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Exercises/ex4_3.md

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+\[ [Index](index.md) | [Exercise 4.2](ex4_2.md) | [Exercise 4.4](ex4_4.md) \]
+
+# Exercise 4.3
+
+*Objectives:*
+
+- Learn about descriptors
+
+*Files Created:* `descrip.py`
+
+*Files Modified:* `validate.py`
+
+## (a) Descriptors in action
+
+Earlier, you created a class `Stock` that made use of
+slots, properties, and other features.   All of these features
+are implemented using the descriptor protocol.  See it in
+action by trying this simple experiment.
+
+First, create a stock object, and try looking up a few attributes:
+
+```python
+>>> s = Stock('GOOG', 100, 490.10)
+>>> s.name
+'GOOG'
+>>> s.shares
+100
+>>>
+```
+
+Now, notice that these attributes are in the class dictionary.
+
+```python
+>>> Stock.__dict__.keys()
+['sell', '__module__', '__weakref__', 'price', '_price', 'shares', '_shares',
+'__slots__', 'cost', '__repr__', '__doc__', '__init__']
+>>> 
+```
+
+Try these steps which illustrate how descriptors get and set values on an instance:
+
+```python
+>>> q = Stock.__dict__['shares']
+>>> q.__get__(s)
+100
+>>> q.__set__(s,75)
+>>> s.shares
+75
+>>> q.__set__(s, '75')
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "stock.py", line 23, in shares
+    raise TypeError('Expected an integer')
+TypeError: Expected an integer
+>>>
+```
+
+The execution of `__get__()` and `__set__()` occurs automatically whenever you access instances.
+
+
+## (b) Make your own descriptor
+
+Define the descriptor class from the notes:
+
+```python
+# descrip.py
+
+class Descriptor:
+    def __init__(self, name):
+        self.name = name
+    def __get__(self, instance, cls):
+        print('%s:__get__' % self.name)
+    def __set__(self, instance, value):
+        print('%s:__set__ %s' % (self.name, value))
+    def __delete__(self, instance):
+        print('%s:__delete__' % self.name)
+```
+
+Now, try defining a simple class that uses this descriptor:
+
+```python
+>>> class Foo:
+        a = Descriptor('a')
+        b = Descriptor('b')
+        c = Descriptor('c')
+
+>>> f = Foo()
+>>> f
+<__main__.Foo object at 0x38e130> <class __main__.Foo>
+>>> f.a
+a:__get__
+>>> f.b
+b:__get__
+>>> f.a = 23
+a:__set__ 23
+>>> del f.a
+a:__delete__
+>>>
+```
+
+Ponder the fact that you have captured the dot-operator for a
+specific attribute.
+
+## (c) From Validators to Descriptors
+
+In the previous exercise, you wrote a series of classes that could perform checking.
+For example:
+
+```python
+>>> PositiveInteger.check(10)
+10
+>>> PositiveInteger.check('10')
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+    raise TypeError('Expected %s' % cls.expected_type)
+TypeError: expected <class 'int'>
+>>> PositiveInteger.check(-10)
+```
+
+You can extend this to descriptors by making a simple change to the `Validator` base
+class.  Change it to the following:
+
+```python
+# validate.py
+
+class Validator:
+    def __init__(self, name):
+        self.name = name
+
+    @classmethod
+    def check(cls, value):
+        return value
+
+    def __set__(self, instance,	value):
+        instance.__dict__[self.name] = self.check(value)
+```
+
+Note: The lack of the `__get__()` method in the descriptor means that
+Python will use its default implementation of attribute lookup.  This
+requires that the supplied name matches the name used in the instance
+dictionary.
+
+No other changes should be necessary.   Now, try modifying the `Stock` class to
+use the validators as descriptors like this:
+
+```python
+class Stock:
+    name   = String('name')
+    shares = PositiveInteger('shares')
+    price  = PositiveFloat('price')
+
+    def __init__(self,name,shares,price):
+        self.name = name
+        self.shares = shares
+        self.price = price
+```
+
+You'll find that your class works the same way as before, involves much
+less code, and gives you all of the desired checking:
+
+```python
+>>> s = Stock('GOOG', 100, 490.10)
+>>> s.name
+'GOOG'
+>>> s.shares
+100
+>>> s.shares = 75
+>>> s.shares = '75'
+... TypeError ...
+>>> s.shares = -50
+... ValueError ...
+>>>
+```
+
+This is pretty cool.  Descriptors have allowed you to greatly simplify the implementation
+of the `Stock` class.  This is the real power of descriptors--you get low level control
+over the dot and can use it to do amazing things.
+
+## (d) Fixing the Names
+
+One annoying thing about descriptors is the redundant name specification. For example:
+
+```python
+class Stock:
+    ...
+    shares = PositiveInteger('shares')
+    ...
+```
+
+We can fix that.  Change the top-level `Validator` class to include a `__set_name__()` method
+like this:
+
+```python
+# validate.py
+
+class Validator:
+    def __init__(self, name=None):
+        self.name = name
+
+    def __set_name__(self, cls, name):
+        self.name = name
+
+    @classmethod
+    def check(cls, value):
+        return value
+
+    def __set__(self, instance,	value):
+        instance.__dict__[self.name] = self.check(value)
+```
+
+Now, try rewriting your `Stock` class so that it looks like this:
+
+```python
+class Stock:
+    name   = String()
+    shares = PositiveInteger()
+    price  = PositiveFloat()
+
+    def __init__(self,name,shares,price):
+        self.name = name
+        self.shares = shares
+        self.price = price
+```
+
+Ah, much nicer.   Be aware that this ability to set the name is a Python 3.6
+feature however.  It won't work on older versions.
+
+\[ [Solution](soln4_3.md) | [Index](index.md) | [Exercise 4.2](ex4_2.md) | [Exercise 4.4](ex4_4.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)