Python parsing module

Here's your definition of g
>>> g = "$" + "as" + Word(alphas)
>>> print g
{"$as" W:(abcd...)}

"$" + "as" gets interpreted by Python as the addition of two strings,
resulting in "$as".

Force the leading "$" to be a pyparsing Literal, and then you'll get a
better parser:
>>> g = Literal("$") + "as" + Word(alphas)
>>> print g
{{"$" "as"} W:(abcd...)}

-- Paul


-----Original Message-----
From: cathal coffey [mailto:coffey.cathal@...] 
Sent: Tuesday, June 21, 2011 5:35 PM
To: pyparsing-users@...
Subject: [Pyparsing] White space

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)

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.

The reason I am confused is that the following grammar accepts
regardless of white space... why doesn't the above?

g = "as" + Word(alphas)
g.parseString('  as abc    ')
Out: (['as', 'abc'], {})

Kind regards,
Cathal

----------------------------------------------------------------------------
--
EditLive Enterprise is the world's most technically advanced content
authoring tool. Experience the power of Track Changes, Inline Image
Editing and ensure content is compliant with Accessibility Checking.
http://p.sf.net/sfu/ephox-dev2dev
_______________________________________________
Pyparsing-users mailing list
Pyparsing-users@...
https://lists.sourceforge.net/lists/listinfo/pyparsing-users

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.

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:eike.welk@...] 
Sent: Tuesday, June 21, 2011 7:24 PM
To: pyparsing-users@...
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.


----------------------------------------------------------------------------
--
EditLive Enterprise is the world's most technically advanced content
authoring tool. Experience the power of Track Changes, Inline Image
Editing and ensure content is compliant with Accessibility Checking.
http://p.sf.net/sfu/ephox-dev2dev
_______________________________________________
Pyparsing-users mailing list
Pyparsing-users@...
https://lists.sourceforge.net/lists/listinfo/pyparsing-users

Hi,

Paul McGuire wrote:
> > from pyparsing import Word, alphas
> > g = "$" + "as" + Word(alphas)
> > g.parseString('$ as abc')
> > ParseException: Expected "$as" (at char 0), (line:1, col:1)
>
> Here's your definition of g
> >>> g = "$" + "as" + Word(alphas)
> >>> print g
> {"$as" W:(abcd...)}
>
> "$" + "as" gets interpreted by Python as the addition of two strings,
> resulting in "$as".

I guess yet another way, keyword issue aside, is to alter the
associativity with parenthesis?

    >>> g = "$" + ("as" + Word(alphas))
    >>> print g
    {"$" {"as" W:(abcd...)}}
    >>> g.parseString('$ as abc')
    (['$', 'as', 'abc'], {})
    >>>

Cheers, Ralph.