Class Example

Classes in Python (1)

Creating a Class

• Classes will be defined using the class keyword followed by the name you want to give it

class Student:
    name = "Ian"

Creating an Object

• An object can be created from the class by calling upon the class name
  • we first create a variable for this class object
  • then we call the class name followed by brackets

student1 = Student()

• The variable name can then be accessed
  • by calling the variable name at the end of the object

Using Class Constructors

• All classes consist of an in-built function which is used to execute code when it is being initiated
  • this is the function known as __init__()
• This initialiser can be used to assign values to an objects properties,
  • or other operations that are necessary to perform when an object is in the process of being created
• __init__() is called automatically each time the class has been used to create a new object

class Student:
    def __init__(self, name, age):
        self.name = name
        self.age = age

student1 = Student("Ian", 33)
student2 = Student("Terry", 1)

Class Functions

• Classes can also consist of functions, and these will belong to the object that is created

class Student:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def greeting(self):
        return "Hello " + self.name + " and welcome to 4061CEM!"

student1 = Student("Ian", 33)

Modifying an Objects Properties (1)

• Objects can be modified by accessing the variables directly
  • this is not a very good method of doing this

class Student:
    def __init__(self, name, age):
        self.name = name
        self.age = age

student1 = Student("Ian", 33)
student1.age = -1

Modifying an Objects Properties (2)

• A better way of modifying a classes variable is by creating a function

class Student:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def change_age(self, new_age):
        self.age = new_age

student1 = Student("Ian", 33)
student1.change_age(-1)

Modifying an Objects Properties (3)

• Variables and functions can be private inside a class
• This is achieved by adding two underscores (‘__’) to the beginning of the variable name

class Student:
    def __init__(self, name, age):
        self.__name = name
        self.__age = age
    def change_age(self, new_age):
        self.__age = new_age
    def get_age(self):
        return self.__age

student1 = Student("Ian", 33)
student1.change_age(-1)

Modifying an Objects Properties (4)

• Objects can be deleted by using the del keyword

del student1

Pass Keyword

• Classes cannot be empty, but if you require a class to be empty you can use the pass keyword

class Student:
    pass

Creating a Parent Class

• Any class you create in Python can be a parent class
  • the syntax is the same as creating any other class

class Student:
    def __init__(self, name, age):
        self.__name = name
        self.__age = age
    def change_age(self, new_age):
        self.__age = new_age
    def get_age(self):
        return self.__age
    def get_name(self):
        return self.__name
    def greeting(self):
        return f"Hello {self.__name} and welcome to 4061CEM!"

student1 = Student("Ian", 33)

Creating a Child Class

• A child class is created by passing through the parent class as parameter when creating the child class
  • the child class will inherit all properties and functions of the parent class

class Person(Student):
    pass

person1 = Person("Ian", 33)

Modifying a Child Class (1)

• When adding an __init__ function to the child class, it will no longer inherit the __init__ function from the parent class
• To keep the variables from the parent class, we need to call the __init__ function from the parent class

class Person(Student):
    def __init__(self, name, age, location):
        Student.__init__(self, name, age)
        self.location = location

person1 = Person("Ian", 33, "Coventry")

Modifying a Child Class (2)

• The same process can be achieved using the super() function, instead of using the parent classes name

class Person(Student):
    def __init__(self, name, age, location):
        super().__init__(name, age)
        self.location = location

person1 = Person("Ian", 33, "Coventry")

Adding a Function to a Child Class

• Functions can be added to the child class in the same manner
  • let’s add a function to greet the person and acknowledge where they have come from

class Person(Student):
    def __init__(self, name, age, location):
        super().__init__(name, age)
        self.location = location
    def greeting(self):
        return f"Hello {self.get_name()} and welcome to 4061CEM from {self.location}!"

person1 = Person("Ian", 33, "Coventry")