new code example audiofile.py showing inhiterance with polymorphism

python_3_oop/chapter03/audiofile.py

@@ -0,0 +1,32 @@
+# small example for inhiterance with polymorphism
+
+class AudioFile:
+
+    def __init__(self, filename):
+        if not filename.endswith(self.ext):
+            raise Exception("Invalid file format")
+        self.filename = filename
+
+
+class MP3File(AudioFile):
+
+    ext = "mp3"
+
+    def play(self):
+        print("Playing {} as mp3".format(self.filename))
+
+
+class WavFile(AudioFile):
+
+    ext = "wav"
+
+    def play(self):
+        print("Playing {} as wav".format(self.filename))
+
+
+class OggFile(AudioFile):
+
+    ext = "ogg"
+
+    def play(self):
+        print("Playing {} as ogg".format(self.filename))