Re: [Pyparsing] A PyParsing hack

[Paul M. <pa...@al...>]

Greg - 

Bwa-ha-ha! Another minion in my evil domain!!! :)  I'm very happy that you
find pyparsing so useful AND enjoyable.

Your concept of passing a class instead of a function to setParseAction
looks pretty good.  Your point is dead-on - why bother returning just a
nested list of strings when you can use parse actions to construct and
return executable objects?  It is an excellent combination of both lexing
and parsing.  And extra credit for using parse actions as part of an
operatorPrecedence expression.

Please don't be disappointed, but yes, pyparsing *can* do this already.  In
fact, the code you posted/pasted, if you just change PP.NoStringBase to
object as the base class for your various parse action/classes, will run
otherwise unchanged with the unpatched pyparsing running on my system.  I
get this result:

[<Assignment 
	<Token 'X'> = <Math [<Token 'Y'>, 
				   '+', 
				   <Math [5, '*', <Token 'Z'>]
                           >]
                    >
>]

For another example of this technique, see:
http://pyparsing.wikispaces.com/space/showimage/simpleBool.py.  In addition
to being online at the pyparsing wiki, this script is included in the
examples directory of the source and doc distibutions.

In "Getting Started With Pyparsing"
(http://www.oreilly.com/catalog/9780596514235), the final chapter shows how
to convert query string expressions into objects that generate Python set
expressions, which can then be intersect'ed and union'ed to correspond to
the and's and or's of the original query string.  And I have a new magazine
article coming out soon that uses this method to compile an esoteric
language to executable objects.

Congratulations on arriving at this powerful concept on your own, it is most
definitely an advanced technique!

And keep proposing patches!  There are now many more brains than just my own
thinking about this module and how it could be improved, and I would be
foolish to limit new work to only the stuff I come up with myself.  Many of
the recent enhancements came as a result of discussions and patches
submitted over discussion lists and forums.  I'm glad that you are not
afraid to get your fingernails dirty, and get right into the pyparsing code
to see what it is doing.

-- Paul

(I'm sorry about unitTests - some of the files that I parse in my testing
are proprietary to some of my business customers, so I can't post them.
I'll try to update the SVN with what is publicly shareable, and
conditionalize the unitTests.py script to step over the private tests.)