class AudioFile:
    # Note self.ext, self.filename, this is how polymorphism works.
    def __init__(self, filename):
        if not filename.endswith(self.ext):
            raise Exception("Invalid file format")
        self.filename = filename

    def play(self):
        print("playing {} as {}".format(self.filename,self.ext))


class MP3File(AudioFile):
    ext = 'mp3'

class WavFile(AudioFile):
    ext = 'wav'

class OggFile(AudioFile):
    ext = 'ogg'

audio_file = MP3File('hello.mp3')
audio_file.play()
"""
playing hello.mp3 as mp3
"""