How to Use SELF in Python

How to Use SELF in Python _ MediaOne Singapore

Python, oh Python! It’s a language loved by many for its simplicity and flexibility. One of the key features that sets Python apart is its support for object-oriented programming (OOP).

Understanding OOP and its concepts is crucial for building modular and reusable code. And one of the fundamental elements of OOP in Python is the SELF keyword.

In this blog post, we’ll explore how to use SELF in Python and harness its power to create elegant and efficient code. So, let’s dive in!

Understanding the SELF Keyword

In Python, the SELF keyword is used to refer to the instance of a class within that class. It allows you to access the attributes and methods of an object from within the class itself. When a method is called on an instance, SELF is implicitly passed as the first argument to the method.

The SELF keyword is not a reserved word in Python; it’s just a convention. However, it’s widely adopted and recommended to use SELF as the first parameter name in instance methods.

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Defining a Class and Accessing Attributes with SELF

To grasp the concept of SELF, let’s start by creating a simple class and defining some attributes:

class Car:
def __init__(self, make, model, year):
self.make = make
self.model = model
self.year = year
def get_details(self):
return f”This is a {self.year} {self.make} {self.model}.”

In the example above, we define a class called “Car” with three attributes: “make,” “model,” and “year.” The __init__ method is a special method called the constructor, which is executed when a new instance of the class is created. The SELF parameter allows us to assign the values passed to the constructor to the object’s attributes.

To access these attributes within the class, we use SELF followed by a dot and the attribute name. In the get_details method, we construct a string that includes the values of the “year,” “make,” and “model” attributes using SELF.

Creating an Instance and Using the SELF Keyword

Now that we have our class defined, let’s create an instance of the Car class and see how we can use the SELF keyword:

my_car = Car("Tesla", "Model S", 2023)


This is a 2023 Tesla Model S.

In the code snippet above, we create a new instance of the Car class called “my_car” and pass the arguments “Tesla,” “Model S,” and 2023 to the constructor. We then call the get_details method on the “my_car” instance, which utilizes the SELF keyword to access the instance attributes and construct the string.

By using SELF, we can access the attributes specific to each instance of the class. It allows us to create multiple objects with different attribute values, making our code more flexible and scalable.

Modifying Attributes with SELF

In addition to accessing attributes, SELF also enables us to modify them within the class. Let’s extend our Car class to include a method that updates the year attribute:

class Car:
def __init__(self, make, model, year):
self.make = make
self.model = model
self.year = year
def get_details(self):
returnf”This is a {self.year} {self.make} {self.model}.”
def update_year(self, new_year):
self.year = new_year
return f"The year of the car has been updated to {self.year}."

In the updated version of the Car class, we've added a new method called `update_year` that takes a new year as an argument. Inside the method, we assign the new year value to the `year` attribute using SELF. By doing so, we can modify the attribute value of a specific instance without affecting other instances of the class.
Let‘s see how this works in practice:“`python
my_car = Car(“Tesla”, “Model S”, 2023)
print(my_car.get_details()) # Output: This is a 2023 Tesla Model S.

print(my_car.get_details()) # Output: This is a 2024 Tesla Model S.

By calling the update_year method on the my_car instance and passing 2024 as the argument, we update the year attribute. When we print the car details again, we can see that the year has been successfully modified.

The SELF keyword allows us to access and modify instance-specific attributes within the class methods, enabling dynamic and interactive behavior in our code.

Using SELF in Method Calls

Apart from accessing attributes, the SELF keyword also plays a crucial role when calling other methods within a class. Let’s enhance our Car class by adding a method called start_engine, which calls another method called check_fuel:

class Car:
def __init__(self, make, model, year):
self.make = make
self.model = model
self.year = year
def get_details(self):
return f”This is a {self.year} {self.make} {self.model}.”def update_year(self, new_year):
self.year = new_year
return f”The year of the car has been updated to {self.year}.”

def check_fuel(self):
return “Fuel level is sufficient. You can start the engine.”

def start_engine(self):
return “Engine started successfully.”

In the modified version of the Car class, we’ve added two new methods: check_fuel and start_engine. The start_engine method calls the check_fuel method using SELF.

Let’s see the SELF keyword in action:

my_car = Car("Tesla", "Model S", 2023)


Fuel level is sufficient. You can start the engine.
Engine started successfully.

