# we define our own exception for this example
class UnknownOperatorExcpetion (Exception) :
    pass

# some helper functions to make the code easier to read
def const(c) :
    return Expr("const",c,c)

def add(a,b) :
    return Expr("add",a,b)

def mult(a,b) :
    return Expr("mult",a,b)

class Expr :
    def __init__(self,op,a,b) :
        self.operand1 = a
        self.operand2 = b
        self.op = op
        
    # even the debug function is recursive, so we can see the whole expression
    def __str__(self) :
        if self.op == "add" :
            return "(" + str(self.operand1) + " + " + str(self.operand2) + ")"
        elif self.op == "mult" :
            return "(" + str(self.operand1) + " * " + str(self.operand2) + ")"
        elif self.op == "const" :
            return str(self.operand1)
        else :
            raise UnknownOperatorException
        
    # evaluating the expression also needs to be recursive, because
    # we need to evaluate from the inside of the brackets outwards
    def eval(self) :
        if self.op == "add" :
            return self.operand1.eval() + self.operand2.eval()
        elif self.op == "mult" :
            return self.operand1.eval() * self.operand2.eval()
        elif self.op == "const" :
            return self.operand1
        else :
            raise UnknownOperatorException