Python: Polymorphism
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Polymorphism allows the same method name to perform different actions depending on the object type. The document discusses polymorphism in the context of playing different audio file types (e.g. MP3, WAV, OGG). It defines an AudioFile parent class with subclasses for each file type that override the play() method. This allows calling play() on any audio file object while the correct playback logic is handled polymorphically based on the file's type.
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Python: Polymorphism
- 2. Polymorphism • Polymorphism simply means that we can call the same method name with parameters, and depending on the parameters, it will do different things. For example: >>> print(6 * 5) >>> print(“Hello” * 5)
- 3. Polymorphism • Polymorphism simply means that we can call the same method name with parameters, and depending on the parameters, it will do different things. For example: >>> print(6 * 5) >>> print(“Hello” * 5) Mult (6, 5) Mult (“Hello”, 5)
- 4. Polymorphism • Polymorphism simply means that we can call the same method name with parameters, and depending on the parameters, it will do different things. For example: >>> print(6 * 5) >>> print(“Hello” * 5) Mult (6, 5) Mult (“Hello”, 5) 30 HelloHelloHelloHelloHello
- 5. Polymorphism • A more complicated example to consider would be to think about creating a method called play() to play an audio file. • A media player will be needed to load the AudioFile object. • The instruction to play the file might be as simple as: >>> audio_file.play( )
- 6. Polymorphism • However, different audio files use different compression algorithms (e.g. .mp3, .wma, .ogg), and some aren’t stored as compressed at all (e.g. .wav). • We can use inheritance with polymorphism to simplify the design. Each filetype is represented as a different subclass of AudioFile, and each of those has a play( ) method.
- 7. Polymorphism AudioFile _ _init_ _( ) MP3File play( ) WAVFile play( ) OGGFile play( )
- 8. Polymorphism class AudioFile: def _ _init_ _(self, filename): if not filename.endswith(self.ext): # THEN raise Exception(“Invalid format”) # ENDIF; self.filename = filename # END init() # END CLASS.
- 9. Polymorphism class AudioFile: def _ _init_ _(self, filename): if not filename.endswith(self.ext): # THEN raise Exception(“Invalid format”) # ENDIF; self.filename = filename # END init() # END CLASS. Check if the file extension of the audio being played is a known extension, self.ext is set in each of the subclasses. Raise an exception if it’s an unknown file extension If it’s a known file extension, then assign the filename passed in to self.filename
- 10. Polymorphism class MP3File(AudioFile): ext = “mp3” def play(self): print(“playing {} as mp3”.format(self.filename)) # END play # END CLASS.
- 11. Polymorphism class WAVFile(AudioFile): ext = “wav” def play(self): print(“playing {} as wav”.format(self.filename)) # END play # END CLASS.
- 12. Polymorphism class OGGFile(AudioFile): ext = “ogg” def play(self): print(“playing {} as ogg”.format(self.filename)) # END play # END CLASS.
- 13. Polymorphism • Here’s how we run it: >>> mp3 = MP3File(“myfile.mp3”) >>> mp3.play() playing myfile.mp3 as mp3
- 14. Polymorphism • Here’s another one: >>> wav = WAVFile(“myfile.wav”) >>> wav.play() playing myfile.wav as wav
- 15. Polymorphism • Here’s an error: >>> ogg_declared_as_mp3 = MP3File("myfile.ogg") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "polymorphic_audio.py", line 4, in __init__ raise Exception("Invalid format") Exception: Invalid format
- 16. etc.