Python — Write Files

Writing text

with open("output.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
    f.write("Second line\n")

write() does NOT add newlines — you supply every \n.

"w" destroys instantly: opening with w erases the existing file before you write a character. Accidental data loss lives here.

Append — the non-destructive default choice

with open("log.txt", "a", encoding="utf-8") as f:
    f.write("2026-08-23 user logged in\n")

Create-only: "x"

try:
    with open("config.json", "x") as f:    # fails if file exists
        f.write("{}")
except FileExistsError:
    print("already exists — refusing to overwrite")

writelines & building content

lines = [f"{name}: {score}\n" for name, score in scores.items()]

with open("scores.txt", "w") as f:
    f.writelines(lines)        # no newlines added — include them yourself

Structured output via modules

import json

with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

import csv
with open("report.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "score"])
    writer.writerows(scores.items())

A small logging helper

def log(msg):
    from datetime import datetime
    stamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open("app.log", "a", encoding="utf-8") as f:
        f.write(f"[{stamp}] {msg}\n")

log("started")
log("error on step 3")

Open-append-close per entry = other programs can read the file live.

Safe replacement pattern

Overwriting important files? Write-then-rename is crash-safe:

import os, tempfile

tmp = tempfile.NamedTemporaryFile("w", delete=False, dir=".")
tmp.write(new_contents)
tmp.close()
os.replace(tmp.name, "important.json")   # atomic on same filesystem

Worst case you keep old OR new file, never half of each.

Gotchas: forgetting \n (one giant line) · writing without encoding on Windows · two processes appending simultaneously can interleave lines · "w" + crash = lost file.

Mini Practice

1. Write a 3-line poem; verify by reading back.
2. Log-helper with timestamps; call it five times.
3. Export dict → JSON file; CSV table via csv module.
4. Prove "w" vs "a" difference on the same file.
5. Use "x" to guard against overwriting an existing config.

Next: delete files →