Module pyparsing :: Class Forward |
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# Class Forward

source code

         object --+
                  |
      ParserElement --+
                      |
    ParseElementEnhance --+
                          |
                         Forward

Known Subclasses:
  - _ForwardNoRecurse

Forward declaration of an expression to be defined later - used for recursive grammars, such as algebraic infix notation. When the expression is known, it is assigned to the `Forward` variable using the '<<' operator.

Note: take care when assigning to `Forward` not to overlook precedence of operators. Specifically, '|' has a lower precedence than '<<', so that:

       fwdExpr << a | b | c

will actually be evaluated as:

       (fwdExpr << a) | b | c

thereby leaving b and c out as parseable alternatives. It is recommended that you explicitly group the values inserted into the `Forward`:

       fwdExpr << (a | b | c)

Converting to use the '<<=' operator instead will avoid this problem.

See ParseResults.pprint for an example of a recursive parser created using `Forward`.

Instance Methods
__init__(self, other=None) x.__init__(...) initializes x; see help(type(x)) for signature | source code
__lshift__(self, other) | source code
__ilshift__(self, other) | source code
leaveWhitespace(self) Disables the skipping of whitespace before matching the characters in the `ParserElement`'s defined pattern. | source code
streamline(self) | source code
validate(self, validateTrace=`[``]`) Check defined expressions for valid structure, check for infinite recursive definitions. | source code
__str__(self) str(x) | source code
copy(self) Make a copy of this `ParserElement`. | source code
Inherited from `ParseElementEnhance`: `checkRecursion`, `ignore`, `parseImpl` Inherited from `ParserElement`: `__add__`, `__and__`, `__call__`, `__eq__`, `__hash__`, `__invert__`, `__mul__`, `__ne__`, `__or__`, `__radd__`, `__rand__`, `__repr__`, `__req__`, `__rmul__`, `__rne__`, `__ror__`, `__rsub__`, `__rxor__`, `__sub__`, `__xor__`, `addCondition`, `addParseAction`, `canParseNext`, `matches`, `parseFile`, `parseString`, `parseWithTabs`, `postParse`, `preParse`, `runTests`, `scanString`, `searchString`, `setBreak`, `setDebug`, `setDebugActions`, `setFailAction`, `setName`, `setParseAction`, `setResultsName`, `setWhitespaceChars`, `split`, `suppress`, `transformString`, `tryParse` Inherited from `object`: `__delattr__`, `__format__`, `__getattribute__`, `__new__`, `__reduce__`, `__reduce_ex__`, `__setattr__`, `__sizeof__`, `__subclasshook__`

Static Methods
Inherited from `ParserElement`: `enablePackrat`, `inlineLiteralsUsing`, `resetCache`, `setDefaultWhitespaceChars`

Class Variables
Inherited from `ParserElement`: `DEFAULT_WHITE_CHARS`, `packrat_cache`, `packrat_cache_lock`, `packrat_cache_stats`, `verbose_stacktrace`

Properties
Inherited from `object`: `__class__`

Method Details

### __init__(self, other=None) (Constructor) | source code
x.__init__(...) initializes x; see help(type(x)) for signature Overrides: object.__init__ (inherited documentation)

### leaveWhitespace(self) | source code
Disables the skipping of whitespace before matching the characters in the `ParserElement`'s defined pattern. This is normally only used internally by the pyparsing module, but may be needed in some whitespace-sensitive grammars. Overrides: ParserElement.leaveWhitespace (inherited documentation)

### streamline(self) | source code
Overrides: ParserElement.streamline

### validate(self, validateTrace=`[``]`) | source code
Check defined expressions for valid structure, check for infinite recursive definitions. Overrides: ParserElement.validate (inherited documentation)

### __str__(self) (Informal representation operator) | source code
str(x) Overrides: object.__str__ (inherited documentation)

### copy(self) | source code
Make a copy of this `ParserElement`. Useful for defining different parse actions for the same parsing pattern, using copies of the original parse element. Example:
       integer = Word(nums).setParseAction(lambda toks: int(toks[0]))
       integerK = integer.copy().addParseAction(lambda toks: toks[0]*1024) + Suppress("K")
       integerM = integer.copy().addParseAction(lambda toks: toks[0]*1024*1024) + Suppress("M")

       print(OneOrMore(integerK | integerM | integer).parseString("5K 100 640K 256M"))
prints:
       [5120, 100, 655360, 268435456]
Equivalent form of `expr.copy()` is just `expr()`:
       integerM = integer().addParseAction(lambda toks: toks[0]*1024*1024) + Suppress("M")
Overrides: ParserElement.copy (inherited documentation)