A naive S-expr parser in python

simplesexpr.py

#!/usr/bin/env python3

# Simple sexpr parser

from collections import namedtuple

Expr = namedtuple('Expr', ('children'));

OPEN = 1
CLOSE = 2

def lex(txt):
    parts = []
    i = 0
    while i < len(txt):
        if txt[i] == '(':
            i += 1
            parts.append(OPEN)
        elif txt[i] == ')':
            i += 1
            parts.append(CLOSE)
        elif txt[i].isspace():
            i += 1
        else:
            j = i
            while not (txt[j].isspace() or txt[j] in ('(', ')')):
                j += 1
            parts.append(txt[i:j])
            i = j
    return parts

def parse(parts):
    while True:
        if parts[0] == OPEN: # Paren open
            children = []
            i = 1
            while parts[i] != CLOSE:
                child, consumed = parse(parts[i:])
                children.append(child)
                i += consumed
            return children, i+1
        if parts[0] == CLOSE: # Paren close
            return [], 1
        else: # Atom
            return parts[0], 1

def find_singletons(tree):
    stons = []
    for l in tree:
        if type(l) is list and len(l) == 1:
            stons.extend(l)
        elif type(l) is list:
            stons.extend(find_singletons(l))
    return stons

print(parse(lex('(abc def (gh i (j)) jkl)')))

tree = parse(lex('(3 (2 Yet) (3 (2 (2 The) (2 Act)) (3 (4 (3 (2 Is) (3 (2 Still) (4 (Charming)))) (2 Here)) (2 .))))'))
print(tree)
print(find_singletons(tree))

# Prints:
# (['abc', 'def', ['gh', 'i', ['j']], 'jkl'], 12)
# (['3', ['2', 'Yet'], ['3', ['2', ['2', 'The'], ['2', 'Act']], ['3', ['4', ['3', ['2', 'Is'], ['3', ['2', 'Still'], ['4', ['Charming']]]], ['2', 'Here']], ['2', '.']]]], 55)
# ['Charming']