What is the difference between '==' and 'is'? | Interview Question

Q: What is the difference between '==' and 'is'?

**Target Audience:** Beginners preparing for Python interviews — no prior Python knowledge assumed. **Difficulty:** Easy **Python version:** Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); the warning text was also checked on 3.12. > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output shown in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First Two people walk into a café wearing the same jacket: same brand, same size, same colour. Are they wearing "the same jacket"? In one sense yes — the jackets are equal. In another sense no — there are two jackets, and if one person spills coffee on theirs, the other jacket stays clean. Python gives you a separate operator for each meaning. `==` asks "are these jackets alike?" and

The problem

Question presented to candidate: "In Python you can compare two things with == or with is. What is the difference, when do you use each, and why does is sometimes seem to work for numbers and strings?"

What a strong answer should cover:

== tests value equality by calling __eq__ (so a class can define it); is tests identity, whether both names point at the same object, and cannot be overridden.
The rule: use == for values and is only for singletons and sentinels, above all x is None (also is True / is False in rare cases, is NotImplemented, custom sentinels).
Why is "works" by accident: CPython caches small ints (-5 to 256), interns some strings and shares equal constants within one compiled unit, so results depend on how the value was made. Python 3.8+ warns with SyntaxWarning for is against a literal.
Equality protocol details: __eq__ can return NotImplemented and Python then tries the other operand, falling back to identity; without __eq__, == is identity.
The NaN edge case: nan == nan is False, but [nan] == [nan] and nan in [nan] are True because containers check identity first.

Code / implementation expected: Optional — two equal lists showing == True and is False, and x is None.

is
equality

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Solution

Target Audience: Beginners preparing for Python interviews — no prior Python knowledge assumed. Difficulty: Easy Python version: Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); the warning text was also checked on 3.12.

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output shown in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

Two people walk into a café wearing the same jacket: same brand, same size, same colour. Are they wearing "the same jacket"? In one sense yes — the jackets are equal. In another sense no — there are two jackets, and if one person spills coffee on theirs, the other jacket stays clea

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10 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonValue vs identity, __eq__, why is None, and NaN in containers

Practice

PythonSmall-int cache, constant sharing, interning and the bytecode

Practice

Discussion

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