Rational Class in Python

#An auxiliary helper function for constructor method of Rational Class
def gcd(a, b):
    n1 = abs(a)
    n2 = abs(b)
    gcd = 1
    for k in range(2, min(n1,n2)+1):
        if (n1 % k == 0) and (n2 % k == 0):
            gcd = k
    return gcd


class Rational:
    def __init__(self, a = 0, b = 1):
        if  b <= 0:
            raise ValueError ("Denom may not be zero or less!")
        else:
            g  =  gcd(a, b)
            self.n  =  int(a / g)
            self.d  =  int(b / g)

# Add a rational number to this rational number r2
    def __add__(self, r2):
        a = self.n * r2.d +  self.d * r2.n
        b = self.d * r2.d
        return Rational(a, b)

# Subtract a rational number from this rational number r2
    def __sub__(self, r2):
        a = self.n * r2.d - self.d * r2.n
        b = self.d * r2.d
        return Rational(a, b)

    def __eq__(self,r2):
        if self.n *  r2.d ==  self.d * r2.n:
            return True
        else:
            return False

    def __str__ ( self ):
        return  str(self.n) + "/" + str(self.d)


#Client Usage
#Application to overcoming roundoff errors
print("Testing evaluation to 0:")
x= 1. - 1. / 3 - 2. / 3
print(x)
if x == 0:
    print("Test Passed")
else:
    print ("Test Failed")

one = Rational(1,1)
third = Rational(1,3)
twothirds = Rational(2,3)
print(str(one) + " - " + str(third) + " - " + str(twothirds) + " = " )
x= one - third - twothirds
print(x)
if x == Rational(0):
    print("Test Passed")
else:
    print ("Test Failed")