6.9 KiB
[ Index | Exercise 3.4 | Exercise 3.6 ]
Exercise 3.5
Objectives:
- Learn how to use inheritance to write extensible code.
- See a practical use of inheritance by writing a program that must output data in a variety of user-selectable formats such as plain-text, CSV, and HTML.
Files Modified: tableformat.py
One major use of classes in Python is in writing code that be
extended/adapted in various ways. To illustrate, in
link:ex3_2.html[Exercise 3.2] you created a function
print_table() that made tables. You used this to make
output from the portfolio list. For example:
>>> import stock
>>> import reader
>>> import tableformat
>>> portfolio = reader.read_csv_as_instances('Data/portfolio.csv', stock.Stock)
>>> tableformat.print_table(portfolio, ['name','shares','price'])
name shares price
---------- ---------- ----------
AA 100 32.2
IBM 50 91.1
CAT 150 83.44
MSFT 200 51.23
GE 95 40.37
MSFT 50 65.1
IBM 100 70.44
>>> Suppose you wanted the print_table() function to be able
to make tables in any number of output formats such as CSV, XML, HTML,
Excel, etc. Trying to modify the function to support all of those output
formats at once would be painful. A better way to do this involves
moving the output-related formatting code to a class and using
inheritance to implement different output formats.
(a) Defining a generic formatter class
Add the following class definition to the tableformat.py
file:
class TableFormatter:
def headings(self, headers):
raise NotImplementedError()
def row(self, rowdata):
raise NotImplementedError()Now, modify the print_table() function so that it
accepts a TableFormatter instance and invokes methods on it
to produce output:
def print_table(records, fields, formatter):
formatter.headings(fields)
for r in records:
rowdata = [getattr(r, fieldname) for fieldname in fields]
formatter.row(rowdata)These two classes are meant to be used together. For example:
>>> import stock, reader, tableformat
>>> portfolio = reader.read_csv_as_instances('Data/portfolio.csv', stock.Stock)
>>> formatter = tableformat.TableFormatter()
>>> tableformat.print_table(portfolio, ['name', 'shares', 'price'], formatter)
Traceback (most recent call last):
...
NotImplementedError
>>>For now, it doesn’t do much of anything interesting. You’ll fix this in the next section.
(b) Implementing a concrete formatter
The TableFormatter isn’t meant to be used by itself.
Instead, it is merely a base for other classes that will implement the
formatting. Add the following class to tableformat.py:
class TextTableFormatter(TableFormatter):
def headings(self, headers):
print(' '.join('%10s' % h for h in headers))
print(('-'*10 + ' ')*len(headers))
def row(self, rowdata):
print(' '.join('%10s' % d for d in rowdata))Now, use your new class as follows:
>>> import stock, reader, tableformat
>>> portfolio = reader.read_csv_as_instances('Data/portfolio.csv', stock.Stock)
>>> formatter = tableformat.TextTableFormatter()
>>> tableformat.print_table(portfolio, ['name','shares','price'], formatter)
name shares price
---------- ---------- ----------
AA 100 32.2
IBM 50 91.1
CAT 150 83.44
MSFT 200 51.23
GE 95 40.37
MSFT 50 65.1
IBM 100 70.44
>>> (c) Adding More Implementations
Create a class CSVTableFormatter that allows output to
be generated in CSV format:
>>> import stock, reader, tableformat
>>> portfolio = reader.read_csv_as_instances('Data/portfolio.csv', stock.Stock)
>>> formatter = tableformat.CSVTableFormatter()
>>> tableformat.print_table(portfolio, ['name','shares','price'], formatter)
name,shares,price
AA,100,32.2
IBM,50,91.1
CAT,150,83.44
MSFT,200,51.23
GE,95,40.37
MSFT,50,65.1
IBM,100,70.44
>>>Create a class HTMLTableFormatter that generates output
in HTML format:
>>> import stock, reader, tableformat
>>> portfolio = reader.read_csv_as_instances('Data/portfolio.csv', stock.Stock)
>>> formatter = tableformat.HTMLTableFormatter()
>>> tableformat.print_table(portfolio, ['name','shares','price'], formatter)
<tr> <th>name</th> <th>shares</th> <th>price</th> </tr>
<tr> <td>AA</td> <td>100</td> <td>32.2</td> </tr>
<tr> <td>IBM</td> <td>50</td> <td>91.1</td> </tr>
<tr> <td>CAT</td> <td>150</td> <td>83.44</td> </tr>
<tr> <td>MSFT</td> <td>200</td> <td>51.23</td> </tr>
<tr> <td>GE</td> <td>95</td> <td>40.37</td> </tr>
<tr> <td>MSFT</td> <td>50</td> <td>65.1</td> </tr>
<tr> <td>IBM</td> <td>100</td> <td>70.44</td> </tr>
>>>(d) Making it Easier To Choose
One problem with using inheritance is the added complexity of picking
different classes to use (e.g., remembering the names, using the right
import statements, etc.). A factory function can simplify
this. Add a function create_formatter() to your
tableformat.py file that allows a user to more easily make
a formatter by specifying a format such as 'text',
'csv', or 'html'. For example:
>>> from tableformat import create_formatter, print_table
>>> formatter = create_formatter('html')
>>> print_table(portfolio, ['name','shares','price'], formatter)
<tr> <th>name</th> <th>shares</th> <th>price</th> </tr>
<tr> <td>AA</td> <td>100</td> <td>32.2</td> </tr>
<tr> <td>IBM</td> <td>50</td> <td>91.1</td> </tr>
<tr> <td>CAT</td> <td>150</td> <td>83.44</td> </tr>
<tr> <td>MSFT</td> <td>200</td> <td>51.23</td> </tr>
<tr> <td>GE</td> <td>95</td> <td>40.37</td> </tr>
<tr> <td>MSFT</td> <td>50</td> <td>65.1</td> </tr>
<tr> <td>IBM</td> <td>100</td> <td>70.44</td> </tr>
>>>Discussion
The TableFormatter class in this exercise is an example
of something known as an “Abstract Base Class.” It’s not something
that’s meant to be used directly. Instead, it’s serving as a kind of
interface specification for a program component–in this case the various
output formats. Essentially, the code that produces the table will be
programmed against the abstract base class with the expectation that a
user will provide a suitable implementation. As long as all of the
required methods have been implemented, it should all just “work”
(fingers crossed).
[ Solution | Index | Exercise 3.4 | Exercise 3.6 ]
>>> Advanced Python Mastery
... A course by dabeaz
... Copyright 2007-2023
.
This work is licensed under a Creative Commons
Attribution-ShareAlike 4.0 International License