Reading and Writing Files

Big idea

open() connects your program to a file. Use it inside a with block, and Python closes the file for you when the block ends — even if something goes wrong.

Writing a file

with open("notes.txt", "w") as f:
    f.write("Buy milk\n")
    f.write("Call Ada\n")

with open("notes.txt") as f:
    print(f.read())

output
Buy milk
Call Ada

• open("notes.txt", "w") — open for writing. Creates the file, or empties it if it already exists.
• as f — f is the open file.
• f.write(text) — writes text. Unlike print, it doesn't add a new line; put \n in yourself.
• When the with block ends, the file is closed and everything is saved.

File modes

Reading line by line

Looping over a file gives one line at a time — the best way to read big files, since only one line is in memory at once:

with open("shopping.txt", "w") as f:
    f.write("eggs\nflour\nmilk\n")

with open("shopping.txt", "a") as f:
    f.write("sugar\n")

count = 0
with open("shopping.txt") as f:
    for line in f:
        count += 1
        print(count, line.strip())

1 eggs
2 flour
3 milk
4 sugar

Each line ends with its \n, which is why .strip() is there — without it, print would add a second line break and you'd see blank lines.

Other ways to read:

• f.read() — the whole file as one string.
• f.readlines() — a list of lines (each still ending in \n).
• f.read().splitlines() — a list of lines without the \n.

When the file isn't there

with open("missing.txt") as f:
    print(f.read())

output
No saved notes yet

Opening a file that doesn't exist for reading raises FileNotFoundError. Wrap it: try: the with block, and except FileNotFoundError: print("No saved notes yet").

Why with?

Without with, you must call f.close() yourself — and if an exception happens first, it never gets called. Unclosed files can lose data that hadn't been saved yet. with makes closing automatic.

Watch it

Think of it like

Borrowing a library book. open checks it out, with is the promise to return it, and the end of the block is the book going back on the shelf — automatically, even if you got distracted.

Key terms

• open() — Opens a file and returns a file object.
• Mode — How the file is opened: "r", "w", "a", "x".
• with — Runs a block and then closes the file automatically.