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Exercises/ex3_4.md

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+\[ [Index](index.md) | [Exercise 3.3](ex3_3.md) | [Exercise 3.5](ex3_5.md) \]
+
+# Exercise 3.4
+
+*Objectives:*
+
+- Learn how to encapsulate object internals using private
+  attributes, properties, and slots
+
+*Files Modified:* `stock.py`
+
+## (a) Private attributes
+
+As a general rule, attributes that are internal to a class should have a leading underscore.
+In the previous exercise, the `Stock` class had a `types` class variable that was
+used for converting rows of data.  Change the code so that this variable has a leading
+underscore on it.
+
+## (b) Properties for computed attributes
+
+Earlier, you defined a class `Stock`.  For example:
+
+```python
+>>> s = Stock('GOOG',100,490.10)
+>>> s.name
+'GOOG'
+>>> s.shares
+100
+>>> s.price
+490.1
+>>> s.cost()
+49010.0
+>>>
+```
+
+Using a property, turn `cost()` into an attribute that no longer requires the parentheses.   For example:
+
+```python
+>>> s = Stock('GOOG', 100, 490.1)
+>>> s.cost               # Property. Computes the cost
+49010.0
+>>> 
+```
+
+## (c) Enforcing Validation Rules
+
+Using properties and private attributes, modify the `shares` attribute
+of the `Stock` class so that it can only be assigned a non-negative
+integer value.    In addition, modify the `price` attribute so that it
+can only be assigned a non-negative floating point value.
+
+The new object should work almost exactly the same as
+the old one except for extra type and value checking.
+
+```python
+>>> s = Stock('GOOG', 100, 490.10)
+>>> s.shares = 50          # OK
+>>> s.shares = '50'
+Traceback (most recent call last):
+...
+TypeError: Expected integer
+>>> s.shares = -10
+Traceback (most recent call last):
+...
+ValueError: shares must be >= 0
+
+>>> s.price = 123.45       # OK
+>>> s.price = '123.45'
+Traceback (most recent call last):
+...
+TypeError: Expected float
+>>> s.price = -10.0
+Traceback (most recent call last):
+...
+ValueError: price must be >= 0
+>>>
+```
+
+## (d) Adding `__slots__`
+
+Modify your new `Stock` class to use `__slots__`.  You will find that
+you have to use a different set of attribute names than
+before--specifically, you will have to list the private attribute
+names (e.g., if a property is storing a value in an attribute
+`_shares`, that is the name you list in `__slots__`).  Verify that the
+class still works and that you can no longer add new attributes.
+
+```python
+>>> s = Stock('GOOG', 100, 490.10)
+>>> s.spam = 42
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+AttributeError: 'Stock' object has no attribute 'spam'
+>>>
+```
+
+## (e) Reconciling Types
+
+In the current `Stock` class, there is a `_types` class variable
+that gives conversions when reading from a file, but there are also
+properties that are enforcing types.  Who is in charge of this show?
+Fix the property definitions so that they use the types specified
+in the `_types` class variable.  Make sure the properties work
+when types are changed via subclassing.   For example:
+
+```python
+>>> from decimal import Decimal
+>>> class DStock(Stock):
+        _types = (str, int, Decimal)
+
+>>> s = DStock('AA', 50, Decimal('91.1'))
+>>> s.price = 92.3
+Traceback (most recent call last):
+...
+TypeError: Expected a Decimal
+>>> 
+```
+
+**Discussion**
+
+The resulting `Stock` class at the end of this exercise is a muddled
+mess of properties, type checking, constructors, and other details.
+Imagine how unpleasant it would be to maintain code that featured
+dozens or hundreds of such class definitions.
+
+We're going to figure out how to simplify things considerably, but it's
+going to take some time and some more advanced techniques. Stay tuned.
+
+\[ [Solution](soln3_4.md) | [Index](index.md) | [Exercise 3.3](ex3_3.md) | [Exercise 3.5](ex3_5.md) \]
+
+----
+`>>>` 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/)