ch11-24: clean up by @eumiro & sync with Atlas

24-descriptor/bulkfood/bulkfood_v4.py

@@ -0,0 +1,70 @@
+"""
+
+A line item for a bulk food order has description, weight and price fields::
+
+    >>> raisins = LineItem('Golden raisins', 10, 6.95)
+    >>> raisins.weight, raisins.description, raisins.price
+    (10, 'Golden raisins', 6.95)
+
+A ``subtotal`` method gives the total price for that line item::
+
+    >>> raisins.subtotal()
+    69.5
+
+The weight of a ``LineItem`` must be greater than 0::
+
+    >>> raisins.weight = -20
+    Traceback (most recent call last):
+        ...
+    ValueError: weight must be > 0
+
+No change was made::
+
+    >>> raisins.weight
+    10
+
+Negative or 0 price is not acceptable either::
+
+    >>> truffle = LineItem('White truffle', 100, 0)
+    Traceback (most recent call last):
+        ...
+    ValueError: price must be > 0
+
+If the descriptor is accessed in the class, the descriptor object is
+returned:
+
+    >>> LineItem.weight  # doctest: +ELLIPSIS
+    <bulkfood_v4.Quantity object at 0x...>
+    >>> LineItem.weight.storage_name
+    'weight'
+
+"""
+
+
+# tag::LINEITEM_V4[]
+class Quantity:
+
+    def __set_name__(self, owner, name):  # <1>
+        self.storage_name = name          # <2>
+
+    def __set__(self, instance, value):   # <3>
+        if value > 0:
+            instance.__dict__[self.storage_name] = value
+        else:
+            msg = f'{self.storage_name} must be > 0'
+            raise ValueError(msg)
+
+    # no __get__ needed
+
+class LineItem:
+    weight = Quantity()  # <5>
+    price = Quantity()
+
+    def __init__(self, description, weight, price):
+        self.description = description
+        self.weight = weight
+        self.price = price
+
+    def subtotal(self):
+        return self.weight * self.price
+# end::LINEITEM_V4[]