The Danger of Overusing "is" Instead of "==" in Python

If you've been working with Python for a while, you've probably used both is and == in your code. They look similar, they even read similarly. But underneath the hood, they do very different things. And using is when you meant to use == can lead to subtle, nasty bugs that are incredibly hard to track down.

What's the Difference Between is and ==?

• == checks if two values are equal.
• is checks if two variables point to the same object in memory.

Here’s a simple way to remember it:

👉 == → “Do these things look the same?”

👉 is → “Are these things literally the same object?”

A Quick Example

a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)  # True: the contents are the same
print(a is b)  # False: different objects in memory

Even though a and b contain the same list values, they’re not the same object. Python created two separate lists.

Where Things Get Dangerous

The trouble starts when developers assume is behaves like ==, or vice versa, especially because sometimes is works "accidentally" due to Python's internal optimizations.

Example 1: Strings

x = "hello"
y = "hello"

print(x == y)  # True
print(x is y)  # True? ...sometimes

This can be True for is, because Python “interns” (reuses) short strings to save memory. But watch this:

a = "hello world! this is a very long string"
b = "hello world! this is a very long string"

print(a == b)  # True
print(a is b)  # False

Suddenly is doesn’t work, even though the strings are equal. Why? Because Python didn’t intern the longer string. Relying on is here gives you flaky, inconsistent results depending on factors you don't control like string length or Python implementation.

Example 2: Integers

Python also caches small integers between -5 and 256.

x = 256
y = 256

print(x is y)  # True

a = 257
b = 257

print(a is b)  # False

It’s easy to write code that “passes” some tests but fails in production. And these bugs can be near-impossible to debug later.

So When Should You Use is?

There are valid uses for is. But they’re specific and rare.

Use is When Checking for Singleton Objects

For example:

if my_var is None:
    ...

This is the correct way to check for None, because None is a singleton. There’s only one None object in Python.

Other valid cases:

• my_obj is True
• my_obj is False

But even for True and False, == is usually safer in data-heavy code (e.g. pandas, NumPy), because those libraries redefine what truthy means.

Real-World Bug Scenarios

Here are a few real bugs I've seen:

Bug 1: Wrong Comparison in Loops

for item in my_list:
    if item is "done":  # oops
        break

This might work for some strings, but fail silently for others. The correct check is:

for item in my_list:
    if item == "done":
        break

Bug 2: False Negatives in Data Validation

user_input = input("Type yes or no: ")

if user_input is "yes":  # might fail even if input was 'yes'
    print("Confirmed!")

You might never know this failed unless your user types "yes" and nothing happens.

Bug 3: Unreliable Conditional Logic

x = 1000
y = 10 * 100

if x is y:  # False! Even though values are the same
    do_something()

This will silently not run do_something() even though most humans would say “1000 is equal to 1000.”

TL;DR - When to Use Which?

Final Advice

• Stick with == unless you're 100% sure you're checking for identity.
• Use is only when checking for None, or comparing against sentinel objects.
• Don’t rely on Python’s internal optimizations like interning or small int caching. They’re implementation details and can change between versions or platforms.