做除法时为什么不能分子除分子，分母除分母？

class Rational(object):

    def __init__(self,p,q):

        self.p = p

        self.q = q

    def __add__(self,r):

        return Rational(self.p*r.q+self.q*r.p,self.q*r.q)

    def __sub__(self,r):

        return Rational(self.p*r.q-self.q*r.p,self.q*r.q)

    def __mul__(self,r):

        return Rational(self.p*r.p,self.q*r.q)

    def __truediv__(self,r):

        return Rational(self.p/r.p,self.q/r.q)

    def __str__(self):

        return '{}/{}'.format(self.p,self.q)

r1 = Rational(1,2)

r2 = Rational(2,3)

print(r1 + r2,r1 - r2,r1 * r2,r1 / r2)

会在打印除法的位置报错