python leetcode 227. Basic Calculator II

class Solution(object):
    def calculate(self, s):
        """
        :type s: str
        :rtype: int
        """
        def helper(op, val):
            if op == '+':
                stack.append(val)
            elif op == '-':
                stack.append(-val)
            elif op == '*':
                stack.append(stack.pop() * val)
            elif op == '/':
                l, r = stack.pop() , val
                if l * r < 0 and l % r != 0:
                    stack.append(l // r  + 1)
                else:
                    stack.append(l // r)
        
        stack, val, op = [], 0, '+'
        
        for i in range(len(s)):
            if s[i].isdigit():
                val = val * 10 + int(s[i])
            elif s[i] in '/*-+':
                helper(op, val)
                op, val = s[i], 0
                
        helper(op, val)
                
        return sum(stack)