Writing Files — Saving Text to Disk

By the end of this lesson you can:

• Write a single line with f.write("text\\n") — and remember the explicit \\n.
• Choose between mode "w" (overwrite) and mode "a" (append) — and predict which one will keep your old data.
• Save many lines at once with f.writelines(lst).
• Round-trip a list to disk and back: write it out, read it in, compare.

Make a fresh folder for this lesson. Don't create the file by hand — Python is about to.

Predict what's in diary.txt after running this:

# Run #1 — start fresh, write three lines
with open("diary.txt", "w") as f:
    f.write("Day 1: started Python.\n")
    f.write("Day 2: built Mad Libs.\n")
    f.write("Day 3: learned about dicts.\n")

# Run #2 — open in "a" mode and add one more
with open("diary.txt", "a") as f:
    f.write("Day 4: saved my first file!\n")

Day 1: started Python.
Day 2: built Mad Libs.
Day 3: learned about dicts.
Day 4: saved my first file!

The mode argument

open(name)            same as open(name, "r")  → read text
open(name, "w")       write text — file wiped clean
open(name, "a")       append text — original kept, new lines added at end
open(name, "x")       create-new only — crashes if file exists
open(name, "r+")      read and write (rarely needed)

For now you only need "r", "w" and "a". "x" is occasionally handy when you want to be sure you're not overwriting.

1 · f.write(text)

The basic call. Pass a string. Python writes it as-is — no newline added. If you want lines, put \\n at the end yourself.

with open("greetings.txt", "w") as f:
    f.write("Hello\n")
    f.write("Hello again\n")
    f.write("Last line\n")

Output file:

Hello
Hello again
Last line

Forget the \\n and all three lines collide on one row — HelloHello againLast line. The newline is on you.

2 · f.writelines(list_of_lines)

Pass a list of strings — they all get written one after another. Same rule: no newlines are added for you.

students = ["Aisyah\n", "Wei Jie\n", "Priya\n", "Iman\n"]
with open("class.txt", "w") as f:
    f.writelines(students)

If your list doesn't already have \\n on each item, glue them on in one line with a list comprehension:

students = ["Aisyah", "Wei Jie", "Priya", "Iman"]
with open("class.txt", "w") as f:
    f.writelines([name + "\n" for name in students])

Or use the .join trick from PY-L2-14 with a single .write:

with open("class.txt", "w") as f:
    f.write("\n".join(students) + "\n")

3 · print(..., file=f)

Surprise: print can write to a file. Pass file= and it writes there instead of the screen — and adds the newline for you.

with open("log.txt", "w") as f:
    print("Booting up...", file=f)
    print("Loaded 42 items", file=f)
    print("Done.", file=f)

Same output as three f.write("...\\n") calls, but no manual newlines. A favourite trick for quick logs.

The append pattern · saving high scores

The "a" mode is the "diary" pattern. Run after run, the file grows.

from datetime import datetime

score = 95
when  = datetime.now().isoformat(timespec="seconds")

with open("scores.txt", "a") as f:
    f.write(f"{when} | score {score}\n")

Run it three times — three lines appear. The same code never wipes earlier runs.

Round-trip — write then read

Sanity-check what you wrote by reading it back:

with open("class.txt", "w") as f:
    f.writelines([n + "\n" for n in ["Aisyah", "Wei Jie"]])

with open("class.txt") as f:
    print(f.read())
# → Aisyah
#   Wei Jie

The story

Build a tiny diary that the user can use any number of times. Each entry adds to diary.txt. A second mode lets them read everything they've written so far.

Save as diary.py:

Code

# diary.py — append-only diary app

from datetime import datetime

def add_entry():
    print()
    print("Type your entry. Press Enter on an empty line to save.")
    lines = []
    while True:
        line = input()
        if line == "":
            break
        lines.append(line)

    if len(lines) == 0:
        print("(nothing to save)")
        return

    now = datetime.now().isoformat(timespec="minutes")
    with open("diary.txt", "a", encoding="utf-8") as f:
        f.write(f"--- {now} ---\n")
        for line in lines:
            f.write(line + "\n")
        f.write("\n")
    print("Saved.")

def show_diary():
    print()
    try:
        with open("diary.txt", encoding="utf-8") as f:
            print(f.read())
    except FileNotFoundError:
        print("(no entries yet)")

while True:
    pick = input("1 add  2 read  q quit: ").lower()
    if pick == "1":
        add_entry()
    elif pick == "2":
        show_diary()
    elif pick == "q":
        print("Bye!")
        break
    else:
        print("Pick 1, 2 or q.")

Sample session

1 add  2 read  q quit: 1

Type your entry. Press Enter on an empty line to save.
Built my first diary app.
It saves between runs!

Saved.
1 add  2 read  q quit: 2

--- 2026-05-27T18:42 ---
Built my first diary app.
It saves between runs!

1 add  2 read  q quit: q
Bye!

Read the diff

One file. Two modes used. add_entry uses "a" so old entries are kept. show_diary uses default read mode. The try/except FileNotFoundError handles the first-ever run, when the file doesn't exist yet. The datetime timestamp comes from a stdlib module we'll dive into in PY-L2-32.

Writing files is one letter different from reading. open(name, "w") wipes the file and writes from scratch. open(name, "a") appends to the end and keeps everything that was there. f.write(text) writes one string — without a newline, so add \\n yourself. f.writelines(list) writes many at once. print(..., file=f) writes through print's nicer interface (and adds the newline for free). Round-trip your output to be sure it loads back the way you saved it.