How do Class and Instance Variables Differ in Python?

Understanding Class and Instance Variables in Python

One of the key concepts in object-oriented programming is the distinction between class and instance variables. In Python, these variables play a crucial role in managing data within a class. However, confusion can arise regarding their behavior based on how they are defined.

Class vs. Instance Variables

• Class Variables: Variables that are declared within a class but outside any method are known as class variables. They are shared among all instances of the class, regardless of their state.
• Instance Variables: Variables that are declared within a method of a class, using the self keyword, are known as instance variables. They are specific to each instance of the class.

Example 1: Class Variable

<code class="python">class testClass():
    list = []
    def __init__(self):
        self.list.append('thing')</code>

In this example, the list variable is a class variable because it is defined outside of any method. As a result, when a new instance of the class is created, it accesses the same list variable and its contents. Thus, the output for both instances (p and f) is:

['thing']
['thing', 'thing']

Example 2: Instance Variable

<code class="python">class testClass():
    def __init__(self):
        self.list = []
        self.list.append('thing')</code>

In this example, the list variable is defined within the __init__() method, using self. Therefore, it becomes an instance variable. Each instance has its own separate list variable. Thus, the output for each instance is:

['thing']
['thing']

Naming Resolution for Class and Instance Variables

The key to understanding the behavior of class and instance variables lies in how Python resolves names. When you access self.list, Python first checks if a list attribute exists in the instance itself. If not found, it then checks in the class definition.

This explains why in the first example, list is treated as a class variable, while in the second example, it becomes an instance variable.

In summary, the placement of variable definitions within a class or method determines whether they are class or instance variables. Understanding this distinction is crucial for effectively managing data within object-oriented Python code.

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