Reading & Writing Files

Programs need to save data and read it back: settings, logs, reports, datasets. Python's built-in open() handles text and binary files, and the with statement makes sure files are always closed properly.

Writing a file

PY write.py

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
    f.write("Second line\n")
    print("Third line", file=f)

print("saved")

Output

saved

open(path, mode) returns a file object. Mode "w" creates the file, or overwrites it if it exists. write() doesn't add newlines for you; print(..., file=f) does. The with block closes the file automatically when it ends — even if an error occurs.

Always pass encoding="utf-8" for text files. Without it, Python uses the system's default encoding, which differs between Windows and other systems and can garble accented characters and emoji.

Reading a file

PY read.py

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("apples\nbananas\ncherries\n")

with open("notes.txt", encoding="utf-8") as f:
    content = f.read()
print(repr(content))

with open("notes.txt", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip("\n").upper())

with open("notes.txt", encoding="utf-8") as f:
    lines = f.read().splitlines()
print(lines)

Output

'apples\nbananas\ncherries\n'
APPLES
BANANAS
CHERRIES
['apples', 'bananas', 'cherries']

f.read() returns the whole file as one string.
Looping over the file gives one line at a time, including its trailing \n. This is memory-efficient even for huge files.
f.read().splitlines() gives a list of lines without newline characters.

File modes

Mode	Meaning
`"r"`	Read (default). Error if the file doesn't exist
`"w"`	Write. Creates or truncates the file
`"a"`	Append to the end. Creates the file if needed
`"x"`	Create; error if the file already exists — safe against overwriting
`"b"`	Binary, combined with the above: `"rb"`, `"wb"`
`"+"`	Read and write: `"r+"`

Appending

PY append.py

from datetime import datetime

def log(message):
    stamp = datetime(2026, 10, 6, 9, 30).strftime("%H:%M")
    with open("app.log", "a", encoding="utf-8") as f:
        f.write(f"{stamp} {message}\n")

open("app.log", "w").close()
log("started")
log("processed 42 rows")

with open("app.log", encoding="utf-8") as f:
    print(f.read(), end="")

Output

09:30 started
09:30 processed 42 rows

Handling missing files

PY missing.py

def read_settings(path):
    try:
        with open(path, encoding="utf-8") as f:
            return f.read()
    except FileNotFoundError:
        print(f"{path} not found, using defaults")
        return ""

read_settings("does-not-exist.ini")

Output

does-not-exist.ini not found, using defaults

Processing a file line by line

PY process.py

data = """name,score
Ana,91
Ben,78
Cleo,85
"""
with open("scores.csv", "w", encoding="utf-8") as f:
    f.write(data)

total = count = 0
with open("scores.csv", encoding="utf-8") as f:
    next(f)
    for line in f:
        name, score = line.strip().split(",")
        total += int(score)
        count += 1

print(f"{count} students, average {total / count:.1f}")

Output

3 students, average 84.7

next(f) skips the header row. For real CSV files with quoted commas, use the csv module (lesson 45).

Binary files

PY binary.py

data = bytes([0x89, 0x50, 0x4E, 0x47])

with open("header.bin", "wb") as f:
    f.write(data)

with open("header.bin", "rb") as f:
    raw = f.read()

print(raw, raw[1:].decode("ascii"), len(raw))

Output

b'\x89PNG' PNG 4

Binary mode reads and writes bytes instead of str — use it for images, PDFs, archives and any non-text format. Never pass encoding in binary mode.

Several files at once

PY two_files.py

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("b\na\nc\n")

with open("notes.txt", encoding="utf-8") as src, open("sorted.txt", "w", encoding="utf-8") as dst:
    dst.writelines(sorted(src))

with open("sorted.txt", encoding="utf-8") as f:
    print(f.read().split())

Output

['a', 'b', 'c']

Try it yourself

The editor runs Python in your browser with a temporary, in-memory file system, so you can create and read files freely:

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