Python self Parameter

The self Parameter

The self parameter is a reference to the current instance of the class.

It is used to access properties and methods that belong to the class.

Example

Use self to access class properties:

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

  def greet(self):
    print("Hello, my name is " + self.name)

p1 = Person("Emil", 25)
p1.greet()

Note: The self parameter must be the first parameter of any method in the class.

Why Use self?

Without self, Python would not know which object's properties you want to access:

Example

The self parameter links the method to the specific object:

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

  def printname(self):
    print(self.name)

p1 = Person("Tobias")
p2 = Person("Linus")

p1.printname()
p2.printname()

self Does Not Have to Be Named "self"

It does not have to be named self, you can call it whatever you like, but it has to be the first parameter of any method in the class:

Example

Use the words myobject and abc instead of self:

class Person:
  def __init__(myobject, name, age):
    myobject.name = name
    myobject.age = age

  def greet(abc):
    print("Hello, my name is " + abc.name)

p1 = Person("Emil", 36)
p1.greet()

Accessing Properties with self

You can access any property of the class using self:

class Car:
  def __init__(self, brand, model, year):
    self.brand = brand
    self.model = model
    self.year = year

  def display_info(self):
    print(f"{self.year} {self.brand} {self.model}")

car1 = Car("Toyota", "Corolla", 2020)
car1.display_info()

Calling Methods with self

You can also call other methods within the class using self:

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

  def greet(self):
    return "Hello, " + self.name

  def welcome(self):
    message = self.greet()
    print(message + " ! Welcome to our website.")

p1 = Person("Tobias")
p1.welcome()

Python Class Properties

Class Properties

Properties are variables that belong to a class. They store data for each object created from the class.

Example

Create a class with properties:

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

p1 = Person("Emil", 36)

print(p1.name)
print(p1.age)

Access Properties

You can access object properties using dot notation:

class Car:
  def __init__(self, brand, model):
    self.brand = brand
    self.model = model

car1 = Car("Toyota", "Corolla")

print(car1.brand)
print(car1.model)

Class Properties vs Object Properties

Properties defined inside __init__() belong to each object (instance properties).

Properties defined outside methods belong to the class itself (class properties) and are shared by all objects:

class Person:
  species = "Human" # Class property

  def __init__(self, name):
    self.name = name # Instance property

p1 = Person("Emil")
p2 = Person("Tobias")

print(p1.name)
print(p2.name)
print(p1.species)
print(p2.species)

Modify Properties

You can modify the value of properties on objects:

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

p1 = Person("Tobias", 25)
print(p1.age)

p1.age = 26
print(p1.age)

Delete Properties

You can delete properties from objects using the del keyword:

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

p1 = Person("Linus", 30)

del p1.age

print(p1.name) # This works
# print(p1.age) # This would cause an error

Add New Properties

You can add new properties to existing objects:

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

p1 = Person("Tobias")

p1.age = 25
p1.city = "Oslo"

print(p1.name)
print(p1.age)
print(p1.city)

Note: Adding properties this way only adds them to that specific object, not to all objects of the class.