Defining classes

A class is defined with the keyword class. The syntax is as follows:

class NameOfClass:
    # class defition goes here

Classes are usually named in camel case. This means that all the words in the class name are written together, without spaces, and each word is capitalised. The following class names follow this convention:

• Weekday
• BankAccount
• LibraryDatabase
• PythonCourseGrades

A single class definition should represent a single whole, the contents of which should be atomically linked together in some way. In more complicated programs classes can contain members of other classes. For example, the class Course could contain objects of class Lecture, ExerciseSession etc.

Lets have a look at a skeleton of a class definition. The functionalities are still missing at this point.

class BankAccount:
    pass

The above piece of code tells Python that here we are defining a class named BankAccount. The class does not contain any functionality yet, but we can still create an object based on the class.

Lets have a look at a program where two variables are added to a BankAccount object: balance and owner. Any variables attached to an object are called its attributes, or more specifically, data attributes, or sometimes instance variables.

The attributes attached to an object can be accessed through the object:

class BankAccount:
    pass

peters_account = BankAccount()
peters_account.owner = "Peter Python"
peters_account.balance = 5.0

print(peters_account.owner)
print(peters_account.balance)

The data attributes are available only through the object they are attached to. Each BankAccount object created based on the BankAccount class has its own values attached to the data attributes. Those values can be accessed by referring to the object in question:

account = BankAccount()
account.balance = 155.50

print(account.balance) # This refers to the data attribute balance attached to the account
print(balance) # THIS CAUSES AN ERROR, as there is no such independent variable available, and the object reference is missing

Adding a constructor

In the above example we saw that a new instance of a class can be created by calling the constructor method of the class like so: NameOfClass(). Above we then attached data attributes to the object separately, but it is often more convenient to pass these initial values of attributes directly as the object is created. In the above example we first had a BankAccount object without these attributes, and the attributes only existed after they were explicitly declared.

Declaring attributes outside the constructor results in a situation where different instances of the same class can have different attributes. The following code produces an error because we now have another BankAccount object, paulas_account, which does not contain the same attributes:

class BankAccount:
    pass

peters_account = BankAccount()
peters_account.owner = "Peter"
peters_account.balance = 1400

paulas_account = BankAccount()
paulas_account.owner = "Paula"

print(peters_account.balance)
print(paulas_account.balance) # THIS CAUSES AN ERROR

So, instead of declaring attributes after each instance of the class is created, it is usually a better idea to initialize the values of the attributes as the class constructor is called. As the BankAccount class definition is currently just a skeleton, the constructor method is implicitly assumed by the Python interpreter, but it is possible to define your own constructor methods, and that is exactly what we will do now.

A constructor method is a method declaration with the special name __init__, usually included at the very beginning of a class definition.

Lets have a look at a BankAccount class with a constructor method added in:

class BankAccount:

    # The constructor
    def __init__(self, balance: float, owner: str):
        self.balance = balance
        self.owner = owner

The name of the constructor method is always __init__. Notice the two underscores on both sides of the word init.

The first parameter in a constructor definition is always named self. This refers to the object itself, and is necessary for declaring any attributes attached to the object. The assignment

self.balance = balance

assigns the balance received as an argument to the balance attribute of the object. It is a common convention to use the same variable names for the parameters and the data attributes defined in a constructor, but the variable names self.balance and balance above refer to two different variables:

• The variable self.balance is an attribute of the object. Each BankAccount object has its own balance.

• The variable balance is a parameter in the constructor method __init__. Its value is set to the value passed as an argument to the method as the constructor is called (that is, when a new instance of the class is created).

Now that we have defined the parameters of the constructor method, we can pass the desired initial values of the data attributes as arguments as a new object is created:

class BankAccount:

    # The constructor
    def __init__(self, balance: float, owner: str):
        self.balance = balance
        self.owner = owner

# As the method is called, no argument should be given for the self parameter
# Python assigns the value for self automatically
peters_account = BankAccount(100, "Peter Python")
paulas_account = BankAccount(20000, "Paula Pythons")

print(peters_account.balance)
print(paulas_account.balance)

It is now much easier to work with the BankAccount objects, as the values can be passed at object creation, and the resulting two separate instances can be handled more predictably and uniformly. Declaring data attributes in the constructor also ensures the attributes are actually declared, and the desired initial values are always given by the programmer using the class.

It is still possible to change the initial values of the data attributes later in the program:

class BankAccount:

    # The constructor
    def __init__(self, balance: float, owner: str):
        self.balance = balance
        self.owner = owner

peters_account = BankAccount(100, "Peter Python")
print(peters_account.balance)

# Change the balance to 1500
peters_account.balance = 1500
print(peters_account.balance)

# Add 2000 to the balance
peters_account.balance += 2000
print(peters_account.balance)

Let's have a look at another example of classes and objects. We'll write a class which models a single draw of lottery numbers:

from datetime import date

class LotteryDraw:

    def __init__(self, round_week: int, round_date: date, numbers: list):
        self.round_week = round_week
        self.round_date = round_date
        self.numbers = numbers


# Create a new LotteryDraw object
round1 = LotteryDraw(1, date(2021, 1, 2), [1,4,8,12,13,14,33])

# Tulostetaan tiedot
print(round1.round_week)
print(round1.round_date)

for number in round1.numbers:
    print(number)

As you can see above, the attributes can be of any type. Here, each LotteryDraw object has attributes of type list and date.

Using objecs formed from your own classes

Objects formed from your own class definitions are no different from any other Python objects. They can be passed as arguments and return values just like any other object. We could, for example, write some helper functions for working with bank accounts:

# this function creates a new bank account object and returns it
def open_account(name: str):
    new_account =  BankAccount(0, name)
    return new_account

# this function adds the amount passed as an argument to the balance of the bank account also passed as an argument
def deposit_money_on_account(account: BankAccount, amount: int):
    account.balance += amount

peters_account = open_account("Peter Python")
print(peters_account.balance)

deposit_money_on_account(peters_account, 500)

print(peters_account.balance)

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