When we call the start_engine method on the my_car instance, it first executes the check_fuel method using SELF. The SELF keyword allows the start_engine method to access and call other methods within the same class. This helps organize our code and promotes code reusability.

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Inheritance and SELF

Inheritance is another crucial concept in object-oriented programming that allows us to create new classes based on existing ones. When it comes to inheritance and using SELF, things work a little differently.

Consider the following example:

class ElectricCar(Car):
def charge_battery(self):
return "Battery charging in progress."
def start_engine(self):
return “Engine cannot be started. Use the power button instead.”

In this example, we define a new class called ElectricCar that inherits from the Car class. The ElectricCar class introduces two new methods: charge_battery and start_engine`.

The charge_battery method is specific to electric cars and returns a string indicating that the battery is charging. The start_engine method overrides the method with the same name in the parent class. Inside the overridden method, we still use SELF to call the check_fuel method, but we modify the return statement to indicate that the engine cannot be started in an electric car.

Let’s see how this works in practice:

my_electric_car = ElectricCar("Tesla", "Model 3", 2023)
print(my_electric_car.start_engine()) # Output: Engine cannot be started. Use the power button instead.
print(my_electric_car.charge_battery()) # Output: Battery charging in progress.

In the code snippet above, we create an instance of the ElectricCar class called my_electric_car. When we call the start_engine method on this instance, it calls the overridden method in the ElectricCar class, which in turn calls the check_fuel method using SELF. The output message indicates that the engine cannot be started and suggests using the power button instead.

The SELF keyword works seamlessly in the context of inheritance, allowing us to access and override methods in parent classes while maintaining the necessary context within the class.

Exploring Advanced Usages of SELF in Python

Now that you have a solid foundation in using the SELF keyword in Python, let’s dive deeper into some advanced techniques and use cases where SELF can be incredibly powerful. These techniques will help you write more expressive and flexible code. So let’s get started!

  1. Chaining Method Calls

One of the advantages of using SELF is the ability to chain method calls. This technique allows you to call multiple methods on an instance in a sequential manner. Each method call modifies the instance and returns SELF, enabling you to chain the next method call. This can lead to more concise and readable code.

Here’s an example to demonstrate method chaining:

class Calculator:
def __init__(self, value):
self.value = value
def add(self, num):
self.value += num
return selfdef multiply(self, num):
self.value *= num
return self

def subtract(self, num):
self.value -= num
return self

def divide(self, num):
self.value /= num
return self

def display(self):

In the Calculator class above, we have defined several methods to perform arithmetic operations on the value attribute. Each method modifies the value attribute and returns SELF, allowing method chaining.

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Let’s see how we can use method chaining with the Calculator class:

calc = Calculator(10)
calc.add(5).multiply(2).subtract(3).divide(4).display() # Output: 6.25

In the example above, we create an instance of the Calculator class with an initial value of 10. By chaining the methods add, multiply, subtract, and divide one after another, we perform a series of arithmetic operations on the value attribute. Finally, we call the display method to print the result.

By leveraging method chaining, you can write more expressive and concise code, especially when dealing with a series of operations or transformations on an object.

  1. Accessing Class Attributes

The SELF keyword is primarily used to access instance attributes, but it can also be used to access class attributes within instance methods. Class attributes are variables that are shared among all instances of a class. By using SELF, you can access and modify these shared attributes.

Let’s illustrate this with an example:

class Circle:
pi = 3.14159 # Class attribute
def __init__(self, radius):
self.radius = radiusdef calculate_area(self):
area = Circle.pi * (self.radius ** 2)
return area

def calculate_circumference(self):
circumference = 2 * Circle.pi * self.radius
return circumference

In the Circle class above, we have a class attribute called pi, which represents the mathematical constant π. The instance methods calculate_area and calculate_circumference use SELF to access the pi class attribute and perform calculations based on the instance’s radius.

Let’s create an instance of the Circle class and calculate its area and circumference:

circle = Circle(5)
print(circle.calculate_area()) # Output: 78.53975
print(circle.calculate_circumference()) # Output: 31.4159
In the example above, we create a Circle instance with a radius of 5. By calling the calculate_area and calculate_circumference methods, we access the class attribute pi using SELF and perform the necessary calculations. This allows us to leverage the shared attribute `pic and perform consistent calculations across all instances of the class.
  1. Using SELF in Static Methods
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In Python, static methods are methods that belong to a class rather than an instance. They are not bound to any specific instance and do not have access to instance attributes or the SELF keyword. However, you can still use SELF in static methods, but with a different purpose.

