Add a solution

CH_04_design_patterns/exercise_03/solution_01.py

@@ -0,0 +1,29 @@
+class Borg:
+
+    def __new__(cls):
+        if not hasattr(cls, '_state'):
+            cls._state = {}
+        instance = super(Borg, cls).__new__(cls)
+        instance.__dict__ = cls._state
+        return instance
+
+
+class SubBorg(Borg):
+    pass
+
+
+class SubBorg2(Borg):
+    pass
+
+
+def main():
+    a = SubBorg()
+    b = SubBorg()
+    c = SubBorg2()
+    a.attr = 10
+    assert hasattr(b, 'attr') and b.attr == 10
+    assert not hasattr(c, 'attr')
+
+
+if __name__ == '__main__':
+    main()