How to Update or access Class variables in Python

# Table of Contents

# Updating class variables in Python

Update class variables by accessing them directly on the class.

Class variables are shared by all instances and can be updated directly on the class or in a class method.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary

    # ✅ update class variable
    @classmethod
    def set_cls_id(cls, new_cls_id):
        cls.cls_id = new_cls_id
        return cls.cls_id

    # ✅ update instance variable
    def set_name(self, new_name):
        self.name = new_name
        return new_name


print(Employee.cls_id)  # 👉️ employee
Employee.set_cls_id('new_employee_id')
print(Employee.cls_id)  # 👉️ new_employee_id

bob = Employee('Bobbyhadz', 100)

bob.set_name('Bobbyhadz2')
print(bob.name) # 👉️ Bobbyhadz2

The code for this article is available on GitHub

The class has a cls_id class variable and name and salary instance variables.

Class variables are shared by all instances and can be accessed directly on the class, e.g. Employee.cls_id.

Instance variables are unique to each instance you create by instantiating the class.

# Updating class variables in class methods

We used a class method to update the cls_id variable.

main.py

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@classmethod
def set_cls_id(cls, new_cls_id):
    cls.cls_id = new_cls_id
    return cls.cls_id

Class methods get passed the class as the first argument.

# Updating class variables directly on the class

Note that you can also set class variables directly on the class, e.g. Employee.cls_id = new_cls_id.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary



print(Employee.cls_id)  # 👉️ employee

# ✅ Update a class variable
Employee.cls_id = 'new_employee_id'

print(Employee.cls_id)  # 👉️ new_employee_id

updating class variable directly on the class

The code for this article is available on GitHub

Class variables are shared by all instances of the class, whereas instance variables are unique to each instance.

# Updating instance variables

You'll more often have to update instance variables in the class. Here is an example.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary


    # ✅ Update an instance variable
    def set_name(self, new_name):
        self.name = new_name
        return new_name


alice = Employee('Alice', 150)
bob = Employee('Bobbyhadz', 100)

bob.set_name('Bobbyhadz2')
print(bob.name)  # 👉️ Bobbyhadz2

print(alice.name) # 👉️ Alice

updating instance variables

The code for this article is available on GitHub

Updating one instance variable doesn't update the attribute for the other instances.

# Updating a class variable is reflected in all instances

On the other hand, when you update a class variable, the value is updated for all instances.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary

alice = Employee('Alice', 150)
bob = Employee('Bobbyhadz', 100)

print(bob.cls_id)  # 👉️ employee

Employee.cls_id = 'NEW_ID'

print(alice.cls_id)  # 👉️ NEW_ID
print(bob.cls_id)  # 👉️ NEW_ID

updating class variable is reflected in all instances

The code for this article is available on GitHub

Updating the cls_id class variable is reflected in all instances.

# Accessing a class variable from an instance of the class

You can use the type() class if you need to access a class variable from an instance of the class.

main.py

Copied!
class Employee():
    cls_id = 'employee'

    def __init__(self, name, salary):
        self.name = name
        self.salary = salary


bob = Employee('Bobbyhadz', 100)

# 👇️ override class variable on the instance
bob.cls_id = 'new'
print(bob.cls_id)  # 👉️ new

# 👇️ access the actual class variable from the instance
result = type(bob).cls_id
print(result)  # 👉️ employee

accessing class variable from instance of class

The code for this article is available on GitHub

The instance overrides the cls_id variable, so to access the actual class variable, we had to use the type() class.

The type class returns the type of an object.

Most commonly the return value is the same as accessing the __class__ attribute on the object.

The following code sample uses the __class__ attribute and achieves the same result.

main.py

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bob = Employee('Bobbyhadz', 100)

bob.cls_id = 'new'
print(bob.cls_id)  # 👉️ new

result = bob.__class__.cls_id
print(result)  # 👉️ employee

# Accessing Class variables in Python

Class variables are accessed directly on the class.

You can access instance variables using the self object, e.g. self.name. Instance variables are unique to each instance.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary

    def get_name(self):
        # 👇️ class variable
        print(Employee.cls_id)

        # 👇️ access instance variable
        return self.name


bob = Employee('Bobbyhadz', 100)

print(bob.get_name())  # 👉️ Bobbyhadz
print(bob.cls_id)  # 👉️ employee

print(Employee.cls_id)  # 👉️ employee

The code for this article is available on GitHub

The class has a cls_id class variable and name and salary instance variables.

Class variables are shared by all instances and can be accessed directly on the class, e.g. Employee.cls_id.

Instance variables are unique to each instance you create by instantiating the class.

# Accessing class variables by defining a class method

You can also access class variables by defining a class method.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary


    @classmethod
    def get_cls_id(cls):

        return cls.cls_id


print(Employee.get_cls_id())  # 👉️ employee

The code for this article is available on GitHub

We marked the get_cls_id method as a class method. The first argument class methods get passed is the class.

Regular methods get passed a self object that gives us a reference to the instance.

main.py

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class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary

    def get_name(self):
        print(Employee.cls_id)

        return self.name

alice = Employee('Alice', 200)
print(alice.get_name())  # 👉️ Alice
print(alice.cls_id) # 👉️ employee

bob = Employee('Bobbyhadz', 100)
print(bob.get_name())  # 👉️ Bobbyhadz
print(bob.cls_id) # 👉️ bob

The code for this article is available on GitHub

The name and salary instance variables are unique to each instance, but the instances share the same cls_id class variable.

# Accessing a class variable from an instance of the class

You can use the type() class if you need to access a class variable from an instance of the class.

main.py

Copied!
class Employee():
    # 👇️ class variable
    cls_id = 'employee'

    def __init__(self, name, salary):
        # 👇️ instance variables
        self.name = name
        self.salary = salary

    def get_name(self):
        # 👇️ class class variable
        print(Employee.cls_id)

        # 👇️ access instance variable
        return self.name


bob = Employee('Bobbyhadz', 100)

# 👇️ override class variable on the instance
bob.cls_id = 'new'
print(bob.cls_id)  # 👉️ new

# 👇️ access the actual class variable from the instance
result = type(bob).cls_id
print(result)  # 👉️ employee

The code for this article is available on GitHub

The instance overrides the cls_id variable, so to access the actual class variable, we had to use the type() class.

The type class returns the type of an object.

Most commonly the return value is the same as accessing the __class__ attribute on the object.

The following code snippet uses the __class__ attribute and achieves the same result.

main.py

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bob = Employee('Bobbyhadz', 100)

bob.cls_id = 'new'
print(bob.cls_id)  # 👉️ new

result = bob.__class__.cls_id
print(result)  # 👉️ employee

Which approach you pick is a matter of personal preference.

The type() class is more elegant because we don't have to access attributes prefixed with two underscores, but the __class__ attribute is more explicit.

# Additional Resources

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How to Update or access Class variables in Python

# Table of Contents

# Updating class variables in Python

# Updating class variables in class methods

# Updating class variables directly on the class

# Updating instance variables

# Updating a class variable is reflected in all instances

# Accessing a class variable from an instance of the class

# Accessing Class variables in Python

# Accessing class variables by defining a class method

# Accessing a class variable from an instance of the class

# Additional Resources

Borislav Hadzhiev