What is the difference between __new__ and __init__?

Q: What is the difference between __new__ and __init__?

**Target Audience:** Python developers who write classes with `__init__` and have seen `__new__` in library code or an interview question. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11.17, 3.12 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First Think of a furniture workshop. One person, the **builder**, takes the order "a chair" and produces a raw wooden chair: it exists, you can point at it, but it has no paint and no cushion. A second person, the **finisher**, takes that chair and paints it, adds the cushion and puts the customer label on it. Python splits object creation the same way. `__new__` is the builder: it make

The problem

Question presented to candidate: "When I write Point(3, 4), Python calls both __new__ and __init__. What does each one do, in what order, and when would you ever override __new__ instead of just writing an __init__?"

What a strong answer should cover:

__new__(cls, ...) runs first: it receives the class, creates the object (normally via super().__new__(cls)) and returns it; it is implicitly a static method
__init__(self, ...) runs second on the object __new__ returned, sets its attributes and must return None
The glue is type.__call__: it calls __new__, and only calls __init__ if the result is an instance of the class (a subclass counts)
When __new__ is the right tool: subclassing immutable types (int, float, str, tuple), instance caching or singletons, and returning a different (sub)class
Gotchas: a cached instance still has __init__ re-run on every call, object.__new__ accepts only the class, and copy / pickle create objects through __new__ without calling __init__

Code / implementation expected: Optional — a class that prints from both methods to show the order, plus one immutable subclass that needs __new__.

new
init

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Python developers who write classes with __init__ and have seen __new__ in library code or an interview question. Difficulty: Medium Python version: Behaviour checked on CPython 3.11.17, 3.12 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3).

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

Think of a furniture workshop. One person, the builder, takes the order "a chair" and produces a raw wooden chair: it exists, you can point at it, but it has no paint and no cushion. A second person, the finisher, takes that chair and paints it, adds the cushion and puts the

The solution is hidden so you can try first

12 min read · 10 sections

Run the code

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

PythonThe two steps of construction and the rules that connect them

Practice

PythonWhen you need __new__: immutable subclasses, instance caching, copy and pickle

What is the difference between new and init?