update from Atlas

descriptors/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
+    >>> raisins.subtotal()
+    -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

descriptors/bulkfood_v2.py

@@ -0,0 +1,63 @@
+"""
+
+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
+
+No change was made::
+
+    >>> raisins.weight
+    10
+
+The check is also performed on instantiation::
+
+    >>> walnuts = LineItem('walnuts', 0, 10.00)
+    Traceback (most recent call last):
+        ...
+    ValueError: value must be > 0
+
+The proteced attribute can still be accessed if needed for some reason, such as
+white box testing)::
+
+    >>> raisins._LineItem__weight
+    10
+
+"""
+
+
+# BEGIN LINEITEM_V2
+class LineItem:
+
+    def __init__(self, description, weight, price):
+        self.description = description
+        self.weight = weight  # <1>
+        self.price = price
+
+    def subtotal(self):
+        return self.weight * self.price
+
+    @property  # <2>
+    def weight(self):  # <3>
+        return self.__weight  # <4>
+
+    @weight.setter  # <5>
+    def weight(self, value):
+        if value > 0:
+            self.__weight = value  # <6>
+        else:
+            raise ValueError('value must be > 0')  # <7>
+# END LINEITEM_V2

descriptors/bulkfood_v3.py

@@ -0,0 +1,60 @@
+"""
+
+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
+
+"""
+
+# BEGIN LINEITEM_V3
+class Quantity:  # <1>
+
+    def __init__(self, storage_name):
+        self.storage_name = storage_name  # <2>
+
+    def __set__(self, instance, value):  # <3>
+        if value > 0:
+            instance.__dict__[self.storage_name] = value  # <4>
+        else:
+            raise ValueError('value must be > 0')
+
+
+class LineItem:
+    weight = Quantity('weight')  # <5>
+    price = Quantity('price')  # <6>
+
+    def __init__(self, description, weight, price):  # <7>
+        self.description = description
+        self.weight = weight
+        self.price = price
+
+    def subtotal(self):
+        return self.weight * self.price
+# END LINEITEM_V3

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

descriptors/bulkfood_v4.py

@@ -0,0 +1,74 @@
+"""
+
+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
+
+No change was made::
+
+    >>> raisins.weight
+    10
+
+The value of the attributes managed by the descriptors are stored in
+alternate attributes, created by the descriptors in each ``LineItem``
+instance::
+
+    >>> raisins = LineItem('Golden raisins', 10, 6.95)
+    >>> dir(raisins)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
+    ['_Quantity_0', '_Quantity_1', '__class__', ...
+     'description', 'price', 'subtotal', 'weight']
+    >>> raisins._Quantity_0
+    10
+    >>> raisins._Quantity_1
+    6.95
+
+"""
+
+
+# BEGIN LINEITEM_V4
+class Quantity:
+    __counter = 0  # <1>
+
+    def __init__(self):
+        cls = self.__class__  # <2>
+        prefix = cls.__name__  # <3>
+        index = cls.__counter  # <4>
+        self.storage_name = '_{}_{}'.format(prefix, index)  # <5>
+        cls.__counter += 1  # <6>
+
+    def __get__(self, instance, owner):  # <7>
+        return getattr(instance, self.storage_name)  # <8>
+
+    def __set__(self, instance, value):  # <9>
+        if value > 0:
+            setattr(instance, self.storage_name, value)  # <10>
+        else:
+            raise ValueError('value must be > 0')
+
+
+class LineItem:
+    weight = Quantity()  # <11>
+    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