Python class variable vs instance variable

In Python, a class variable is a variable that is shared among all instances of a class. It is defined within the class, but outside of any method, and can be accessed using the class name or an instance of the class.

An instance variable, on the other hand, is a variable that is specific to an instance of a class. It is defined within the class, usually in the constructor method (__init__) and can be accessed using the dot notation on an instance of the class.

For example, the following code defines a class called Person with a class variable species and an instance variable name:

1class Person:
2    species = "Homo sapiens"
3
4    def __init__(self, name):
5        self.name = name

You can access the class variable using the class name:

1print(Person.species) # Output: Homo sapiens

You can also access the class variable using an instance of the class:

1person = Person("William")
2print(person.species) # Output: Homo sapiens

You can access the instance variable using the dot notation on an instance of the class:

1person = Person("William")
2print(person.name) # Output: William

It's important to note that, if you change the value of a class variable using an instance, the change will be reflected across all instances of the class.

1person1 = Person("William")
2person2 = Person("Jane")
3
4print(person1.species) # Output: Homo sapiens
5print(person1.name) # Output: William
6print(person2.species) # Output: Homo sapiens
7print(person2.name) # Output: Jane
8
9person1.species = "Human"
10
11print(person1.species) # Output: Human
12print(person1.name) # Output: William
13print(person2.species) # Output: Human
14print(person2.name) # Output: Jane

In Python, a class variable is a variable that is shared among all instances of a class and is defined within the class but outside of any method. An instance variable is a variable that is specific to an instance of a class, defined within the class and usually in the constructor method and can be accessed using the dot notation on an instance of the class. Understanding the difference between class variables and instance variables is important for creating reusable and maintainable code in Python.