metaprogramming examples

descriptors/model_v4c.py

@@ -0,0 +1,23 @@
+# BEGIN MODEL_V4
+class Quantity:
+    __counter = 0
+
+    def __init__(self):
+        cls = self.__class__
+        prefix = cls.__name__
+        index = cls.__counter
+        self.storage_name = '_{}_{}'.format(prefix, index)
+        cls.__counter += 1
+
+    def __get__(self, instance, owner):
+        if instance is None:
+            return self
+        else:
+            return getattr(instance, self.storage_name)
+
+    def __set__(self, instance, value):
+        if value > 0:
+            setattr(instance, self.storage_name, value)
+        else:
+            raise ValueError('value must be > 0')
+# END MODEL_V4