parser.py

def add_token(ast, token):
    if token != "":
      ast.append(token)
    return ast, ""

def parse_list(code, i):
  ast = []
  current_token = ""
  while i < len(code):
    char = code[i]
    i += 1
    if char == "(":
      sublist, i = parse_list(code, i)
      ast.append(sublist)
    elif char == ")":
      ast, current_token = add_token(ast, current_token)
      return ast, i
    elif char == " ":
      ast, current_token = add_token(ast, current_token)
    else:
      current_token += char

def parser(code):
  ast, i = parse_list(code, 1)
  return ast

code = "(first (list 1 (+ 2 3) 9))"
ast = parser(code)
ast == ['first', ['list', '1', ['+', '2', '3'], '9']]