how to get more info from parse results

[Klaas Hofstra]

Hi,

I really like the power and simplicity of pyparsing and have a question about how the library should be used.

I'll first give a small example that hopefully helps to describe my problem.

As an example, lets say the input looks like:

  const a = 1
  const b = 2
  c = a + b
  d = a - b
  e = d

Now, The grammer could look like this:

  plus = Literal("+")
  minus = Literal("-")
  eq = Literal("=").suppress()

  constName = Word(alphanums)
  constValue = Word(nums)
  constExpr = Group(Literal("const") + constName + eq + constValue)

  varName = Word(alphanums)
  varOp = plus | minus
  opExpr = Group(varName + eq + varName + varOp + varName)
  assignExpr = Group(varName + eq + varName)

  grammer = ZeroOrMore(constExpr) + ZeroOrMore(opExpr) + ZeroOrMore(assignExpr)

  result = grammer.parseString(str)

OK, now my problem/question is that I need to a little 'parsing' of the result in order to extract the constants/plus-min/assigment expressions. There surely must be a better way of doing this because the parser 'knows' which rule matched, so it should be able to give me that information. Is there a way for me to specify the grammar in a different way so I don't need to do the extra 'parsing'?

So I'd like to avoid stuff like this:
  constants = {}
  plusmin = []
  assign = {}
  for x in result :
      if x[0] == "const" :
          constants[x[1]] = x[2]
      else :
          if len(x) == 2 :
              assign[x[0]] = x[1]
          else :
              plusmin.append(x)

  print "constants: ", constants
  print "assignments: ", assign
  print "add/sub: ", plusmin

and replace it with:
  results.get_me_the_constants()
  etc.

I hope y'all understand my question, and thanks in advance!

Cheers,

Klaas

[Paul McGuire]

Klaas -

Welcome to the growing world of pyparsing!

You are absolutely on the right track - the parser does indeed 'know' which rule matched the incoming text.

I think the feature you are looking for is another method on ParserElement, setParseAction.  This allows you to define a method to be called when the rule matches.  The method must accept 3 arguments: a string (the original parse text string), an integer (the location in the string at which the matching text was found), and a ParseResults object containing the matching tokens.  You can access a ParseResults as a list (as you were doing in your example), or as a dictionary (if results names were defined in the grammar).

Since you are delving into the world of arithmetic expressions and assignments, take a look at the fourFn.py example that comes with pyparsing.

Good luck, and keep us informed on your progress!

-- Paul McGuire

[Klaas Hofstra]

Paul,

Thanks for your reply. I've tried setParseAction and it seems to be the way to go.

I'm not really working on arithmetic expressions, that was just the simplest example I could come up with :) I'm working on a parser that takes a custom hardware template description and converts that to a VHDL framework.

Cheers,

Klaas