Exercise 5.1
(a) Objects as Data Structures
In section 2 and 3, we worked with data represented as tuples and dictionaries. For example, a holding of stock could be represented as a tuple like this:
s = ('GOOG',100,490.10)
or as a dictionary like this:
s = { 'name' : 'GOOG',
'shares' : 100,
'price' : 490.10
}
Another approach for representing data would be to define a class.
Create a file called stock.py and define a class Stock that
represents a single holding of stock. Have the instances of Stock
have name, shares, and price attributes. For example:
>>> ================================ RESTART ================================
>>> import stock
>>> s = stock.Stock('GOOG',100,490.10)
>>> s.name
'GOOG'
>>> s.shares
100
>>> s.price
490.1
>>>
Create a few more Stock objects and manipulate them. For example:
>>> a = stock.Stock('AAPL',50,122.34)
>>> b = stock.Stock('IBM',75,91.75)
>>> a.shares*a.price
6117.0
>>> b.shares*b.price
6881.25
>>> stocks = [a,b,s]
>>> stocks
[<stock.Stock object at 0x37d0b0>, <stock.Stock object at 0x37d110>, <stock.Stock object at 0x37d050>]
>>> for t in stocks:
print '%10s %10d %10.2f' % (t.name, t.shares, t.price)
... look at the output ...
>>>
One thing to emphasize here is that the class Stock acts like a
factory for creating instances of objects. Basically, you just call
it as a function and it creates a new object for you. Also, it needs
to be emphasized that each object is distinct---they each have their
own data that is separate from other objects that have been created.
An object defined by a class is somewhat similar to a dictionary—just
with somewhat different syntax. For example, instead of writing
s['name'] or s['price'], you now write s.name and s.price.
(b) Reading Data into a List of Objects
In your stock.py program, write a function read_portfolio(filename) that
reads portfolio data from a file into a list of Stock objects. This function
is going to mimic the behavior of earlier code you have written.
Here’s how your function will behave:
>>> ================================ RESTART ================================
>>> import stock
>>> portfolio = stock.read_portfolio('Data/portfolio.csv')
>>> portfolio
[<stock.Stock object at 0x81d70>, <stock.Stock object at 0x81cf0>, <stock.Stock object at 0x81db0>,
<stock.Stock object at 0x81df0>, <stock.Stock object at 0x81e30>, <stock.Stock object at 0x81e70>,
<stock.Stock object at 0x81eb0>]
>>>
It is important to emphasize that read_portfolio() is a top-level
function, not a method of the Stock class. This function is merely
creating a list of Stock objects; it’s not an operation on an
individual Stock instance.
Try performing some calculations with the above data. First, try printing a formatted table:
>>> for s in portfolio:
print '%10s %10d %10.2f' % (s.name, s.shares, s.price)
... look at the output ...
>>>
Try a list comprehension:
>>> more100 = [s for s in portfolio if s.shares > 100]
>>> for s in more100:
print '%10s %10d %10.2f' % (s.name, s.shares, s.price)
... look at the output ...
>>>
Again, notice the similarity between Stock objects and
dictionaries---they’re basically the same idea, but the syntax for
accessing values differs.
(c) Adding some Methods
With classes, you can attach functions to your objects. These are
known as methods and are simply functions that operate on the data
stored inside an object. Add a cost() and sell() method to your
Stock object. They should work like this:
>>> ================================ RESTART ================================
>>> import stock
>>> s = stock.Stock('GOOG',100,490.10)
>>> s.cost()
49010.0
>>> s.shares
100
>>> s.sell(25)
75
>>> s.shares
75
>>> s.cost()
36757.5
>>>