Python S-exp arithmetic evaluation

gistfile1.py

import operator

Symbol = str

def tokenize(exp):
    return exp.replace('(', ' ( ').replace(')', ' ) ').split()
    
def parser(exp):
    return read_from_tokens(tokenize(exp))
    
def read_from_tokens(tokens):
    if len(tokens) == 0:
        raise SyntaxError(' Unexpected EOF of exp ')
    
    token = tokens.pop(0)
    if token == '(':
        L = []
        while tokens[0] != ')':
            L.append(read_from_tokens(tokens))
        tokens.pop(0)
        return L
    elif token == ')':
        raise SyntaxError(' Unbalanced bracket ')
    else:
        return atom(token)
        
def atom(token):
    try: return int(token)
    except ValueError:
        try: return float(token)
        except ValueError:
            if token in {'+', '*', '/', '-'}:
                return Symbol(token)
            else:
                raise SyntaxError(' Unsupport operator ')

def evaluate(parser_tree):
    if isinstance(parser_tree, int) or isinstance(parser_tree, float):
        return parser_tree
    else:
        tmp = map(evaluate, parser_tree[1:])
        if parser_tree[0] == '+':
            return reduce(operator.add, tmp, 0)
        elif parser_tree[0] == '-':
            return reduce(operator.sub, tmp, 0)
        elif parser_tree[0] == '*':
            return reduce(operator.mul, tmp, 1)
        elif parser_tree[0] == '/':
            ret = tmp[0]
            return reduce(operator.div, tmp[1:], ret)
            
def run(exp):
    parser_tree = parser(exp)
    return evaluate(parser_tree)
     
def test():
    assert run('(+ 3 4)') == 7
    assert run('(+ 3 4 5)') == 12
    assert run('(+ (* 3 4) 5)') == 17
    assert run('(* 3 (+ 4 5))') == 27
    assert run('(* 3 (/ 15 7) 4)') == 24
    return 'test pass!'