added new file with examples of extending built-in types like integer

python_3_oop/chapter06/extend_built-ins.py

@@ -0,0 +1,44 @@
+from collections import KeysView, ItemsView, ValuesView
+
+
+class SillyInt(int):
+
+    def __add__(self, num):
+        return 0
+
+
+a = SillyInt(1)
+b = SillyInt(2)
+print("{} + {} = {}".format(a, b, a+b))
+
+
+class DictSorted(dict):
+
+    def __new__(*args, **kwargs):
+        new_dict = dict.__new__(*args, **kwargs)
+        new_dict.ordered_keys = []
+        return new_dict
+
+    def __setitem__(self, key, value):
+        """self[key] = value syntax"""
+        if key not in self.ordered_keys:
+            self.ordered_keys.append(key)
+        super().__setitem__(key, value)
+
+    def setdefault(self, key, value):
+        if key not in self.ordered_keys:
+            self.ordered_keys.append(key)
+        return super().setdefault(key, value)
+
+    def keys(self):
+        return KeysView(self)
+
+    def values(self):
+        return ValuesView(self)
+
+    def items(self):
+        return ItemsView(self)
+
+    def __iter__(self):
+        """for x in self syntax"""
+        return self.ordered_keys.__iter__()