What\'s the Difference: Instance vs. Class Attributes in Python?

Impact of Variable Definition Within and Outside __init__() in Classes

In Python, class variables declared within the constructor function __init__() exhibit distinct behavior compared to those defined outside. Variables declared inside __init__() are instance attributes, while variables defined outside are class attributes.

Instance Attributes (Defined Inside __init__() with 'self')

Variables prepended with self within __init__() are associated with specific object instances. Each instance has its own copy of these variables. Therefore, changes made to these variables within one instance won't affect other instances.

Example:

<code class="python">class WithClass:
    def __init__(self):
        self.value = "Bob"

    def my_func(self):
        print(self.value)</code>

Here, value is an instance attribute. Each instance of WithClass will have its own value.

Class Attributes (Defined Outside __init__)

Variables defined outside __init__() are class attributes. These variables are shared among all instances of the class. Any changes made to them within one instance are reflected across all other instances.

Example:

<code class="python">class WithoutClass:
    value = "Bob"

    def my_func(self):
        print(self.value)</code>

In this case, value is a class attribute. All instances of WithoutClass will refer to the same value variable.

Consequences of Variable Definition

The choice between defining variables inside or outside __init__() depends on the desired functionality:

• Instance Attributes: Use when you want each instance to have its own unique version of a variable.
• Class Attributes: Use when the variable value should be shared among all instances of the class.

Improper use of these attributes can lead to unexpected behavior or errors. For instance, defining an attribute as an instance attribute when it should be a class attribute may inadvertently create multiple copies of the variable, leading to performance issues or data inconsistencies.

The above is the detailed content of What\'s the Difference: Instance vs. Class Attributes in Python?. For more information, please follow other related articles on the PHP Chinese website!