[ Index | Exercise 3.3 | Exercise 3.5 ]
Objectives:
- Learn how to encapsulate object internals using private attributes, properties, and slots
Files Modified: stock.py
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.
Earlier, you defined a class Stock. For example:
>>> 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:
>>> s = Stock('GOOG', 100, 490.1)
>>> s.cost # Property. Computes the cost
49010.0
>>> 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.
>>> 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
>>>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.
>>> 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'
>>>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:
>>> 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.
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