added wikipedia example for duck typing with python

python_3_oop/chapter03/duck_typing_wikipedia.py

@@ -0,0 +1,48 @@
+# duck typing example taken from wikipedia
+# https://de.wikipedia.org/wiki/Duck-Typing
+
+
+class Bird:
+    "Birds have a name that they return in string represention"
+
+    def __init__(self, name):
+        self.name = name
+
+    def __str__(self):
+        return self.__class__.__name__+' '+self.name
+
+
+class Duck(Bird):
+    "Ducks are birds which can quak"
+
+    def quak(self):
+        print(str(self)+': quak')
+
+
+class Frog:
+    "Frogs also can quak"
+    def quak(self):
+        print(str(self)+': quak')
+
+
+def main():
+
+    ducks = [Bird('Gustav'), Duck('Donald'), object()]
+    for duck in ducks:
+        # exception handling when object hasn't quak method
+        try:
+            duck.quak()
+        except AttributeError:
+            print('No duck:', duck)
+
+    ducks.append(Frog())
+    for duck in ducks:
+        # we also can use hasattr() function instead of exception handling
+        if hasattr(duck, 'quak'):
+            duck.quak()
+        else:
+            print('No duck:', duck)
+
+
+if __name__ == "__main__":
+    main()