dynamic attributes, descriptors and first concurrency examples

descriptors/model_v5.py

@@ -9,7 +9,7 @@ class AutoStorage:  # <1>
         cls = self.__class__
         prefix = cls.__name__
         index = cls.__counter
-        self.storage_name = '_{}_{}'.format(prefix, index)
+        self.storage_name = '_{}:{}'.format(prefix, index)
         cls.__counter += 1
 
     def __get__(self, instance, owner):
@@ -34,6 +34,7 @@ class Validated(abc.ABC, AutoStorage):  # <3>
 
 
 class Quantity(Validated):  # <7>
+    """a number greater than zero"""
 
     def validate(self, instance, value):
         if value <= 0:
@@ -42,6 +43,7 @@ class Quantity(Validated):  # <7>
 
 
 class NonBlank(Validated):
+    """a string with at least one non-space character"""
 
     def validate(self, instance, value):
         value = value.strip()