renumbering chapters >= 19

24-class-metaprog/bulkfood/model_v6.py

@@ -0,0 +1,60 @@
+import abc
+
+
+class AutoStorage:
+    __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)
+
+
+class Validated(abc.ABC, AutoStorage):
+
+    def __set__(self, instance, value):
+        value = self.validate(instance, value)
+        super().__set__(instance, value)
+
+    @abc.abstractmethod
+    def validate(self, instance, value):
+        """return validated value or raise ValueError"""
+
+
+class Quantity(Validated):
+    """a number greater than zero"""
+
+    def validate(self, instance, value):
+        if value <= 0:
+            raise ValueError('value must be > 0')
+        return value
+
+
+class NonBlank(Validated):
+    """a string with at least one non-space character"""
+
+    def validate(self, instance, value):
+        value = value.strip()
+        if len(value) == 0:
+            raise ValueError('value cannot be empty or blank')
+        return value
+
+# tag::MODEL_V6[]
+def entity(cls):  # <1>
+    for key, attr in cls.__dict__.items():  # <2>
+        if isinstance(attr, Validated):  # <3>
+            type_name = type(attr).__name__
+            attr.storage_name = '_{}#{}'.format(type_name, key)  # <4>
+    return cls  # <5>
+# end::MODEL_V6[]