How to parse nested function calls using Pyparsing?

Question

I'm developing a css parser but cannot figure out how to parse nested function calls like alpha(rgb(1, 2, 3), 0.5).

Here is my code:

# -*- coding: utf-8 -*-


from pyparsing import * #, Word, alphas, OneOrMore, countedArray, And, srange, hexnums,  Combine, cStyleComment

from string import punctuation
from app.utils.file_manager import loadCSSFile, writeFile
from pyparsing import OneOrMore, Optional, Word, quotedString, delimitedList,  Suppress
from __builtin__ import len

# divide tudo por espaços, tabulações e novas linhas
words = ZeroOrMore(cStyleComment | Word(printables))

digit = '0123456789'; underscore = '_'; hyphen =  '-'
hexDigit = 'abcdefABCDEF' + digit
letter = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
az_AZ_underscore = underscore + letter;  
az_AZ_09_underscore = az_AZ_underscore + digit; 
az_AZ_hyphen = hyphen + letter;
az_AZ_09_hiphen = az_AZ_hyphen + digit;
LPAR = Suppress('('); RPAR = Suppress(')')

# identifiers
identifier = Combine(Word(az_AZ_underscore) + Optional(Word(az_AZ_09_underscore)))
# identifiers
identifier_reserved = Combine(Word(az_AZ_hyphen) + Optional(Word(az_AZ_09_hiphen)))

# numbers
hexadecimal = Word(hexnums, min=1)
integer = Word(digit, min=1)
decimal = Combine('.' + integer | integer + Optional('.' + integer))


# value values
color_hex = Combine('#' +  hexadecimal ) 
at_identifier =  Combine('@' + identifier)
arg = at_identifier | color_hex | decimal | quotedString 




function_call = identifier + LPAR + Optional(delimitedList(arg)) + RPAR

value =  Group(color_hex | at_identifier | function_call)

print(value.parseString('a(b())'))

I whould like to do something like arg = at_identifier | color_hex | decimal | quotedString | function_call but it's not possible because the variable is function_call is not declared yet.

How could I parse nested function calls using Pyparsing?

PaulMcG · Accepted Answer · 2014-04-28 04:27:19Z

3

You are really very close. To define a recursive grammar like this, you need to forward-declare the nested expression.

As you have already seen, this:

arg = at_identifier | color_hex | decimal | quotedString
function_call = identifier + LPAR + Optional(delimitedList(arg)) + RPAR

only parses function calls with args that are not themselves function calls.

To define this recursively, first define an empty placeholder expression, using Forward():

function_call = Forward()

We don't know yet what will go into this expression, but we do know that it will be a valid argument type. Since it has been declared now we can use it:

arg = at_identifier | color_hex | decimal | quotedString | function_call

Now that we have arg defined, we can define what will go into a function_call. Instead of using ordinary Python assignment using '=', we have to use an operator that will modify the function_call instead of redefining it. Pyparsing allows you to use <<= or << operators (the first is preferred):

function_call <<= identifier + LPAR + Optional(delimitedList(arg)) + RPAR

This is now sufficient to parse your given sample string, but you've lost visibility to the actual nesting structure. If you group your function call like this:

function_call <<= Group(identifier + LPAR + Group(Optional(delimitedList(arg))) + RPAR)

you will always get a predictable structure from a function call (a named and the parsed args), and the function call itself will be grouped.

answered Apr 28, 2014 at 4:27

PaulMcG

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3 Comments

Natanael Over a year ago

Thanks! It's works perfectly! Are you the author of pyparsing? Thanks again for this great tool!

ArtOfWarfare Over a year ago

Hi Paul - I found it difficult to find your talk from PyCon 2006. It's mentioned here, but the link is dead: wiki.python.org/moin/PyCon2006/Talks - I ultimately found it by going to archive.org. They had the files that used to be hosted there, but either the JS scripts are buggy or something about the archive process messed them up, making it difficult to view in the browser (the whole page flickers a lot). Saving the archive as a PDF gave me the presentation in a usable form. Just thought you might want to fix that dead link so other people don't have so much trouble like I did.

PaulMcG Over a year ago

ptmcg.com/geo/python/confs/TxUnconf2008Pyparsing.html (PyCon 2006 recycled/updated for TX Unconference in 08) - mouse over the bottom right corner to navigate slides

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