A simple Python s-expression parser.

sexp.py

from string import whitespace

atom_end = set('()"\'') | set(whitespace)

def parse(sexp):
    stack, i, length = [[]], 0, len(sexp)
    while i < length:
        c = sexp[i]

        print c, stack
        reading = type(stack[-1])
        if reading == list:
            if   c == '(': stack.append([])
            elif c == ')': 
                stack[-2].append(stack.pop())
                if stack[-1][0] == ('quote',): stack[-2].append(stack.pop())
            elif c == '"': stack.append('')
            elif c == "'": stack.append([('quote',)])
            elif c in whitespace: pass
            else: stack.append((c,))
        elif reading == str:
            if   c == '"': 
                stack[-2].append(stack.pop())
                if stack[-1][0] == ('quote',): stack[-2].append(stack.pop())
            elif c == '\\': 
                i += 1
                stack[-1] += sexp[i]
            else: stack[-1] += c
        elif reading == tuple:
            if c in atom_end:
                atom = stack.pop()
                if atom[0][0].isdigit(): stack[-1].append(eval(atom[0]))
                else: stack[-1].append(atom)
                if stack[-1][0] == ('quote',): stack[-2].append(stack.pop())
                continue
            else: stack[-1] = ((stack[-1][0] + c),)
        i += 1
    return stack.pop()

Yeah, pretty nice.