literal string or expression is given for this argument, it defines an expression that should cause the SkipTo expression to fail, and not skip over that expression

    SkipTo can also be written using ...:

    LBRACE, RBRACE = map(Literal, "{}")
    brace_expr = LBRACE + SkipTo(RBRACE) + RBRACE
    # can also be written as
    brace_expr = LBRACE + ... + RBRACE
    
  • White - also similar to Word, but matches whitespace characters. Not usually needed, as whitespace is implicitly ignored by pyparsing. However, some grammars are whitespace-sensitive, such as those that use leading tabs or spaces to indicating grouping or hierarchy. (If matching on tab characters, be sure to call parseWithTabs on the top-level parse element.)
  • Empty - a null expression, requiring no characters - will always match; useful for debugging and for specialized grammars
  • NoMatch - opposite of Empty, will never match; useful for debugging and for specialized grammars

1.2.3   Expression subclasses

  • 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; ZeroOrMore(expr) can also be written as expr[...].

  • OneOrMore - similar to ZeroOrMore, but at least one match must be present; OneOrMore(expr) can also be written as expr[1, ...].

  • 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

1.2.4   Expression operators

  • ~ - creates NotAny using the expression after the operator
  • + - creates And using the expressions before and after the operator
  • | - creates MatchFirst (first left-to-right match) using the expressions before and after the operator
  • ^ - creates Or (longest match) using the expressions before and after the operator
  • & - creates Each using the expressions before and after the operator
  • * - creates And by multiplying the expression by the integer operand; if expression is multiplied by a 2-tuple, creates an And of (min,max) expressions (similar to “{min,max}” form in regular expressions); if min is None, intepret as (0,max); if max is None, interpret as expr*min + ZeroOrMore(expr)
  • - - like + but with no backup and retry of alternatives
  • == - matching expression to string; returns True if the string matches the given expression
  • <<= - inserts the expression following the operator as the body of the Forward expression before the operator (<< can also be used, but <<= is preferred to avoid operator precedence misinterpretation of the pyparsing expression)

1.2.5   Positional subclasses

  • StringStart - matches beginning of the text
  • StringEnd - matches the end of the text
  • LineStart - matches beginning of a line (lines delimited by \n characters)
  • LineEnd - matches the end of a line
  • WordStart - matches a leading word boundary
  • WordEnd - matches a trailing word boundary

1.2.6   Converter subclasses

  • Combine - joins all matched tokens into a single string, using specified joinString (default joinString=""); expects all matching tokens to be adjacent, with no intervening whitespace (can be overridden by specifying adjacent=False in constructor)
  • Suppress - clears matched tokens; useful to keep returned results from being cluttered with required but uninteresting tokens (such as list delimiters)

1.2.7   Special subclasses

  • Group - causes the matched tokens to be enclosed in a list; useful in repeated elements like ZeroOrMore and OneOrMore to break up matched tokens into groups for each repeated pattern
  • Dict - like Group, but also constructs a dictionary, using the [0]’th elements of all enclosed token lists as the keys, and each token list as the value
  • SkipTo - catch-all matching expression that accepts all characters up until the given pattern is found to match; useful for specifying incomplete grammars
  • Forward - placeholder token used to define recursive token patterns; when defining the actual expression later in the program, insert it into the Forward object using the <<= operator (see fourFn.py for an example).

1.2.8   Other classes

  • 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:

    • as a list
      • total list of elements can be found using len()
      • individual elements can be found using [0], [1], [-1], etc., or retrieved using slices
      • elements can be deleted using del
      • the -1th element can be extracted and removed in a single operation using pop(), or any element can be extracted and removed using pop(n)
    • as a dictionary
      • if setResultsName() is used to name elements within the overall parse expression, then these fields can be referenced as dictionary elements or as attributes
      • the Dict class generates dictionary entries using the data of the input text - in addition to ParseResults listed as [ [ a1, b1, c1, ...], [ a2, b2, c2, ...]  ] it also acts as a dictionary with entries defined as { a1 : [ b1, c1, ... ] }, { a2 : [ b2, c2, ... ] }; this is especially useful when processing tabular data where the first column contains a key value for that line of data
      • list elements that are deleted using del will still be accessible by their dictionary keys
      • supports get(), items() and keys() methods, similar to a dictionary
      • a keyed item can be extracted and removed using pop(key). Here key must be non-numeric (such as a string), in order to use dict extraction instead of list extraction.
      • new named elements can be added (in a parse action, for instance), using the same syntax as adding an item to a dict (parseResults["X"] = "new item"); named elements can be removed using del parseResults["X"]
    • as a nested list
      • results returned from the Group class are encapsulated within their own list structure, so that the tokens can be handled as a hierarchical tree

    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 viewed by calling dump(). dump()` will first show the ``asList() output, followed by an indented structure listing parsed tokens that have been assigned results names.

1.2.9   Exception classes and Troubleshooting

  • 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 as 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 searchString() or scanString().

  • Diagnostics can be enabled using pyparsing.enable_diag and passing one of the following enum values defined in pyparsing.Diagnostics

    • warn_multiple_tokens_in_named_alternation - flag to enable warnings when a results name is defined on a MatchFirst or Or expression with one or more And subexpressions
    • warn_ungrouped_named_tokens_in_collection - flag to enable warnings when a results name is defined on a containing expression with ungrouped subexpressions that also have results names
    • warn_name_set_on_empty_Forward - flag to enable warnings when a Forward is defined with a results name, but has no contents defined
    • warn_on_parse_using_empty_Forward - flag to enable warnings when a Forward is defined in a grammar but has never had an expression attached to it
    • warn_on_assignment_to_Forward - flag to enable warnings when a Forward is defined but is overwritten by assigning using '=' instead of '<<=' or '<<'
    • warn_on_multiple_string_args_to_oneof - flag to enable warnings when oneOf is incorrectly called with multiple str arguments
    • enable_debug_on_named_expressions - flag to auto-enable debug on all subsequent calls to ParserElement.setName

    All warnings can be enabled by calling pyparsing.enable_all_warnings(). Sample:

    import pyparsing as pp
    pp.enable_all_warnings()
    
    fwd = pp.Forward().setResultsName("recursive_expr")
    
    >>> UserWarning: warn_name_set_on_empty_Forward: setting results name 'recursive_expr'
                     on Forward expression that has no contained expression
    

1.3   Miscellaneous attributes and methods

1.3.1   Helper methods

  • 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))).

