update from Atlas

descriptors/bulkfood_v3_broken.py

@@ -0,0 +1,59 @@
+"""
+WARNING: Broken implementation for demonstration purposes.
+
+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: value must be > 0
+
+Negative or 0 price is not acceptable either::
+
+    >>> truffle = LineItem('White truffle', 100, 0)
+    Traceback (most recent call last):
+        ...
+    ValueError: value must be > 0
+
+
+No change was made::
+
+    >>> raisins.weight
+    10
+
+"""
+
+class Quantity:
+
+    def __init__(self, storage_name):
+        self.storage_name = storage_name
+
+    def __set__(self, instance, value):
+        if value > 0:
+            instance.__dict__[self.storage_name] = value
+        else:
+            raise ValueError('value must be > 0')
+
+
+class LineItem:
+    weight = Quantity('weight')
+    price = Quantity('weight')  # <-- this is the bug discussed in the book
+
+    def __init__(self, description, weight, price):
+        self.description = description
+        self.weight = weight
+        self.price = price
+
+    def subtotal(self):
+        return self.weight * self.price