pyparsing: crash on empty element

[gry]

[python 2.3.3, pyparsing 1.3]
I have:

def unpack_sql_arra y(s):
# unpack a postgres "array", e.g. "{'w1','w2','w3 '}" into a
list(str)
import pyparsing as pp
withquotes = pp.dblQuotedStr ing.setParseAct ion(pp.removeQu otes)
withoutquotes = pp.CharsNotIn(' ",{}')
parser = pp.StringStart( ) + \
pp.Literal('{') .suppress() + \
pp.delimitedLis t(withquotes | withoutquotes) + \
pp.Literal('}') .suppress() + \
pp.StringEnd()
return parser.parseStr ing(s).asList()

which works beautifully, except on the input: "{}". How can I neatly
modify the parser to return an empty list in this case?
Yes, obviously, I could say
if s=='{}': return []
It just seems like I'm missing some simple intrinsic way to get this
out of the parser. I am hoping to become more skillful in using the
wonderful pyparsing module!

-- George Young

[Paul McGuire]

To indicate that a particular parse expression may be empty, pyparsing
includes the Optional element.

Change:
pp.delimitedLis t(withquotes | withoutquotes) + \

to:
pp.Optional( pp.delimitedLis t(withquotes | withoutquotes) ) + \

and I think this will have the desired effect.

-- Paul
<gr*@ll.mit.edu > wrote in message
news:11******** **************@ u72g2000cwu.goo glegroups.com.. .

[python 2.3.3, pyparsing 1.3]
I have:

def unpack_sql_arra y(s):
# unpack a postgres "array", e.g. "{'w1','w2','w3 '}" into a
list(str)
import pyparsing as pp
withquotes = pp.dblQuotedStr ing.setParseAct ion(pp.removeQu otes)
withoutquotes = pp.CharsNotIn(' ",{}')
parser = pp.StringStart( ) + \
pp.Literal('{') .suppress() + \
pp.delimitedLis t(withquotes | withoutquotes) + \
pp.Literal('}') .suppress() + \
pp.StringEnd()
return parser.parseStr ing(s).asList()

which works beautifully, except on the input: "{}". How can I neatly
modify the parser to return an empty list in this case?
Yes, obviously, I could say
if s=='{}': return []
It just seems like I'm missing some simple intrinsic way to get this
out of the parser. I am hoping to become more skillful in using the
wonderful pyparsing module!

-- George Young