What is string interning in Python? | Interview Question

Q: What is string interning in Python?

**Target Audience:** Python developers who know strings, dictionaries and the difference between `==` and `is`. No C knowledge is needed. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11 (and repeated on 3.12 and 3.13 where noted). Both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Every idea is explained in plain words first. When a word appears that an interviewer expects you to know, it is tagged with `📌 Interview term:` and defined on the spot. All of them are collected in the Quick Glossary at the end. Every output shown was produced by running the code. ## 1. Why This Even Matters — A Story First A big office building has a reception desk that prints visitor badges. On a normal day a thousand people come in, but almost all of them are visiting one of five departments: "Sales", "Legal", "IT", "HR", "Finance". The receptionist could print a brand-new "Sales" sticker for every Sales visitor. Or she could

The problem

Question presented to candidate: "Sometimes two equal strings in Python are the same object and sometimes they are not. What is string interning, which strings does CPython intern on its own, when would you call sys.intern yourself, and why is comparing strings with is still a mistake?"

What a strong answer should cover:

Interning means keeping one canonical object per distinct string value in an interpreter-wide table and reusing it, so equal interned strings are also identical (is)
CPython interns automatically: identifiers (variable, function, attribute and keyword-argument names), string constants that look like identifiers (ASCII letters, digits, underscore), the empty string, and attribute names passed to setattr/getattr; it does not intern strings built at run time, read from files or with spaces/punctuation/non-ASCII
Why: dict lookups compare by identity before calling __eq__ and strings cache their hash, so interned keys (namespaces, __dict__, keyword arguments) are found with a pointer comparison; it also saves memory for repeated values
sys.intern(s) returns the canonical copy; useful when the same few strings are repeated a huge number of times (parsed logs, CSV columns, tokens), with the cost of one hash lookup per call
It is a CPython implementation detail: compare strings with ==; is works only by accident and the compiler warns about is with a literal

Code / implementation expected: Optional — demonstrating a run-time-built string that is equal but not identical, and fixing it with sys.intern.

str
interning

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Solution

Target Audience: Python developers who know strings, dictionaries and the difference between == and is. No C knowledge is needed. Difficulty: Medium Python version: Behaviour checked on CPython 3.11 (and repeated on 3.12 and 3.13 where noted). Both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

How to read this doc: Every idea is explained in plain words first. When a word appears that an interviewer expects you to know, it is tagged with 📌 Interview term: and defined on the spot. All of them are collected in the Quick Glossary at the end. Every output shown was produced by running the code.

1. Why This Even Matters — A Story First

A big office building has a reception desk that prints visitor badges. On a normal day a thousand people come in, but almost all of them are visiting one of five departments: "Sales", "Legal", "IT", "HR", "Fina

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12 min read · 10 sections

Run the code

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PythonWhich strings get interned, and sys.intern

Practice

PythonWhy interning helps: identity-first lookups and deduplication

Practice

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