def get_radians(self):
        """ Return the angle between this vector and the positive x-axis 
        measured in radians. Result will be between -pi and pi. """
        if not self: raise NullVectorError()
        return math.atan2(self.y, self.x)
Return the angle between this vector and the positive x-axis 
        measured in radians. Result will be between -pi and pi.