Re: [Pyparsing] Forward() causing odd behaviour

[Ralph C. <ra...@in...>]

Hi Cathal,

> I have found something very weird. When I type the below into my
> python console I get the following error: ParseException: Expected ")"
> (at char 3), (line:1, col:4)
> 
> from pyparsing import *
> function = Forward()
> function = '#' + Word(nums) + Group(Literal('(') + Optional(function)) + Literal(')')
> function.parseString('#0(#1())', True)

I get that too, and it seems correct.

> However, if I then re-enter everything accept the line: function =
> Forward() it works as I initially expected it to.
> 
> from pyparsing import *
> function = '#' + Word(nums) + Group(Literal('(') + Optional(function)) + Literal(')')
> function.parseString('#0(#1())', True)
> Out[6]: (['#', '0', (['(', '#', '1', (['('], {}), ')'], {}), ')'], {})

I get

    >>> from pyparsing import *
    >>> function = '#' + Word(nums) + Group(Literal('(') + Optional(function)) + Literal(')')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'function' is not defined
    >>> 

which again seems correct.  It seems you're entering these lines into
the same Python interpreter in which you've already defined function.
IOW, your two tests are not isolated;  the second is altered by the
leftovers of the first.

Again, printing function in both cases may have given a clue.  First
time,

    >>> print function
    {"#" W:(0123...) Group:({"(" [Forward: None]}) ")"}

second time,

    >>> print function
    {"#" W:(0123...) Group:({"(" [{"#" W:(0123...) Group:({"(" [Forward: None]}) ")"}]}) ")"}

So the second one still isn't right:

    >>> function.parseString('#0(#1(#2()))', True)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/pymodules/python2.6/pyparsing.py", line 1076, in parseString
        raise exc
    pyparsing.ParseException: Expected ")" (at char 3), (line:1, col:4)
    >>> 

You haven't got a recursive grammar, just one two levels deep.

> How do I make this work without having to enter the first sequence and
> then the second sequence of modified statements?

    from pyparsing import *
    f = Forward()
    function = '#' + Word(nums) + Group(Literal('(') + Optional(f)) + Literal(')')
    f << function
    function.parseString('#0(#1())', True)

You must keep a reference to the Forward() so that you can "fill it in"
later.  You can't do this is you through it away by immediately
re-assigning to function, hence my use of f.  Note, the Forward is no
longer None.

    >>> print function
    {"#" W:(0123...) Group:({"(" [Forward: {"#" W:(0123...) Group:({"(" [...]}) ")"}]}) ")"}

Cheers, Ralph.