Everything you've written so far vanishes when the program ends. Files change that — they're how data survives.

The open() function gives you a file object to work with.

with open("notes.txt", "w") as f:
    f.write("Line one\nLine two\nLine three\n")

f = open("notes.txt")
content = f.read()
f.close()

print(content)

(The first block creates a file to read — writing is properly covered in the next lesson.)

open() takes a filename and returns a file object. .read() returns the whole contents as one string. .close() releases the file.

Modes

The second argument sets the mode:

| Mode | Meaning | | --- | --- | | "r" | read (the default) — errors if the file is missing | | "w" | write — creates or completely overwrites | | "a" | append — adds to the end | | "x" | create — errors if the file already exists |

open("notes.txt") and open("notes.txt", "r") are the same thing.

Three ways to read

.read() gives you everything as one string:

with open("data.txt", "w") as f:
    f.write("apple\nbanana\ncherry\n")

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

.readlines() gives you a list, one item per line — note each still carries its \n:

with open("data.txt", "w") as f:
    f.write("apple\nbanana\ncherry\n")

with open("data.txt") as f:
    lines = f.readlines()

print(lines)
print(len(lines))

Looping over the file gives one line at a time, and is the best option for large files because it never loads the whole thing into memory:

with open("data.txt", "w") as f:
    f.write("apple\nbanana\ncherry\n")

with open("data.txt") as f:
    for line in f:
        print(line.strip())

.strip() removes the trailing newline — without it you'd get a blank line between each.

A file can only be read once

The file object tracks a position. Once you reach the end, there's nothing left:

with open("data.txt", "w") as f:
    f.write("hello\n")

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

The second .read() returns an empty string. Store the contents in a variable if you need them twice.

Missing files

Reading a file that doesn't exist raises FileNotFoundError:

f = open("nope.txt")

Lesson 6.6 shows how to handle that gracefully instead of crashing.

Try it yourself

Write three lines, then read them back both as one string and as a list.

Checkpoint

• open(path, mode) returns a file object; "r" is the default mode.
• .read() gives one string, .readlines() a list, looping gives line by line.
• Lines keep their trailing \n — use .strip().
• A file object can only be read through once.
• A missing file raises FileNotFoundError.