metaprogramming examples

descriptors/bulkfood_v2.py

@@ -60,4 +60,4 @@ class LineItem:
             self.__weight = value  # <6>
         else:
             raise ValueError('value must be > 0')  # <7>
-# END LINEITEM_V2
+# END LINEITEM_V2

descriptors/bulkfood_v3.py

@@ -33,6 +33,7 @@ No change was made::
 
 """
 
+
 # BEGIN LINEITEM_V3
 class Quantity:  # <1>
 

descriptors/bulkfood_v3_broken.py

@@ -34,6 +34,7 @@ No change was made::
 
 """
 
+
 class Quantity:
 
     def __init__(self, storage_name):

descriptors/bulkfood_v4.py

@@ -45,23 +45,23 @@ class Quantity:
 
     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>
+        prefix = cls.__name__
+        index = cls.__counter
+        self.storage_name = '_{}_{}'.format(prefix, index)  # <3>
+        cls.__counter += 1  # <4>
 
-    def __get__(self, instance, owner):  # <7>
-        return getattr(instance, self.storage_name)  # <8>
+    def __get__(self, instance, owner):  # <5>
+        return getattr(instance, self.storage_name)  # <6>
 
-    def __set__(self, instance, value):  # <9>
+    def __set__(self, instance, value):
         if value > 0:
-            setattr(instance, self.storage_name, value)  # <10>
+            setattr(instance, self.storage_name, value)  # <7>
         else:
             raise ValueError('value must be > 0')
 
 
 class LineItem:
-    weight = Quantity()  # <11>
+    weight = Quantity()  # <8>
     price = Quantity()
 
     def __init__(self, description, weight, price):
@@ -71,4 +71,4 @@ class LineItem:
 
     def subtotal(self):
         return self.weight * self.price
-# END LINEITEM_V4
+# END LINEITEM_V4

descriptors/bulkfood_v4b.py

@@ -0,0 +1,86 @@
+"""
+
+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
+
+If the descriptor is accessed in the class, the descriptor object is
+returned:
+
+    >>> LineItem.price  # doctest: +ELLIPSIS
+    <bulkfood_v4b.Quantity object at 0x...>
+    >>> br_nuts = LineItem('Brazil nuts', 10, 34.95)
+    >>> br_nuts.price
+    34.95
+
+"""
+
+
+# BEGIN LINEITEM_V4B
+class Quantity:
+    __counter = 0
+
+    def __init__(self):
+        cls = self.__class__
+        prefix = cls.__name__
+        index = cls.__counter
+        self.storage_name = '_{}_{}'.format(prefix, index)
+        cls.__counter += 1
+
+    def __get__(self, instance, owner):
+        if instance is None:
+            return self  # <1>
+        else:
+            return getattr(instance, self.storage_name)  # <2>
+
+    def __set__(self, instance, value):
+        if value > 0:
+            setattr(instance, self.storage_name, value)
+        else:
+            raise ValueError('value must be > 0')
+# END LINEITEM_V4B
+
+
+class LineItem:
+    weight = Quantity()
+    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

descriptors/bulkfood_v4c.py

@@ -0,0 +1,65 @@
+"""
+
+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
+
+If the descriptor is accessed in the class, the descriptor object is
+returned:
+
+    >>> LineItem.price  # doctest: +ELLIPSIS
+    <model_v4c.Quantity object at 0x...>
+    >>> br_nuts = LineItem('Brazil nuts', 10, 34.95)
+    >>> br_nuts.price
+    34.95
+
+"""
+
+# BEGIN LINEITEM_V4C
+import model_v4c as model  # <1>
+
+
+class LineItem:
+    weight = model.Quantity()  # <2>
+    price = model.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_V4C

descriptors/model_v4c.py

@@ -0,0 +1,23 @@
+# BEGIN MODEL_V4
+class Quantity:
+    __counter = 0
+
+    def __init__(self):
+        cls = self.__class__
+        prefix = cls.__name__
+        index = cls.__counter
+        self.storage_name = '_{}_{}'.format(prefix, index)
+        cls.__counter += 1
+
+    def __get__(self, instance, owner):
+        if instance is None:
+            return self
+        else:
+            return getattr(instance, self.storage_name)
+
+    def __set__(self, instance, value):
+        if value > 0:
+            setattr(instance, self.storage_name, value)
+        else:
+            raise ValueError('value must be > 0')
+# END MODEL_V4