What does the 'w' mode do to an existing file?

It overwrites it — everything already in the file is lost the moment you open it in 'w' mode. Use 'a' (append) to add to a file without wiping it.

Why use with open(...) as f?

The with block closes the file automatically when you're done, even if an error happens inside. It's the safest way to handle files and the pattern expected in GCSE exam answers.

Do I need to add \n when writing?

Yes. .write() does not add newlines for you. If you want each write on its own line, include \n at the end of the string.

Python file handling

Reading and writing files is a required topic in every UK GCSE Computer Science specification. This unit covers Python's file I/O API: opening a file with open(), the read/readlines/write methods, the modes 'r', 'w' and 'a', and — most importantly — using with blocks so files always close cleanly. Every example runs in the browser via RunPy's in-browser file system, so learners can see the file contents update as their code runs. Perfect for classroom teaching, GCSE and A Level revision, and independent or home-schooled learners.

What you'll learn

Open a file with open(filename, mode)
Understand the modes: 'r' (read), 'w' (write, overwrites), 'a' (append)
Read a whole file with .read(); line by line with .readlines() or a for loop
Write to a file with .write() — remember it doesn't add a newline
Use with open(...) as f: so the file always closes

UK GCSE topic coverage

File writing
Multi-line writing
File reading
splitlines
for line in f
File + cast + accumulate
with statement
CSV parsing
Append mode
Applied file handling

Worked example

Append a line to a log file, then read it back

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

with open("log.txt", "r") as f:
    for line in f:
        print(line.strip())

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Lessons in this unit

Full interactive lessons with checked tasks are available to signed-in RunPy users. Unit 1 and a selection of exam-style questions are free with a RunPy account.

Writing to a file
Writing several lines
Reading a file
Counting the lines
Reading line by line
Processing numbers from a file
A safer way to open files: with
Parsing data with .split()
Adding to a file: append mode
Bringing it together: a file tool

Frequently asked questions

What does the 'w' mode do to an existing file?: It overwrites it — everything already in the file is lost the moment you open it in 'w' mode. Use 'a' (append) to add to a file without wiping it.
Why use with open(...) as f?: The with block closes the file automatically when you're done, even if an error happens inside. It's the safest way to handle files and the pattern expected in GCSE exam answers.
Do I need to add \n when writing?: Yes. .write() does not add newlines for you. If you want each write on its own line, include \n at the end of the string.