Prac06: Modelling the World with Objects

Last updated on 2024-10-02 | Edit this page

Overview

Questions

  • How can we set up classes to represent real-world objects?
  • How do we write methods to communicate with our objects?
  • How can our code help to maintain a valid state in the object?

Objectives

  1. Understand the main concepts in object- oriented programming and their value
  2. Read and explain object-oriented code
  3. Apply and create simple object-oriented Python code

Introduction

In this practical you will write Python programs to implement the objects used as examples in the lecture. We will see how to create and manipulate objects, and how to group them in useful ways.

Activity 1 - Animals as Objects

We will keep our class definitions in separate python files. In your Practical 6 directory, edit animals.py with vim. Enter the class definition for Cat into animals.py.

If you try to run animals.py, what happens?

We need a driver program to test out our Cat class. Edit testCat.py and enter the following code:

PYTHON 

from animals import Cat

garfield = Cat(‘Garfield’, ‘1/1/1978’, ‘Orange’, ‘Tabby’)

garfield.printit()

print(garfield)

Note the difference in the output of garfield.printit() and print(garfield).

Activity 2. More Animals!

Edit animals.py and add in the class definitions for Dog and Bird. Create some test data and code in testAnimals.py to test all three classes. Use testCat.py and the lecture notes as a starting point.

Activity 3 - Even more animals!

Following the general outline for our programs, we will now create a program to read in animals from a file into a list of animal objects. The file will hold comma-separated values. Our algorithm will be:

  1. Openfile
  2. Read data from file into animal list
  3. Print animal list using printit() method

More detail of what we will need in our code is:

  1. Import classes
  2. Create variables
  3. Open file
  • For each line in the file
  • Create animal object using class matching first field in the entry
  • Add it to the animal list
  1. Print animal list

Have a look at animals.csv (below) to see how you can read in the file.

Dog,Dude,1/1/2011,Brown,Jack Russell
Cat,Oogie,1/1/2006,Grey,Fluffy
Bird,Big Bird,10/11/1969,Yellow,Canary
Cat,Garfield,1/1/1978,Orange,Tabby

Work through each part of the algorithm and test it out before moving on to the next step.

Extend the animals.csv file to include animals of your choice, then re-run your code.

Activity 4 - Building bank accounts

Type in the code from Version 3 of the Bank Accounts example in the lecture slides. The class definitions should be in accounts.py and the driver code in testAccounts.py. Test (run) the code to see that it works as shown in the lecture.

Activity 5 - Simulating bank account transactions

We are going write some new code banking.py to allow the user to choose a transaction, account and optional amount to run transactions on one account. This will be in a loop that also allows for printing all the current balance. We are working towards something similar to the bucketlist builder…

  1. Import classes
  2. Create variables
  3. build bank account object (‘Everyday’,‘007’,3000)
  4. Request transaction selection: withdrawal, deposit, interest, balance or exit. e.g. W for Withdrawal, D for deposit, I for interest, B for balance and X for eXit
  5. While exit is not selected
  6. If ‘W’ – ask for amount then call withdraw method
  7. Else if ‘D’ – ask for amount then call deposit method
  8. Else if ‘I’ – call interest method
  9. Else if ‘B’ – print balance
  10. Request transaction selection

Activity 6 - Improving bank account code

The code for requesting the transaction selection is quite cumbersome. Copy banking.py to banking2.py and add a new function: chooseTrans() which will:

  1. Prompt for the transaction selection
  2. Check that it is valid (while not valid, ask again)
  3. Return the choice

Update banking2.py to use the function in both places it is required.

Submission

Update the README file to include all files created in this practical.

All of your work for this week’s practical should be submitted via Blackboard using the Practical 06 link. This should be done as a single “zipped” file. Submit the resulting file through Blackboard. (refer to Practical 00 or 01 for instructions on zipping files.

There are no direct marks for these submissions, but they may be taken into account when finalising your mark for the unit. Go to the Assessment link on Blackboard and click on Practical 03 for the submission page.

And that’s the end of Practical 06!

Key Points

  • We can model real world items as objects
  • The template for an object is its class - which we can use to make lots of objects
  • Objects know their state (data) and behaviour (methods) and are responsible for choosing who can access their data, and how… we can trust them to maintain a valid state
  • The convention is to use a capital letter for the class name, and lowercase for variables (which can hold objects)
  • If we create a dog object d1 = Dog("Brutus"), we call its methods as d1.sit() and can access data as d1.name or through a method e.g. d1.getName(). Accessing the data directly is often easier, but can be risky…
    1. Could alter data and make it invalid
    2. Requires knowledge of the internal workings of the object - which is against OO principles

Reflection

  1. Knowledge: What is the difference between a class and an object?
  2. Comprehension: What is the difference in the code used to define a class, and the code used to define an object?
  3. Application: How would you write a new class to represent a rabbit?
  4. Analysis: What is the difference between garfield.printit() and print(garfield)? Which would you use and why?
  5. Synthesis: How would you modify the pet classes in animals.py to include a microchip number for cats and dogs?
  6. Evaluation: Task 5 takes in user input to determine the transactions and amounts if required. What parts of this should have validation to make the code more robust? (less likely to crash if given the wrong input)

Challenge

For those who want to explore a bit more of the topics covered in this practical. Note that the challenges are not assessed but may form part of the prac tests or exam.

  1. Add class variables and methods to Dog, Cat and Bird to represent how they move (self.moves), and to getMoves(). These will be class variables as they will apply to all objects of that class.
  2. Extend banking.py to have three bankaccounts in a list. Modify your code to ask for the account for each transaction