pyparsing

[Bo¹tjan Jerko]

Hello !

I am trying to understand pyparsing. Here is a little test program
to check Optional subclass:

from pyparsing import Word,nums,Liter al,Optional

lbrack=Literal( "[").suppress ()
rbrack=Literal( "]").suppress ()
ddot=Literal(": ").suppress ()
start = Word(nums+".")
step = Word(nums+".")
end = Word(nums+".")

sequence=lbrack +start+Optional (ddot+step)+ddo t+end+rbrack

tokens = sequence.parseS tring("[0:0.1:1]")
print tokens

tokens1 = sequence.parseS tring("[1:2]")
print tokens1

It works on tokens, but the error message is showed on
the second string ("[1:2]"). I don't get it. I did use
Optional for ddot and step so I guess they are optional.

Any hints what I am doing wrong?

The versions are pyparsing 1.1.2 and Python 2.3.3.

Thanks,

B.

[Daniel 'Dang' Griffith]

Re: pyparsing

On Thu, 13 May 2004 08:05:32 +0200, bostjan.jerko@m f.uni-lj.si
(Bo¹tjan Jerko) wrote:
[color=blue]
>Hello !
>
>I am trying to understand pyparsing. Here is a little test program
>to check Optional subclass:
>
>from pyparsing import Word,nums,Liter al,Optional
>
>lbrack=Literal ("[").suppress ()
>rbrack=Literal ("]").suppress ()
>ddot=Literal(" :").suppress ()
>start = Word(nums+".")
>step = Word(nums+".")
>end = Word(nums+".")
>
>sequence=lbrac k+start+Optiona l(ddot+step)+dd ot+end+rbrack
>
>tokens = sequence.parseS tring("[0:0.1:1]")
>print tokens
>
>tokens1 = sequence.parseS tring("[1:2]")
>print tokens1
>
>It works on tokens, but the error message is showed on
>the second string ("[1:2]"). I don't get it. I did use
>Optional for ddot and step so I guess they are optional.
>
>Any hints what I am doing wrong?
>
>The versions are pyparsing 1.1.2 and Python 2.3.3.
>
>Thanks,
>
>B.[/color]
I don't see anything "obviously" wrong to me, but changing it thusly
seems to resolve the problem (I added a few intermediate rules to
make it more obvious):

pref = lbrack + start
midf = ddot + step
suff = ddot + end + rbrack
sequence = pref + midf + suff | pref + suff

I've run into "this kind of thing" now and again, and have always
been able to resolve it by reorganizing my rules.

--dang

[Paul McGuire]

Re: pyparsing

"Bo¹tjan Jerko" <bostjan.jerko@ mf.uni-lj.si> wrote in message
news:87fza46evn .fsf@bostjan-pc.mf.uni-lj.si...[color=blue]
> Hello !
>
> I am trying to understand pyparsing. Here is a little test program
> to check Optional subclass:
>
> from pyparsing import Word,nums,Liter al,Optional
>
> lbrack=Literal( "[").suppress ()
> rbrack=Literal( "]").suppress ()
> ddot=Literal(": ").suppress ()
> start = Word(nums+".")
> step = Word(nums+".")
> end = Word(nums+".")
>
> sequence=lbrack +start+Optional (ddot+step)+ddo t+end+rbrack
>
> tokens = sequence.parseS tring("[0:0.1:1]")
> print tokens
>
> tokens1 = sequence.parseS tring("[1:2]")
> print tokens1
>
> It works on tokens, but the error message is showed on
> the second string ("[1:2]"). I don't get it. I did use
> Optional for ddot and step so I guess they are optional.
>
> Any hints what I am doing wrong?
>
> The versions are pyparsing 1.1.2 and Python 2.3.3.
>
> Thanks,
>
> B.[/color]
Bostjan -

Here's how pyparsing is processing your input strings:

[0:0.1:1]
[ = lbrack
0 = start
:0.1 = ddot + step (Optional match)
: = ddot
1 = end
] = rbrack

[1:2]
[ = lbrack
1 = start
:2 = ddot + step (Optional match)
] = oops! expected ddot -> failure


Dang Griffith proposed one alternative construct, here's another, perhaps
more explicit:
lbrack + ( ( ddot + step + ddot + end ) | (ddot + end) ) + rbrack

Note that the order of the inner construct is important, so as to not match
ddot+end before trying ddot+step+ddot+ end; '|' is a greedy matching
operator, creating a MatchFirst object from pyparsing's class library. You
could avoid this confusion by using '^', which generates an Or object:
lbrack + ( (ddot + end) ^ ( ddot + step + ddot + end ) ) + rbrack
This will evaluate both subconstructs, and choose the longer of the two.

