rpn

class RPN:
    operations = {'+': lambda x, y: x + y, '-': lambda x, y: x - y, '*': lambda x, y: x * y, '/': lambda x, y: x / y, '^': lambda x, y: x ** y}
    def calculate(self):
        stack = []
        for e in self._rpn:
            if type(e) is Decimal:
                stack.append(e)
            else:
                y = stack.pop()
                x = stack.pop()
                stack.append(RPN.operations[e](x, y))
        return stack[0]