Re: [Pyparsing] White space

[Paul M. <pt...@au...>]

Eike -

Good point!  It isn't necessary to make '$' the Literal expression, it is
just as good (and if using Keyword, probably more appropriate) to make 'as'
the expression using the Keyword class.

-- Paul


-----Original Message-----
From: Eike Welk [mailto:eik...@gm...] 
Sent: Tuesday, June 21, 2011 7:24 PM
To: pyp...@li...
Subject: Re: [Pyparsing] White space

Hello Cathal!

I'm trying to answer your question without testing anything. So it might be 
completely wrong. 

On Wednesday 22.06.2011 00:34:49 cathal coffey wrote:
> Hello,
> 
> I have a quick question. Shouldn't the below grammar accept the given
> string?
> 
> from pyparsing import Word, alphas
> g = "$" + "as" + Word(alphas)
> g.parseString('$ as abc')
> ParseException: Expected "$as" (at char 0), (line:1, col:1)

It the order with which the ``+`` operators are applied in the first 
statement, that fools you.

The left ``+`` operator is applied first so that two strings are added::

	"$" + "as" 

Therefor your first statement is equivalent to::

	g = "$as" + Word(alphas)

You get the desired result with::

	g = Literal("$") + "as" + Word(alphas)

Look also at ``Keyword``. You probably want::

	from pyparsing import Word, alphas, Keyword
	Kw = Keyword
	g = "$" + Kw("as") + Word(alphas)

The string ``"$"`` is now implicitly converted to ``Literal("$")`` because
to 
apply the left ``+`` operator, Python calls the method ``Keyword.__radd__``.

This method converts strings to literals. For the behavior of Python's 
operators look at the official (but somewhat cryptic) explanation:

http://docs.python.org/reference/datamodel.html#emulating-numeric-types
 
> 
> I want the grammar to accept $ followed by: any number of spaces
> followed by: as followed by: any number of spaces followed by: any
> string of characters.


HTH,
Eike.


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