What does __slots__ do? | Interview Question

Q: What does __slots__ do?

**Target Audience:** Python developers who know classes and instance attributes and want to understand what `__slots__` changes under the hood. **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). Memory figures are from our CPython runs and vary by version. > **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 Picture two kinds of suitcase. The first is a duffel bag: you can stuff anything into it, any time, and it stretches. The second is a moulded case with exactly two cut-outs, one for a camera and one for a lens. You cannot add a sandwich, but it is smaller, nothing rattles, and if you try to put the lens in the camer

The problem

Question presented to candidate: "Some classes in our codebase declare __slots__ = ('x', 'y'). What does that actually change about the objects, why would you do it, and what breaks when you do?"

What a strong answer should cover:

__slots__ declares a fixed set of attribute names; instances get fixed storage for them instead of a per-instance __dict__
Effects: less memory per object (no dict), and assigning an undeclared attribute raises AttributeError
How: each slot is a member descriptor on the class (type(Cls.__dict__['x']) is member_descriptor) that reads and writes a fixed offset in the object
Inheritance rules: every class in the chain must define __slots__ (only new names) or a __dict__ comes back; two bases with non-empty slots cannot be combined
Costs: no weak references unless '__weakref__' is listed, no class-attribute defaults with the same name, no vars() or cached_property; @dataclass(slots=True) (3.10+) generates them for you

Clarifying questions expected:

How many instances are alive at once? (the saving only matters with many objects)
Is the class part of a hierarchy, or used with weak references or cached_property?

Code / implementation expected: Optional — a slotted class next to a plain one showing the missing __dict__ and the AttributeError on a typo.

slots

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 know classes and instance attributes and want to understand what __slots__ changes under the hood. 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). Memory figures are from our CPython runs and vary by version.

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

Picture two kinds of suitcase. The first is a duffel bag: you can stuff anything into it, any time, and it stretches. The second is a moulded case with exactly two cut-outs, one for a camera and one for a lens. You ca

The solution is hidden so you can try first

10 min read · 12 sections

Run the code

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

PythonNo __dict__, slot descriptors, unknown attributes rejected and the memory saving

What does slots do?