added chapter map to README

22-dyn-attr-prop/bulkfood/bulkfood_v1.py

@@ -0,0 +1,37 @@
+"""
+A line item for a bulk food order has description, weight and price fields.
+A ``subtotal`` method gives the total price for that line item::
+
+    >>> raisins = LineItem('Golden raisins', 10, 6.95)
+    >>> raisins.weight, raisins.description, raisins.price
+    (10, 'Golden raisins', 6.95)
+    >>> raisins.subtotal()
+    69.5
+
+But, without validation, these public attributes can cause trouble::
+
+# BEGIN LINEITEM_PROBLEM_V1
+
+    >>> raisins = LineItem('Golden raisins', 10, 6.95)
+    >>> raisins.subtotal()
+    69.5
+    >>> raisins.weight = -20  # garbage in...
+    >>> raisins.subtotal()    # garbage out...
+    -139.0
+
+# END LINEITEM_PROBLEM_V1
+
+"""
+
+
+# BEGIN LINEITEM_V1
+class LineItem:
+
+    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_V1