Writing & Appending Files

Writing to a file

Open a file with "w" to write. This overwrites the file if it exists.

python
with open("output.txt", "w") as f:
    f.write("Hello
")
    f.write("World
")

Appending to a file

Open with "a" to add new content to the end.

python
with open("log.txt", "a") as f:
    f.write("New entry
")

Writing multiple lines

writelines() writes a list of strings.

python
lines = ["one
", "two
", "three
"]
with open("nums.txt", "w") as f:
    f.writelines(lines)

Creating a new file only

Use "x" to create a file and fail if it already exists.

python
with open("unique.txt", "x") as f:
    f.write("first time")

Common mistakes to avoid

• Forgetting that "w" erases the file first.
• Writing numbers without converting them to strings.
• Forgetting newline characters when you want separate lines.

What you should understand after this lesson

• The difference between write and append modes.
• How to write one line or many lines.
• How to avoid accidental overwrites.