How do you read and write files in Python? | Interview Question

Q: How do you read and write files in Python?

**Target Audience:** Beginners learning Python, and anyone who wants the precise answer an interviewer expects **Difficulty:** Easy **Python version:** Behaviour checked on CPython 3.11 (and in the page's Pyodide 3.11 runtime) > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First Think of a file on disk as a notebook locked in a cupboard. To use it you have to unlock the cupboard (open the file), say what you plan to do (read it, write a fresh page, add to the end), do the work, and then lock the cupboard again (close the file) so nobody else finds it half-written. Two things go wrong in real programs. People forget to lock the cupboard, so changes never reach the disk or the program runs out of file handles. And people

The problem

Question presented to candidate: "Walk me through how you read and write files in Python. Which modes would you use, how do you make sure the file gets closed, and what goes wrong with encodings and large files?"

What a strong answer should cover:

open(path, mode, encoding=...) returns a file object; the common modes are r, w (truncates), a (appends), x (create only, fails if it exists), plus b for binary and + for read/write
with open(...) as f: closes the file even when an exception is raised, because the file object is a context manager
Text mode decodes bytes to str using an encoding and translates newlines; binary mode gives raw bytes. Always pass encoding="utf-8" because the default depends on the locale (PEP 686 makes UTF-8 the default only from Python 3.15)
read() loads everything; iterating for line in f streams one line at a time with flat memory; readlines() builds a list
pathlib.Path gives read_text, write_text, read_bytes, write_bytes, mkdir(parents=True) and / path joining
Safe overwrites: write to a temp file, then os.replace it over the target

Clarifying questions expected:

Is the data text or binary, and if text, which encoding does it use?
How large can the file get: does it fit in memory?

Code / implementation expected: Yes — a short with open(...) read/write, ideally showing streaming and an explicit encoding.

files
pathlib
io

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Solution

Target Audience: Beginners learning Python, and anyone who wants the precise answer an interviewer expects Difficulty: Easy Python version: Behaviour checked on CPython 3.11 (and in the page's Pyodide 3.11 runtime)

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

Think of a file on disk as a notebook locked in a cupboard. To use it you have to unlock the cupboard (open the file), say what you plan to do (read it, write a fresh page, add to the end), do the work, and then lock the cupboard again (close the file) so nobody else finds it half-written.

Two things go wrong in real programs. People forget to loc

The solution is hidden so you can try first

14 min read · 12 sections

Run the code

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

PythonModes, with blocks, reading line by line, and the two common errors

Practice

Pythonpathlib, binary mode, newlines, encodings and an atomic replace

Practice

Discussion

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