how to make a warning to user if they input inappropriate value

warning_method.py

#Using "raise" in python to warning user

def set(self, new_x, new_y, new_orientation):
    if new_x < 0 or new_x >= world_size:
        raise ValueError, 'X coordinate out of bound'
    if new_y < 0 or new_y >= world_size:
        raise ValueError, 'Y coordinate out of bound'
    if new_orientation < 0 or new_orientation >= 2 * pi:
        raise ValueError, 'Orientation must be in [0..2pi]'
    self.x = float(new_x)
    self.y = float(new_y)
    self.orientation = float(new_orientation)