Worksheet

A1.7 Classes and objects

Programming techniques and object-oriented programming · A level · OCR H446 1.2.4, AQA 7517 4.1.2.3, Eduqas A500QS 1.4 · about 20 min

What this lesson is about

Classes, objects, attributes, methods and constructors; encapsulation, access specifiers, getters and setters, and class diagrams.

• Class, object, attribute, method
• Encapsulation and information hiding
• Class diagrams
• In the exam languages

Questions 5 marks in all

1. [1 mark]What is instantiation?

   1. ACreating an object from a class
   2. BDefining the methods of a class
   3. CHiding attributes from other code
   4. DMaking a subclass

2. [1 mark]What does this program print?

   class Counter:
       def __init__(self, start):
           self.__count = start
   
       def up(self):
           self.__count = self.__count + 1
           return self.__count
   
   a = Counter(0)
   b = Counter(10)
   a.up()
   a.up()
   print(a.up(), b.up())

3. [1 mark]In a class diagram, what does the symbol - before an attribute mean?

   1. APrivate
   2. BPublic
   3. CProtected
   4. DStatic

4. [1 mark]Which are reasons for making an attribute private and providing a setter method?

   Tick every answer that is true.

   1. AThe setter can refuse invalid values
   2. BThe way the value is stored can change without breaking code that uses the class
   3. COther code can change the attribute directly
   4. DThe object cannot be put into an invalid state from outside

5. [1 mark]What is the name of the method that runs automatically when an object is created, to give its attributes their starting values?

The task: the odometer class

Write the class Odometer from the class diagram. - The constructor __init__(self, name) stores name (a string) in the private attribute self.__name and sets the private attribute self.__total to 0. - drive(self, cm) takes a whole number of cm. If cm is 0 or more it drives forward cm; if it is negative it drives backward by -cm. Either way it adds the size of the move, abs(cm), to self.__total. - get_total(self) returns the total. - report(self) returns the string <name>: <total> cm. Then make two objects, trip_a = Odometer("trip A") and trip_b = Odometer("trip B"), and call trip_a.drive(20), trip_b.drive(-10) and trip_a.drive(15) in that order. Print trip_a.report() then trip_b.report(), which should show trip A: 35 cm and trip B: 10 cm.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

class Odometer:
    def __init__(self, name):
        self.__name = name

trip_a = Odometer("trip A")

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/a1-7-classes-and-objects/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Add reset(self). Should it be public or private? Draw the new class diagram.
2. Try print(trip_a.__total) outside the class. Then try print(trip_a._Odometer__total). What does that tell you about how private Python's privacy is?
3. Add a setter for the name that refuses an empty string with a ValueError.