Or you can use another pyparsing helper, the delimited list
lbrack + delimitedlist( Word(nums+"."), delim=":") + rbrack
This implicitly suppresses delimiters, so that all you will get back are
["1","0.1"," 1"] in the first case and ["1","2"] in the second.

Happy pyparsing!
-- Paul

[Paul McGuire]

Re: pyparsing (errata)

> Dang Griffith proposed one alternative construct, here's another, perhaps[color=blue]
> more explicit:
> lbrack + ( ( ddot + step + ddot + end ) | (ddot + end) ) + rbrack
>[/color]

should be:
lbrack + start + ( ( ddot + step + ddot + end ) | (ddot + end) ) +
rbrack
[color=blue]
> Note that the order of the inner construct is important, so as to not[/color]
match[color=blue]
> ddot+end before trying ddot+step+ddot+ end; '|' is a greedy matching
> operator, creating a MatchFirst object from pyparsing's class library.[/color]
You[color=blue]
> could avoid this confusion by using '^', which generates an Or object:
> lbrack + ( (ddot + end) ^ ( ddot + step + ddot + end ) ) + rbrack[/color]

should be:
lbrack + start + ( (ddot + end) ^ ( ddot + step + ddot + end ) ) +
rbrack
[color=blue]
> This will evaluate both subconstructs, and choose the longer of the two.
>
> Or you can use another pyparsing helper, the delimited list
> lbrack + delimitedlist( Word(nums+"."), delim=":") + rbrack[/color]

at least this one is correct! No, wait, I mis-cased delimitedList!
should be:
lbrack + delimitedList( Word(nums+"."), delim=":") + rbrack
[color=blue]
> This implicitly suppresses delimiters, so that all you will get back are
> ["1","0.1"," 1"] in the first case and ["1","2"] in the second.
>
> Happy pyparsing!
> -- Paul
>
>[/color]
Sorry for the sloppiness,
-- Paul

[Bo¹tjan Jerko]

Re: pyparsing (errata)

Paul,

thanks for the explanation.

Bo¹tjan

On Fri, 14 May 2004, ptmcg@austin.rr ._bogus_.com spake:[color=blue][color=green]
>> Dang Griffith proposed one alternative construct, here's another, perhaps
>> more explicit:
>> lbrack + ( ( ddot + step + ddot + end ) | (ddot + end) ) +
>> rbrack
>>[/color]
>
> should be:
> lbrack + start + ( ( ddot + step + ddot + end ) | (ddot + end)
> ) +
> rbrack
>[color=green]
>> Note that the order of the inner construct is important, so as to
>> not[/color]
> match[color=green]
>> ddot+end before trying ddot+step+ddot+ end; '|' is a greedy matching
>> operator, creating a MatchFirst object from pyparsing's class
>> library.[/color]
> You[color=green]
>> could avoid this confusion by using '^', which generates an Or
>> object: lbrack + ( (ddot + end) ^ ( ddot + step + ddot + end )
>> ) + rbrack[/color]
>
> should be:
> lbrack + start + ( (ddot + end) ^ ( ddot + step + ddot + end )
> ) +
> rbrack
>[color=green]
>> This will evaluate both subconstructs, and choose the longer of the
>> two.
>>
>> Or you can use another pyparsing helper, the delimited list
>> lbrack + delimitedlist( Word(nums+"."), delim=":") + rbrack[/color]
>
> at least this one is correct! No, wait, I mis-cased delimitedList!
> should be:
> lbrack + delimitedList( Word(nums+"."), delim=":") + rbrack
>[color=green]
>> This implicitly suppresses delimiters, so that all you will get
>> back are ["1","0.1"," 1"] in the first case and ["1","2"] in the
>> second.
>>
>> Happy pyparsing!
>> -- Paul
>>
>>[/color]
> Sorry for the sloppiness,
> -- Paul[/color]