simple calculator

import re
import collections
import itertools

RE_TOKEN = re.compile('([()])|([+-])|([0-9]+)')

class Solution:
    def calculate(self, s: str) -> int:
        stack = []
        for val in itertools.chain('(', (t.group(0) for t in RE_TOKEN.finditer(s)), ')'):
            if val == ')':
                cur = 0
                while True:
                    el = stack.pop()
                    if el == '(':
                        stack.append(cur+pp)
                        break
                    elif el == '+':
                        cur += pp
                    elif el == '-':
                        cur -= pp
                    else:
                        pp = int(el)
            else:
                stack.append(val)
        return stack[0]