Python id() Function

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The id() function in Python returns a unique identifier for a specified object. This identifier is a numerical value (integer) that corresponds to the object’s identity and is constant for the object during its lifetime.

Syntax of id()

The syntax of the id() function is straightforward:


Here, object is the item for which you want to obtain the identity.

Understanding Object Identity in Python

Every object in Python has a unique identity that distinguishes it from other objects, akin to a person’s social security number. The identity is a number that guarantees uniqueness, which means no two objects will have the same identity at the same time.

Why is Object Identity Important?

Object identity is vital for several reasons:

  • Comparing Objects: In Python, you can use the is operator to check if two variables point to the same object, which is effectively comparing their identities.
  • Object Lifecycle: The identity is used internally by Python to manage objects, especially when two or more variables reference the same object in memory.
  • Debugging: Knowing the identity of objects can help debug issues related to object mutability and aliasing.

How id() Works

Under the hood, when id() is called on an object, Python returns the address of the object in memory. However, there’s no guarantee that this memory address is the actual location of the data. The returned ‘address’ is a unique constant that can be thought of as the object’s address in Python’s memory management system.

Examples of Using id()

Let’s walk through some practical examples of using the id() function.

Basic Usage

number = 42
print(id(number))  # Outputs a unique identifier for the integer 42

This basic example demonstrates the id() function applied to an integer object.

Identity of Variables

a = 'Hello, World!'
b = 'Hello, World!'
print(id(a) == id(b))  # Output could be True or False

In this example, two strings contain the same value. Whether they have the same identity depends on Python’s internal optimizations like string interning.

Mutable vs. Immutable Objects

list1 = [1, 2, 3]
list2 = [1, 2, 3]
print(id(list1) == id(list2))  # Output will be False

tuple1 = (1, 2, 3)
tuple2 = (1, 2, 3)
print(id(tuple1) == id(tuple2))  # Output could be True

For the lists (which are mutable), they will always have different identities. However, the tuples (which are immutable) might have the same identity due to Python’s optimization.

id() with Different Data Types

The id() function works with various data types, whether they’re built-in types like integers, floats, strings, lists, tuples, dictionaries, etc., or instances of custom classes.

Uniqueness of the id() Value

The unique identifier returned by id() is unique for the object during its lifetime. After the object is deleted or goes out of scope, the id may be reused for a new object.

The id() Function and Garbage Collection

Understanding id() can also provide insight into Python’s garbage collection process. As long as an object’s id exists, the object is alive. Once there are no more references to the object, its id (and associated memory) can be recycled by the Python interpreter.

Limitations and Considerations

The id() function is powerful, but there are some considerations:

  • Interpreter Specific: The actual value of the id is interpreter-specific. Different Python interpreters may provide different values for the same object.
  • Lifetime: The id value is guaranteed to be unique only during the lifetime of the object. After the object no longer exists, the id may be reassigned.
  • Not Object’s Value: The id() does not reflect the value of the object and should not be used for hashing or comparing values.

Best Practices When Using id()

  • Use for Identity, Not Value: Utilize id() when you are interested in an object’s identity rather than its value.
  • Debugging: Employ id() to understand object referencing and aliasing issues during debugging.
  • Performance: Be aware that unnecessary use of id() may impact performance, especially in cases where object identity is irrelevant.


The id() function is a critical part of Python’s introspection capabilities, allowing programmers to access the identity of objects. It plays a crucial role in object comparison, debugging, and understanding Python’s memory management and garbage collection mechanisms.

By providing programmers with the ability to differentiate between objects that may seem similar but are distinct entities within Python’s memory space, id() is an essential tool for advanced Python programming.

In essence, id() offers more than just a number; it gives Python developers a glimpse into the inner workings of the Python runtime environment, enabling more precise and controlled coding techniques.

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