dynamic attributes, descriptors and first concurrency examples

descriptors/model_v6.py

@@ -0,0 +1,72 @@
+# BEGIN MODEL_V5
+import abc
+
+
+class AutoStorage:  # <1>
+    __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):
+        setattr(instance, self.storage_name, value)  # <2>
+
+
+class Validated(abc.ABC, AutoStorage):  # <3>
+
+    def __set__(self, instance, value):
+        value = self.validate(instance, value)  # <4>
+        super().__set__(instance, value)  # <5>
+
+    @abc.abstractmethod
+    def validate(self, instance, value):  # <6>
+        """return validated value or raise ValueError"""
+
+INVALID = object()
+
+class Check(Validated):
+
+    def __init__(self, checker):
+        super().__init__()
+        self.checker = checker
+        if checker.__doc__ is None:
+            doc = ''
+        else:
+            doc = checker.__doc__ + '; '
+        self.message = doc + '{!r} is not valid.'
+
+
+    def validate(self, instance, value):
+        result = self.checker(value)
+        if result is INVALID:
+            raise ValueError(self.message.format(value))
+        return result
+
+
+class Quantity(Validated):  # <7>
+
+    def validate(self, instance, value):
+        if value <= 0:
+            raise ValueError('value must be > 0')
+        return value
+
+
+class NonBlank(Validated):
+
+    def validate(self, instance, value):
+        value = value.strip()
+        if len(value) == 0:
+            raise ValueError('value cannot be empty or blank')
+        return value  # <8>
+
+# END MODEL_V5