def calc(self, arg):

      op = arg["op"]

      if op == "C":
         self.clear()
         return str(self.current)

      num = decimal.Decimal(arg["num"])
      if self.op:
         if self.op == "+":
            self.current += num
         elif self.op == "-":
            self.current -= num
         elif self.op == "*":
            self.current *= num
         elif self.op == "/":
            self.current /= num
         self.op = op
      else:
         self.op = op
         self.current = num

      res = str(self.current)
      if op == "=":
         self.clear()

      return res