08 Classes

Question 1

What is object-oriented programming (OOP)?

Answer 1

It’s a way of programming based on the concept of objects. That increases reusability and modularity.

Question 2

In Python, what is an object?

Answer 2

A combination of variables, functions and data structures. It can contain data (a state) and methods (behavior).

Question 3

What are attributes in a Python class?

Answer 3

They describe a state of an object, i.e. the size, the amount, the colour,…

Question 4

What are methods in a Python class?

Answer 4

They describe the behavior of an object, i.e. what it does, how it grows,…

Question 5

Using the example of the class “Dog”, what could be some fitting attributes?

Answer 5

name, fur colour, size, age, race, …

Question 6

Using the example of the class “Dog”, what could be some fitting methods?

Answer 6

wagTail()
bark()
givePaw()
eatKnibble()
…

Question 7

Use a house and its blue-print to give an example on how classes and objects work.

Answer 7

When you create a class “House”, this would be a blue-print you draw for a house you want to build.
The class contains attributes like “numberOfWindows” and “numberOfRooms”.
It also contains methods like “calculateCostOfLiving(numberOfPersonsLivingHere)”.

You can use this blue-print to actually build a house, this would be the object of the class. It exists uniquely among other objects/houses.

You can use the same blue-print to build another house, then you have two objects/houses. If you give them both the same number of rooms and windows, then they look the same. They have the same attributes and methods but two unique addresses (because you can’t put the second house on the same address where the first is already standing).

But you can also use the blue-print to build the same house with a different amount of rooms, turn it into a skyscraper. It’s still a house (still of the same class/type) but it looks a bit different.

Question 8

What are subclasses/child classes in object-oriented programming (OOP)?

Answer 8

A class that inherits attributes and method definitions from another class (superclass/base class/parent class).
These attributes and methods can be changed or extended without having an effect on the attributes and methods of the superclass.

Question 9

Using the example superclass “Animal”, what could be fitting subclasses?

Answer 9

Dog, Cat, Bird, Dinosaur, Elephant,..

Question 10

What’s the syntax to create a new class?

Answer 10

class ClassName:
attribute_1 = “value”

    def \_\_init\_\_(self, attr_2):
          self.attr_2 = attr_2
   
    def method(self):
          #do smth

Question 11

Using the following example, match the various terms to their name.

class Dog:
species = “canine”

 def \_\_init\_\_(self, name):
     self.name = name
  
 def speak(self):
     print(f"{self.name}: woof!")

1. Dog
2. species = “canine”
3. def __init__(self, name)
4. self.name = name
5. def speak(self)
6. {self.name}

Answer 11

1. class name
2. class attribute (has the same value for all objects)
3. special method for object initialization
4. object attribute (is individually set for each object)
5. method (is called by an object of this class)
6. placeholder that accesses the above created object attribute “name”

Question 12

Using this class “Dog” as an example, what’s the code to access the species?

class Dog:
species = “canine”

 def \_\_init\_\_(self, name):
     self.name = name
  
 def speak(self):
     print(f"{self.name}: woof!")

Answer 12

Dog.species

Question 13

Using this class “Dog” as an example, what’s the code to access the name?

class Dog:
species = “canine”

 def \_\_init\_\_(self, name):
     self.name = name
  
 def speak(self):
     print(f"{self.name}: woof!")

Answer 13

my_dog = Dog(“Rex”)
my_dog.name

Question 14

Using this class “Dog” as an example, what’s the code to access the method and what is its output?

class Dog:
species = “canine”

 def \_\_init\_\_(self, name):
     self.name = name
  
 def speak(self):
     print(f"{self.name}: woof!")

Answer 14

my_dog = Dog(“Rex”)
my_dog.speak()

output:
Rex: woof!

Question 15

What’s the syntax to extend a base class (create a subclass)?

Answer 15

class SubClass(SuperClass)

Question 16

Using this class “Dog” as an example, what’s the code to creating a subclass “GuardDog” with the additional attribute “power”?

class Dog:
species = “canine”

 def \_\_init\_\_(self, name):
     self.name = name
  
 def speak(self):
     print(f"{self.name}: woof!")

Answer 16

class GuardDog(Dog):

  def \_\_init\_\_(self, power):
       super().\_\_init\_\_(name)
       self.power = power

Question 17

True or False: Python supports multiple inheritances.

Answer 17

True

Question 18

What is a Graphical Class Notation?

Answer 18

It’s a class diagram using UML (unified modeling language). It shows a class as a table with the name in the title and the attributes and methods listed below. Arrows show inheritances to other class diagrams.

Question 19

What is the method resolution order (MRO) and how does it work for this example?

my_object.some_method()

Answer 19

It determines which method is invoked when inheritence is involved.
It works like a bottom-to-top search:
1. get the class of my_object
2. does the class contain an implementation for some_method() ?
3. if yes: execute this method
if no: get the parent class and start again from step 2
4. repeat until the first implementation is found

Question 20

What built-in function returns the class of an object?

Answer 20

type(object)

Question 21

What built-in function checks if x is an instance of class y or an instance of a subclass of y?

Answer 21

isinstance(x, y)

Question 22

What built-in function checks if x is a subclass of class y?

Answer 22

issubclass(x, y)