Define or override the comparison operators for the class. http://docs.python.org/reference/expressions.html#notin
Looks like you are on the right track, except you only need to pass the second circle object to your comparison. self refers to the first circle object. So self.r would give you the r of the first circle. Also you need to return True or False from the method.
def __gt__(self, circle2):
return self.r > circle2.r
Note that this is just comparing the r's of the circles.
