renumbering chapters >= 19

23-descriptor/bulkfood/model_v5.py

@@ -0,0 +1,36 @@
+# tag::MODEL_V5_VALIDATED_ABC[]
+import abc
+
+class Validated(abc.ABC):
+
+    def __set_name__(self, owner, name):
+        self.storage_name = name
+
+    def __set__(self, instance, value):
+        value = self.validate(self.storage_name, value)  # <1>
+        instance.__dict__[self.storage_name] = value  # <2>
+
+    @abc.abstractmethod
+    def validate(self, name, value):  # <3>
+        """return validated value or raise ValueError"""
+# end::MODEL_V5_VALIDATED_ABC[]
+
+# tag::MODEL_V5_VALIDATED_SUB[]
+class Quantity(Validated):
+    """a number greater than zero"""
+
+    def validate(self, name, value):  # <1>
+        if value <= 0:
+            raise ValueError(f'{name} must be > 0')
+        return value
+
+
+class NonBlank(Validated):
+    """a string with at least one non-space character"""
+
+    def validate(self, name, value):
+        value = value.strip()
+        if len(value) == 0:
+            raise ValueError(f'{name} cannot be blank')
+        return value  # <2>
+# end::MODEL_V5_VALIDATED_SUB[]