Reading and writing files

Every automation script eventually touches files: reading a list of tasks, writing a report, loading a config. Python's built-in open() handles all of it. There is no library to install — this is in the language itself.

Reading a file

The core pattern is three lines:

with open("data.txt", "r") as f:
    content = f.read()
print(content)

• "r" is read mode. The file must already exist.
• f.read() returns the entire file contents as a single string.
• The with block ensures the file is closed when the block ends.

If you want lines as a list instead of one big string, use f.readlines(), or iterate directly:

with open("data.txt", "r") as f:
    for line in f:
        print(line.strip())   # strip() removes the trailing newline

Writing a file

with open("output.txt", "w") as f:
    f.write("Done.\n")

Mode "w" creates the file if it doesn't exist and overwrites it if it does. If you want to add to an existing file without erasing it, use "a" (append) instead.

Why with is safer than manual close

You can open and close files without with:

f = open("data.txt", "r")
content = f.read()
f.close()   # must remember this

The problem: if an exception happens between open() and close(), the file stays open. Open file handles leak resources and can cause data loss. The with statement handles closing automatically — even if an exception is raised inside the block. Always prefer it.

Try it

The runner below uses io.StringIO to simulate a file as an in-memory string — the same read/write API, no real filesystem needed in the browser:

The .seek(0) call rewinds to the start of the in-memory buffer before reading it back. Real files on disk behave the same way — the file cursor tracks your position.

Where to go next

Next: paths and directories — before you can reliably open the right file, you need to understand how the filesystem tree works and why hardcoded paths break on other machines.