Reading and writing files

What you'll learn

Write and read files with open() and with
Process the lines of a file
Append data to a file

Data that stays

Variables forget everything as soon as the program ends. If you want to keep something – high scores, settings, measurements from your robot – you write it into a file.

Good to know: On Codelix the files live in a small, invisible storage inside the browser. They only last while the page is open. On your computer the code works exactly the same – there the files really end up on the disk.

Writing

open(name, "w") opens a file for writing. With with it’s closed automatically at the end:

with open("shopping.txt", "w") as file:
    file.write("Milk\n")
    file.write("Bread\n")

print("Saved!")

\n is a line break. Without it everything would be in one long line.

Reading

Without a second value open() opens for reading. With a for loop you get the file line by line:

with open("shopping.txt", "w") as file:
    file.write("Milk\nBread\nApples\n")

with open("shopping.txt") as file:
    for number, line in enumerate(file, start=1):
        print(number, line.strip())

strip() removes the line break at the end of each line.

Appending

"w" deletes the old content. If you want to add something, use "a" (for append):

with open("log.txt", "w") as file:
    file.write("Start\n")

for round_number in range(3):
    with open("log.txt", "a") as file:
        file.write(f"Round {round_number}\n")

print(open("log.txt").read())

Mode	Meaning
`"r"`	read (default)
`"w"`	write – old content is deleted
`"a"`	append to the end

Remember: Files always contain text. When reading, you have to turn numbers back with int() or float().

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

with open("note.txt", "w") as f:
    f.write("A\nB\nC\n")
with open("note.txt") as f:
    print(len(f.readlines()))

Your task

Your turn

Write the high scores from the list scores into the file highscores.txt – each number on its own line. Then read the file back in, add the numbers up and print Total: 52. The test also tries a different list.

scores = [12, 7, 33]

# 1. Write

# 2. Read and add up

Something to think about
Write with file.write(f"{s}\n") – the \n makes the line break.
Which command helps?
Calculate with the numbers you read from the file – not with a total you typed in.

Gap code – fill in the ___

                        scores = [12, 7, 33]

with ___ as file:
    for s in ___:
        file.write(___)

total = ___
with ___ as file:
    for line in ___:
        total += ___
print(___)

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

scores = [12, 7, 33]

with open("highscores.txt", "w") as file:
    for s in scores:
        file.write(f"{s}\n")

total = 0
with open("highscores.txt") as file:
    for line in file:
        total += int(line)
print(f"Total: {total}")