Writing Files & pathlib

Writing and appending

with open("out.txt", "w", encoding="utf-8") as f:
    f.write("first line\n")     # YOU supply the newlines
    f.write("second line\n")

"w" replaces the whole file. "a" appends to the end:

with open("log.txt", "a", encoding="utf-8") as f:
    f.write("2026-09-13 ran report\n")

Two beginner traps: forgetting \n (everything lands on one line), and using "w" on a log (yesterday's data is gone). Write mode for reports, append mode for logs.

Paths with pathlib

from pathlib import Path

p = Path("data") / "notes.txt"   # join paths portably
print(p.exists())                # False or True
print(Path.cwd())                # where the program runs

Path joins with /, checks existence, and gives you .name, .suffix, and .parent — use it instead of gluing strings with +.