Rotate vector by angle in radians.

def rot_rads(self, rads):
        """ Rotate vector by angle in radians."""

        new_x = self.x * math.cos(rads) - self.y * math.sin(rads)
        self.y = self.x * math.sin(rads) + self.y * math.cos(rads)
        self.x = new_x