Write, Overwrite, Append: How Python Handles File Output

The idea

Reading gets data out of a file. Writing puts data in.

Python gives you two distinct modes for this — and the difference between them matters.

write() — create or overwrite

Mode "w" opens a file for writing. If the file doesn't exist, Python creates it. If it does — it's wiped clean first.

with open("roster.txt", "w", encoding="utf-8") as f:
    f.write("Raven\n")
    f.write("Wolf\n")
    f.write("Ghost\n")

After this runs, roster.txt contains exactly three lines. Whatever was there before is gone.

f.write() doesn't add a newline automatically — you add \n yourself.

Writing multiple lines at once

writelines() takes a list and writes each element — no newlines added automatically:

soldiers = ["Raven\n", "Wolf\n", "Ghost\n"]
with open("roster.txt", "w", encoding="utf-8") as f:
    f.writelines(soldiers)

Same result as before. Cleaner when you already have a list.

append() — add without destroying

Mode "a" opens a file and adds to the end. Existing content is untouched.

with open("roster.txt", "a", encoding="utf-8") as f:
    f.write("Viper\n")
    f.write("Bull\n")

Now roster.txt has five lines — the original three plus two new ones.

If the file doesn't exist, "a" creates it. Same as "w" in that regard.

Writing data from variables

soldiers = ["Raven", "Wolf", "Ghost", "Viper", "Bull"]

with open("roster.txt", "w", encoding="utf-8") as f:
    for soldier in soldiers:
        f.write(soldier + "\n")

Loop through your data, write each value, add a newline. That's the standard pattern.

Checking what was written

After writing, read the file back to verify:

with open("roster.txt", "r", encoding="utf-8") as f:
    print(f.read())

write mode vs append mode — side by side

• "w" — starts fresh every time. Use when you want to replace the file completely.
• "a" — adds to what's already there. Use when you want to keep existing data and add more.

Running a script in "w" mode twice produces one file with one set of data.

Running a script in "a" mode twice produces one file with two sets of data.

Heads up!

• "w" overwrites silently — there's no confirmation, no warning, no undo
• f.write() requires a string — convert numbers with str() or f-strings
• writelines() does not add newlines — include \n in your strings
• The file is created in the same folder as your script if you use just a filename

What you should understand now

• "w" creates or overwrites — existing content is gone
• "a" appends — existing content is preserved
• f.write(string) writes one string — add \n manually
• f.writelines(list) writes a list of strings — add \n in the strings
• Loop over your data and write each item for the most common pattern