Re: [Pyparsing] Lists and Groups

[Michael F. <nob...@gm...>]

Thanks!

I've confirmed it works, even after suppressing 'get' and 'AT', and grouping
the shopping list.

--Michael

On Sat, Nov 13, 2010 at 8:25 PM, Paul McGuire <pt...@au...> wrote:

> I don't think you've gone overboard here.  Your BNF *will* be somewhat
> informal, but don't give up on it.
>
> Your basic form is:
>
> (get) (stuff) (at) (location)
>
> Given that you have some entries where you just get stuff, or specify a
> location just based on a leading '@' symbol, this gets a little more
> complex, (using []'s for optional parts:
>
> (get) (stuff) [[(at)] (location)]
>
> (stuff) and (location) are going to be pretty unstructured, but
> fortunately,
> you're defining some specific forms for (get) and (at).
>
> Your (stuff) can contain a list of items separated by commas, (and), or (,
> and), so I think you can define it as pretty open for (item), and then
> define a delimited list for the list of items.  You'll need to specify the
> lookahead (as you already picked up in your posted pyparsing code) to avoid
> parsing "at" or "and" or a store label with a leading '@' as an item word.
> And using Keyword's for your delimiting words is a good choice, to guard
> against accidentally reading the leading 'at' in 'athletic socks' as 'at',
> and the remaining 'hletic socks' as some kind of store.
>
> So I'd informally write this as:
>
> shopping-list ::= get stuff [[at] location]
> get ::= "get" | "buy" | "pick up"
> at ::= "at" | "from"
> item ::= (~(and | at | '@') Word(alphas+'-'))...
> stuff ::= item [ (', and' | ',' | 'and') item ]...
> location ::= ['@'] Word(alphas)...
>
> Rendered into pyparsing, it ends up very similar to your posted code:
>
> COMMA,AT = map(Literal, ',@')
> KW = CaselessKeyword
> get = KW('get') | KW('buy') | KW('pick') + KW('up') | KW('need')
> at = KW('at') | KW('from')
> location = Combine(Optional(AT) + OneOrMore(Word(alphas)), ' ',
> adjacent=False)
> and_ = KW('and')
> itemdelim = COMMA + and_ | COMMA | and_
> item = Combine(OneOrMore(~(at | itemdelim | '@') + Word(alphas)), ' ',
> adjacent=False)
> stuff = delimitedList(item, itemdelim)
> shoppingList = get + stuff("items") + Optional(Optional(at) +
> location("location"))
>
> And this seems to work ok for your posted tests.
>
> Please try to avoid defining Literals with embedded spaces, and
> *especially*
> not with leading spaces (as you do with " and ") - pyparsing's default
> whitespace skipping will almost surely make your leading-whitespace literal
> unmatchable.  Note how I defined "pick up" as an option for (get) as two
> joined keywords - this immunizes us against cases with extra whitespace
> between the two words, at very little cost.
>
> Thanks for writing, and welcome to the World of Pyparsing!  - :)
>
> -- Paul
>
>
>