Writing to Files in Python

Opening a File Safely

Always use a context manager (with ... as ...). It makes sure the file is closed properly, even if something goes wrong.

try:
    with open("notes.txt", "w") as f:
        f.write("Hello, world!\n")
        f.write("Second line.\n")
    print("Done.")
except PermissionError:
    print("No permission to write there.")

This is the cleanest and safest way to write to a file.

Try this:

• Change the filename to somewhere you can’t write and trigger PermissionError.
• Forget to close a file (by removing with) and see why it’s risky.

File Modes: w, a, and x

• w (write): creates the file or clears it if it already exists.
• a (append): adds text to the end without removing what’s already there.
• x (exclusive create): makes a new file, but fails if the file already exists.

# Overwrite
with open("report.txt", "w") as f:
    f.write("Fresh report\n")

# Append
with open("report.txt", "a") as f:
    f.write("Another entry\n")

# Create only if missing
try:
    with open("unique.txt", "x") as f:
        f.write("Special file\n")
except FileExistsError:
    print("File already exists.")

Try this:

• Use x twice and see FileExistsError.
• Write with w twice and notice the previous text disappears.

Writing Text

You can write with write, writelines, or even print.

lines = ["Alice\n", "Bob\n", "Cara\n"]

with open("people.txt", "w") as f:
    f.writelines(lines)
    print("Dora", file=f)   # adds newline automatically

Try this:

• Save names without \n and see how they look in the file.
• Add print statements and compare the difference.

Common Exceptions

• FileExistsError: using x on an existing file.
• PermissionError: you don’t have rights to write there.
• IsADirectoryError: you passed a folder instead of a file.