Is Python pass-by-value or pass-by-reference?

Q: Is Python pass-by-value or pass-by-reference?

**Target Audience:** Beginners preparing for Python interviews — especially if you learned C, C++ or Java first and want to map the terms. **Difficulty:** Easy **Python version:** Behaviour checked on CPython 3.11.17 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 You lend a friend the address of your house written on a sticky note. They now hold **their own note** with the **same address**. - If they go to the house and paint the door red, you will see a red door when you get home. There is only one house. - If they cross out the address on their note and write a different one, nothing happens to your note or your house. That is exactly what Python does wi

The problem

Question presented to candidate: "If I pass a list into a function and the function changes it, the caller sees the change. If I pass an integer and the function changes it, the caller does not. So is Python pass-by-value or pass-by-reference?"

What a strong answer should cover:

Neither in the classic sense: the function receives a reference to the same object, and that reference is copied into a new local name — called call by sharing, pass by object reference, or, in the Python FAQ, pass by assignment
Mutating the object through the parameter (items.append(x)) is visible to the caller; rebinding the parameter (items = ...) only changes the local name
Why immutable arguments (int, str, tuple) look like pass-by-value: nothing can change them, so every "change" is a rebinding
Proof points: param is original is True inside the function, and a swap(a, b) function cannot work, which it could with true pass-by-reference
The += subtlety (in place for a list, a new object for an int or tuple) and the defensive-copy fix, including why a shallow copy may not be enough

Code / implementation expected: Optional — a short function that mutates vs one that rebinds, showing the caller's object afterwards.

functions
objects

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: Beginners preparing for Python interviews — especially if you learned C, C++ or Java first and want to map the terms. Difficulty: Easy Python version: Behaviour checked on CPython 3.11.17 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

You lend a friend the address of your house written on a sticky note. They now hold their own note with the same address.

If they go to the house and paint the door red, you will see a red door when you get home. There is only one house.
If they cross out the address on their note and wri

The solution is hidden so you can try first

9 min read · 10 sections

Run the code

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

PythonMutate vs rebind: what the caller sees

Practice

PythonDefensive copies, tuples holding lists, and returning instead of out-parameters