What is a weak reference in Python?

Q: What is a weak reference in Python?

**Target Audience:** Python developers building caches, event systems or object graphs who need objects to be freed on time **Difficulty:** Hard **Python version:** Checked on CPython 3.11, 3.12 and 3.13 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); all three gave identical output > **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 block in the **Verified** section was printed by actually running the examples, not typed from memory. ## 1. Why This Even Matters — A Story First You keep a list of your friends' phone numbers. Holding a number does not keep your friend in town: if they move away, the number simply stops working, and you find out when you call it. You did not prevent them from leaving; you only remembered how to reach them *while* they were around. Compare that with holding your friend's house keys. As long as you have the

The problem

Question presented to candidate: "What is a weak reference in Python, and how is it different from a normal reference? Give me a situation where you would use one, and tell me what the weakref module offers beyond weakref.ref."

What a strong answer should cover:

A weak reference refers to an object without increasing its reference count, so it does not keep the object alive; calling the ref returns the object, or None once it has been collected.
Use cases: caches that should not keep values alive (WeakValueDictionary), attaching data to objects you do not own (WeakKeyDictionary), observer/listener lists (WeakSet, WeakMethod), and back-pointers in trees that would otherwise create cycles.
Callbacks on weakref.ref, weakref.proxy (raises ReferenceError when dead), and weakref.finalize for cleanup.
Limits: int, str, tuple, list and dict instances cannot be weakly referenced (subclasses of list/dict can); classes with __slots__ need "__weakref__" in the slots.
The bound-method trap: weakref.ref(obj.method) dies immediately, which is why WeakMethod exists.

Code / implementation expected: Yes — a small WeakValueDictionary cache or a listener registry that cleans itself up.

weakref
memory

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 building caches, event systems or object graphs who need objects to be freed on time Difficulty: Hard Python version: Checked on CPython 3.11, 3.12 and 3.13 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); all three gave identical output

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 block in the Verified section was printed by actually running the examples, not typed from memory.

1. Why This Even Matters — A Story First

You keep a list of your friends' phone numbers. Holding a number does not keep your friend in town: if they move away, the number simply stops working, and you find out when you call it. You did not prevent them from leaving; you only remembered how to reach them *wh

The solution is hidden so you can try first

12 min read · 12 sections

Run the code

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

Pythonref, callbacks, proxy, what can be weakly referenced, and WeakMethod

Practice

PythonA WeakValueDictionary cache, WeakKeyDictionary, WeakSet and a weak parent pointer