Recursive parsing for lisp like syntax

Question

im trying to parse lines in the form:

(OP something something (OP something something ) ) ( OP something something )

Where OP is a symbol for a logical gate (AND, OR, NOT) and something is the thing i want to evaluate.

The output im looking for is something like:

{ 'OPERATOR': [condition1, condition2, .. , conditionN] }

Where a condition itself can be a dict/list pair itself (nested conditions). So far i tried something like:

        tree = dict()
        cond = list()
        tree[OP] = cond


    for string in conditions:
        self.counter += 1
        if string.startswith('('):
            try:
                OP = string[1]
            except IndexError:
                OP = 'AND'
            finally:
                if OP == '?':
                    OP = 'OR'
                elif OP == '!':
                    OP = 'N'

            # Recurse
            cond.append(self.parse_conditions(conditions[self.counter:], OP))
            break

        elif not string.endswith(")"):
            cond.append(string)

        else:
            return tree

    return tree

I tried other ways aswell but i just can't wrap my head around this whole recursion thing so im wondering if i could get some pointers here, i looked around the web and i found some stuff about recursive descent parsing but the tutorials were all trying to do something more complicated than i needed.

PS: i realize i could do this with existing python libraries but what would i learn by doing that eh?

Answer 1

I'm posting this without further comments, for learning purposes (in the real life please do use a library). Note that there's no error checking (a homework for you!)

Feel free to ask if there's something you don't understand.

# PART 1. The Lexer

symbols = None

def read(input):
    global symbols
    import re
    symbols = re.findall(r'\w+|[()]', input)

def getsym():
    global symbols
    return symbols[0] if symbols else None

def popsym():
    global symbols
    return symbols.pop(0)

# PART 2. The Parser
# Built upon the following grammar:
#  
#     program = expr*
#     expr    = '(' func args ')'
#     func    = AND|OR|NOT
#     args    = arg*
#     arg     = string|expr
#     string  = [a..z]

def program():
    r = []
    while getsym():
        r.append(expr())
    return r

def expr():
    popsym() # (
    f = func()
    a = args()
    popsym() # )
    return {f: a}

def func():
    return popsym()

def args():
    r = []
    while getsym() != ')':
        r.append(arg())
    return r

def arg():
    if getsym() == '(':
        return expr()
    return string()

def string():
    return popsym()

# TEST = Lexer + Parser

def parse(input):
    read(input)
    return program()

print parse('(AND a b (OR c d)) (NOT foo) (AND (OR x y))')
# [{'AND': ['a', 'b', {'OR': ['c', 'd']}]}, {'NOT': ['foo']}, {'AND': [{'OR': ['x', 'y']}]}]