Let’s look at an example to understand the use of SELF in static methods:

class MathUtils:
def add_numbers(num1, num2):
return num1 + num2
def multiply_numbers(num1, num2):
return num1 * num2 @staticmethod
def calculate_average(numbers):
return sum(numbers) / len(numbers)

def calculate_hypotenuse(side1, side2):
return (side1 ** 2 + side2 ** 2) ** 0.5

In the MathUtils class, we have defined several static methods that perform mathematical operations. These methods do not require an instance of the class to be called and can be accessed directly from the class itself. However, we can still use the SELF keyword within static methods to refer to the class itself.

Let’s see an example of using the SELF keyword in a static method:

print(MathUtils.calculate_average([5, 8, 10, 12])) # Output: 8.75
print(MathUtils.calculate_hypotenuse(3, 4)) # Output: 5.0

In the example above, we directly call the static methods calculate_average and calculate_hypotenuse using the class name MathUtils. Even though the SELF keyword is used within the static methods, it refers to the class itself. The SELF keyword in static methods is an alternate way to refer to the class, similar to using the class name directly.

By using SELF in static methods, you can create more cohesive and reusable code, especially when the static method needs to reference other static methods or class attributes.

  1. Using SELF in Class Methods

In addition to instance methods and static methods, Python also supports class methods. Class methods are methods that are bound to the class rather than the instance. They receive the class itself as the first argument, conventionally named “cls” (short for “class”).

Using the SELF keyword in class methods allows you to access class attributes and modify them. It provides a convenient way to operate on class-level data and perform actions specific to the class itself.

Let’s see an example of using SELF in a class method:

class BankAccount:
total_accounts = 0
def __init__(self, balance):
self.balance = balance
BankAccount.total_accounts += 1 @classmethod
def get_total_accounts(cls):
return cls.total_accounts

def deposit(self, amount):
self.balance += amount

def withdraw(self, amount):
if amount <= self.balance:
self.balance -= amount
print(“Insufficient funds.”)

def display_balance(self):
print(f”Current balance: £{self.balance})

In the BankAccount class above, we have a class attribute called total_accounts, which keeps track of the total number of bank accounts created. The class method get_total_accounts uses SELF (in this case, “cls”) to access the class attribute.

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Unlocking the Power of SELF: Best Practices and Tips

Now that you have a solid understanding of how to use the SELF keyword in Python, let’s delve into some best practices and tips to further enhance your usage of SELF.

By following these guidelines, you can write cleaner, more maintainable, and efficient code. Let’s explore these best practices!

Be Consistent with Naming Conventions

When using the SELF keyword, it’s important to be consistent with naming conventions. By convention, the first parameter of an instance method is named “self.”

While Python doesn’t enforce this naming, following the convention makes your code more readable and understandable for other developers.

Similarly, when using the SELF keyword as the class method first parameter, the convention is to name it “cls” (short for “class”). Following consistent naming conventions helps maintain code consistency and improves code readability.

Avoid Misusing SELF in Static Methods

Although you can use the SELF keyword in static methods, it’s important to use it judiciously. Since static methods are not bound to instances, using SELF in a static method can be confusing and lead to potential errors.

In general, static methods should be self-contained and operate solely on the inputs passed as arguments. Avoid relying on or modifying instance attributes within static methods. If you find yourself needing access to instance attributes, consider converting the method to an instance method instead.

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Use SELF Sparingly in Class Methods

When using the SELF keyword in class methods, also known as “classmethods,” remember that the first parameter is conventionally named “cls” to indicate the class itself. It’s important to use SELF sparingly within class methods and only when accessing or modifying class-level data.

Class methods are primarily used for operations that pertain to the entire class, such as creating alternate constructors or performing class-wide calculations. Using SELF within a class method to access instance-specific data can lead to confusion and potentially unintended behavior.

About the Author

Tom Koh

Tom is the CEO and Principal Consultant of MediaOne, a leading digital marketing agency. He has consulted for MNCs like Canon, Maybank, Capitaland, SingTel, ST Engineering, WWF, Cambridge University, as well as Government organisations like Enterprise Singapore, Ministry of Law, National Galleries, NTUC, e2i, SingHealth. His articles are published and referenced in CNA, Straits Times, MoneyFM, Financial Times, Yahoo! Finance, Hubspot, Zendesk, CIO Advisor.


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