Exercise 6.2
(a) Simple Properties
Properties are a useful way to add "computed attributes" to an object.
In Exercise 5.1, you created an object Stock. Notice that on your
object there is a slight inconsistency in how different kinds of data
are extracted:
>>> from stock import Stock
>>> s = Stock('GOOG', 100, 490.1)
>>> s.shares
100
>>> s.price
490.1
>>> s.cost()
49010.0
>>>
Specifically, notice how you have to add the extra () to cost because it is a method.
You can get rid of the extra () on cost() if you turn it into a property.
Take your Stock class and modify it so that the cost calculation works like this:
>>> s.cost
49010.0
>>>
Try calling s.cost() as a function and observe that it
doesn’t work now that cost has been defined as a property.
>>> s.cost()
... fails ...
>>>
(b) Properties and Setters
Modify the shares attribute so that the value is stored in a
private attribute and that a pair of property functions are used to ensure
that it is always set to an integer value. Here is an example of the expected
behavior:
>>> s = Stock('GOOG',100,490.10)
>>> s.shares = 50
>>> s.shares = 'a lot'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: expected an integer
>>>
(c) Adding slots
Modify the Stock class so that it has a __slots__ attribute. Then,
verify that new attributes can’t be added:
>>> ================================ RESTART ================================
>>> from stock import Stock
>>> s = Stock('GOOG', 100, 490.10)
>>> s.name
'GOOG'
>>> s.blah = 42
... see what happens ...
>>>
When you use __slots__, Python actually uses a more efficient
internal representation of objects. What happens if you try to
inspect the underlying dictionary of s above?
>>> s.__dict__
... see what happens ...
>>>
It should be noted that __slots__ is most commonly used as an
optimization on classes that serve as data structures. Using slots
will make such programs use far-less memory and run a bit faster.