Python | id() function

Python | id() function

In the jungle of Python’s functionalities, the id() function stands out as a unique and interesting tool. Behind its simplicity lies a powerful mechanism for understanding the identity of objects in the Python runtime. In this article, we’ll start a journey to reveal the mystery of the id() function, exploring its purpose, behavior, and providing informative examples.

What is the id() Function?

The id() function in Python is designed to return the identity of an object. This identity is a unique integer that represents the object and remains constant during its lifetime. Think of it as the object’s address in the Python runtime—a fingerprint that distinguishes it from other objects.

Let’s start with a simple example to illustrate the basic usage of the id() function:

# Basic usage of id()
value = 42
identity = id(value)
print(f"The identity of {value} is {identity}")
# The identity of 42 is 140709939872096

In this example, the id() function is used to retrieve the identity of the integer 42.

Understanding Object Identity:

The id() function becomes particularly valuable when dealing with mutable objects, such as lists or dictionaries. Consider the following example:

# Object identity with lists
list_a = [1, 2, 3, 4]
list_b = list_a

print(id(list_a)) # Output: Unique identifier for list_a
print(id(list_b)) # Output: Same as the identifier for list_a

In this case, both list_a and list_b have the same identity because they reference the same underlying object. Changes to one list will be reflected in the other.

Changing Identity: Immutable vs. Mutable Objects:

The identity of immutable objects, such as integers and strings, remains constant throughout their lifetime. However, for mutable objects like lists, the identity can change if the object is modified.

# Changing identity of a list
original_list = [1, 2, 3, 5]
first_identity = id(original_list)

changed_identity = id(original_list)

print(first_identity != changed_identity) # Output: True

In this example, appending an element to the list changes its identity.

Use Cases: When and Why to Use id():

Object Comparison: id() can be used to compare the identity of two objects, helping to determine if they reference the same underlying data.

Detecting Changes: When dealing with mutable objects, comparing identities before and after modifications can help detect changes.

Memory Optimization: Understanding object identity is crucial for memory optimization, especially when dealing with large data structures.

Caution and Considerations: While id() is powerful, it’s important to use it judiciously. In many cases, other comparison methods or techniques may be more appropriate.The uniqueness of an object’s identity is only guaranteed during its lifetime. Once the object is deleted or goes out of scope, its identity might be reused for a new object.


The id() function in Python is a tool that reveal the unique identity of objects, providing insights into their runtime behavior. By understanding the basics of id() and its applications, developers can navigate through the details of object identity and harness its power for efficient and effective programming. So, go ahead, reveal the mystery, and let the id() function guide you through the jungle of Python’s object-oriented world!

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