  • 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.

  • infixNotation(baseOperand, operatorList) - convenience function to define a grammar for parsing infix notation expressions with a hierarchical precedence of operators. To use the infixNotation helper:

    1. Define the base “atom” operand term of the grammar. For this simple grammar, the smallest operand is either an integer or a variable. This will be the first argument to the infixNotation method.
    2. Define a list of tuples for each level of operator precendence. Each tuple is of the form (opExpr, numTerms, rightLeftAssoc, parseAction), where:
      • opExpr - the pyparsing expression for the operator; may also be a string, which will be converted to a Literal; if None, indicates an empty operator, such as the implied multiplication operation between ‘m’ and ‘x’ in “y = mx + b”.
      • numTerms - the number of terms for this operator (must be 1, 2, or 3)
      • rightLeftAssoc is the indicator whether the operator is right or left associative, using the pyparsing-defined constants opAssoc.RIGHT and opAssoc.LEFT.
      • parseAction is the parse action to be associated with expressions matching this operator expression (the parseAction tuple member may be omitted)
    3. Call infixNotation passing the operand expression and the operator precedence list, and save the returned value as the generated pyparsing expression. You can then use this expression to parse input strings, or incorporate it into a larger, more complex grammar.
  • 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.

    • opener - opening character for a nested list (default=”(“); can also be a pyparsing expression
    • closer - closing character for a nested list (default=”)”); can also be a pyparsing expression
    • content - expression for items within the nested lists (default=None)
    • ignoreExpr - expression for ignoring opening and closing delimiters (default=quotedString)

    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:

    • statementExpr - the expression defining a statement that will be found in the indented block; a valid indentedBlock must contain at least 1 matching statementExpr
    • indentationStackVar - a Python list variable; this variable should be common to all indentedBlock expressions defined within the same grammar, and should be reinitialized to [1] each time the grammar is to be used
    • indent - a boolean flag indicating whether the expressions within the block must be indented from the current parse location; if using indentedBlock to define the left-most statements (all starting in column 1), set indent to False
  • 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))
    
  • ungroup(expr) - function to “ungroup” returned tokens; useful to undo the default behavior of And to always group the returned tokens, even if there is only one in the list. (New in 1.5.6)

  • 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)

  • traceParseAction(fn) - decorator function to debug parse actions. Lists each call, called arguments, and return value or exception

1.3.2   Helper parse actions

  • 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("&nbsp;").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:

    • keyword arguments, as in (class="Customer", align="right"), or
    • a list of name-value tuples, as in (("ns1:class", "Customer"), ("ns2:align", "right"))

    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

1.3.3   Common string and token constants

  • 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

1.4   Generating Railroad Diagrams

Grammars are conventionally represented in what are called “railroad diagrams”, which allow you to visually follow the sequence of tokens in a grammar along lines which are a bit like train tracks. You might want to generate a railroad diagram for your grammar in order to better understand it yourself, or maybe to communicate it to others.

1.4.1   Usage

To generate a railroad diagram in pyparsing, you first have to install pyparsing with the diagrams extra. To do this, just run pip install pyparsing[diagrams], and make sure you add pyparsing[diagrams] to any setup.py or requirements.txt that specifies pyparsing as a dependency.

Next, run pyparsing.diagrams.to_railroad to convert your grammar into a form understood by the railroad-diagrams module, and then pyparsing.diagrams.railroad_to_html to convert that into an HTML document. For example:

from pyparsing.diagram import to_railroad, railroad_to_html

with open('output.html', 'w') as fp:
    railroad = to_railroad(my_grammar)
    fp.write(railroad_to_html(railroad))

This will result in the railroad diagram being written to output.html

1.4.2   Example

You can view an example railroad diagram generated from a pyparsing grammar for SQL SELECT statements.

1.4.3   Customization

You can customize the resulting diagram in a few ways.

Firstly, you can pass in additional keyword arguments to pyparsing.diagrams.to_railroad, which will be passed into the Diagram() constructor of the underlying library, as explained here.

Secondly, you can edit global options in the underlying library, by editing constants:

from pyparsing.diagram import to_railroad, railroad_to_html
import railroad

railroad.DIAGRAM_CLASS = "my-custom-class"
my_railroad = to_railroad(my_grammar)

These options are documented here.

Finally, you can edit the HTML produced by pyparsing.diagrams.railroad_to_html by passing in certain keyword arguments that will be used in the HTML template. Currently, these are:

  • head: A string containing HTML to use in the <head> tag. This might be a stylesheet or other metadata
  • body: A string containing HTML to use in the <body> tag, above the actual diagram. This might consist of a heading, description, or JavaScript.

If you want to provide a custom stylesheet using the head keyword, you can make use of the following CSS classes:

  • railroad-group: A group containing everything relating to a given element group (ie something with a heading)
  • railroad-heading: The title for each group
  • railroad-svg: A div containing only the diagram SVG for each group
  • railroad-description: A div containing the group description (unused)