[Pyparsing] Recursive grammar

[thomas_h <th...@go...>]

Hi all,

I'm new to pyparsing, and I'm struggling with recursive production
rules. As an example, I want to parse expressions that are either
numbers or additions of numbers.

Grammar:

number :: '0'..'9'*
aexp      :: number | number '+' aexp


The following straight-forward implementation doesn't work:

from pyparsing import *
number = Word(nums)
aexp   = Forward()
aexp   = Or ([
          number,
          number + '+' + aexp ,
          ])

Running the code yields:

aexp.parseString("3+4") => (['3'], {})

I probably miss something simple here, but I can't figure out what. If
I make aexp non-recursive ("..., number + '+' + number,])") it works,
but this doesn't solve the general problem (re-arranging the cases in
the 'Or' argument doesn't help either). I looked into the "fourFn.py"
example, but this is much too elaborate to answer my question.

Thanks, Thomas