Identity

What Will You Learn
In this section, you’ll explore how Python’s identity operators—is and is not—work under the hood. You’ll learn to distinguish between object equality and object identity, a key concept when working with mutable and immutable types. Through hands-on examples, you'll see when two variables point to the same object in memory and how this differs from value comparison. Understanding identity operators will help you avoid subtle bugs, especially when working with None, caching, or complex data structures.

In programming, understanding how objects are compared is crucial. While equality (==) checks whether the values of two variables are the same, identity operators (is and is not) are used to determine if two variables point to the same object in memory. This concept is essential for writing accurate, bug-free Python code — especially when working with complex structures, singletons, or default mutable arguments.

If you're just starting with Python, it's common to confuse is with ==. This confusion can lead to subtle and hard-to-debug issues. Mastering identity comparisons will help you avoid such pitfalls and improve the clarity and correctness of your code.

What Are Identity Operators in Python?

Identity operators in Python are used to compare the memory location of two objects. Instead of comparing the values stored inside variables, these operators check whether both operands refer to the exact same object in memory.

There are two identity operators in Python:

• is checks if two variables point to the same memory address.
• is not is the inverse: it checks that they are not the same object.
• These operators are especially useful when comparing to singleton objects like None.
• Identity comparisons can be used for optimization, especially when object uniqueness matters.
• They are not interchangeable with equality checks (== / !=) and should be used intentionally.

How to Use Identity Operators in Python?

Identity operators in Python, is and is not, are used to determine if two variables reference the same object in memory. These operators are not used to compare the contents of objects, but rather their identities — the internal memory address. This is especially relevant when working with immutable vs. mutable types, or when you're comparing to singleton objects like None.

To use them, write expressions like a is b or a is not b. If both variables point to the same object, is returns True. If they point to different objects — even if the values inside are the same — is returns False. It’s common to use is when checking for None, e.g., if x is None:. Avoid using is to compare string values or numbers, unless you're deliberately checking identity for performance or memory control.

Use these operators thoughtfully and in the right context to avoid confusing or incorrect logic.

Where Identity Operators Are Used:

• Checking if a variable is None. Use is None instead of == None for clarity and performance.
• Verifying object identity. Useful when you need to know if two variables refer to the exact same object, such as in caching or object pooling.
• Comparing immutable values like small integers or interned strings. Python may reuse memory for small integers or string literals, so identity comparisons can sometimes be True.
• Ensuring a singleton pattern. Used to confirm that an instance is the same singleton object used elsewhere.
• Detecting unwanted aliasing in mutable objects. Helps you ensure that you're not accidentally modifying shared references.
• Testing object references in debugging. A powerful tool to check whether two variables are truly connected.
• Performance-sensitive checks. Identity comparisons are faster than equality checks when used appropriately.
• Control flow logic. Can be part of decisions when you only want to act if a specific, unique object is passed.

Common Beginner Mistakes When Using Identity Operators in Python

Working with identity operators (is, is not) often leads to confusion for newcomers. These mistakes are subtle but can cause significant bugs if not understood clearly. Below are five typical errors and how to avoid them.

Confusing is with ==

Beginners often use is instead of ==, assuming both check for equality. But is checks whether two variables refer to the same object, while == checks if their values are equal.

Fix: Use == for comparing values and is only when checking identity, like x is None.

Using is for comparing strings or numbers

Python may cache small integers and strings, making is sometimes return True by accident. This is implementation-dependent and unsafe.

Fix: Always use == when comparing numeric or string values, unless you specifically need identity comparison.

Comparing to None using ==

Although x == None works, it's less explicit and potentially slower.

Fix: Prefer x is None and x is not None for clarity and performance.

Expecting is to work for identical-looking containers

Two lists or dictionaries may look the same, but if they're different objects, is will return False.

Fix: Use == when comparing the contents of sequences or mappings.

Not checking for object aliasing after copying

When copying a list or dictionary, a shallow copy may still share references with the original.

Fix: Use is not to ensure the copy is a distinct object, and consider deep copies when needed.

    import copy
    a = [1, 2, 3]
    b = copy.copy(a)
    print(a is b)  # False — different objects

Understanding identity versus equality is essential to writing safe and predictable Python code.

FAQ: Identity Operators in Python