Re: [Pyparsing] Supporting Spelling Variations

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

"And if you have a priori knowledge of which variation will be more common,
put it first in the list." 

Oh, damn, that came out wrong.

There is a caveat with using MatchFirst that some alternatives *must* be
placed in a particular order.  I learned this when I created the simple IDL
parser, and had to specify an expression for the 3 parameter passing
keywords, "in", "out", and "inout".  I did it like this:

paramPassingMechanism = Literal("in") | "out" | "inout"

(The '|' operator generates MatchFirst.)

Unfortunately, when actually parsing an inout parameter, the MatchFirst will
match the leading "in", and "inout" will never match.  This is a big part of
the reason for the oneOf helper method.  oneOf reorders the input strings as
necessary to avoid these kinds of masking collisions.  So I changed my
definition to:

paramPassingMechanism = oneOf("in out inout")

oneOf splits the input string, and then reorders the options to match the
long of two overlapping alternatives.  If you print this expression out from
the Python prompt, you get:

Re:('inout|in|out')

showing that pyparsing has created a Regex, and reordered 'inout' to be
ahead of 'in'.

So to avoid inadvertent masking of overlapping alternatives, I recommend
using oneOf over explicit MatchFirst's.

-- Paul



 
-----Original Message-----
From: pyp...@li...
[mailto:pyp...@li...] On Behalf Of Paul
McGuire
Sent: Monday, September 10, 2007 8:51 AM
To: 'Tim Cook'; 'PyParsing List'
Subject: Re: [Pyparsing] Supporting Spelling Variations

Tim -

Avoid Or in favor of MatchFirst when you have unambiguous alternatives like
this.  And if you have a priori knowledge of which variation will be more
common, put it first in the list.

Here are a couple of options:
- Suppress( oneOf("specialize specialise") )
- MatchFirst( map(Suppress,["specialize","specialise"]) )
- Regex("speciali[sz]e").suppress()

I've not had a chance to test any of these for performance, but they should
be equivalent functionally.

-- Paul



-----Original Message-----
From: pyp...@li...
[mailto:pyp...@li...] On Behalf Of Tim Cook
Sent: Monday, September 10, 2007 3:29 AM
To: PyParsing List
Subject: [Pyparsing] Supporting Spelling Variations

Hi All,

I need to support both 'specialise' and 'specialize'  (actually there are
many British/Australian/US spelling variations like this) 

My first attempts were to use either an Or list or a MatchFirst list.

i.e. Suppress("specialise" Or "specialize")

Each of those raised: TypeError: unsupported operand type(s) for ^:
'str' and 'str'

My solution (so far) is:
specialise = Suppress("specialise")
specialize = Suppress("specialize")
specialiseSection = (specialise | specialize + ...

Is there a better/more efficient approach?

Cheers,
Tim



--
Timothy Cook, MSc
Health Informatics Research & Development Services
http://timothywayne.cook.googlepages.com/home
01-904-322-8582


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