Reading and Writing Files

Variables disappear when a program ends. To keep data, such as notes, scores, settings or logs, you save it to a file. Python makes reading and writing files straightforward.

About this page: the code here runs in your browser, so files are saved in a temporary in-browser file system. They're kept while this page is open, so one example can read a file another example wrote, and they disappear when you reload the page. On your own computer, the same code reads and writes real files.

Writing a file

open(filename, mode) opens a file. The mode "w" means write: it creates the file, or replaces it if it already exists. The with statement closes the file automatically when the block ends:

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

print("Saved!")

write() doesn't add a newline for you, so add \n yourself at the end of each line.

Reading a whole file

Mode "r" (read) is the default, so you can leave it out:

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

print(content)
print(type(content), len(content), "characters")

Reading line by line

Looping over a file gives you one line at a time. That's memory-friendly even for huge files:

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

Each line still ends with \n, which is why we use strip(). Use readlines() to get all the lines as a list:

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

print(lines)
print([line.strip() for line in lines])

Appending

Mode "a" (append) adds to the end of a file instead of replacing it:

with open("notes.txt", "a") as f:
    f.write("Go for a run\n")

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

File modes

Add b for binary files such as images ("rb", "wb").

Careful with "w": opening an existing file in write mode erases its contents immediately, even if you never write anything.

Why use with?

Without with, you must remember to call close() yourself, and if an error happens first, the file stays open:

f = open("notes.txt")
print(f.readline().strip())
f.close()

with guarantees the file is closed, even if an exception happens inside the block. Always prefer it.

with open("menu.txt", "w", encoding="utf-8") as f:
    f.write("Café crème ☕\n")

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

from pathlib import Path

path = Path("notes.txt")
path.write_text("Buy milk\nCall Ada\n", encoding="utf-8")

print(path.exists())
print(path.name, path.stem, path.suffix)
print(path.read_text(encoding="utf-8"))

from pathlib import Path

folder = Path("reports")
folder.mkdir(exist_ok=True)

report = folder / "2026" / "summary.txt"
print(report)
print(report.parent)

Handling a missing file

Reading a file that doesn't exist raises FileNotFoundError. Catch it like any other exception:

try:
    with open("missing.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("The file doesn't exist yet.")

The os module can check first, and list or delete files:

import os

print(os.path.exists("notes.txt"))
print(os.path.exists("missing.txt"))
print(os.listdir("."))

Writing and reading numbers

Files hold text, so convert numbers to strings when writing and back again when reading:

scores = [88, 92, 75, 64]

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

with open("scores.txt") as f:
    loaded = [int(line) for line in f]

print(loaded, "average:", sum(loaded) / len(loaded))

CSV files

CSV (comma-separated values) is the simplest spreadsheet format. The csv module handles the details, such as commas inside quoted values:

import csv

rows = [
    ["name", "city", "score"],
    ["Ada", "London", 92],
    ["Grace", "New York", 88],
]

with open("people.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

with open("people.csv") as f:
    for row in csv.DictReader(f):
        print(f"{row['name']} from {row['city']} scored {row['score']}")

JSON files

JSON is the standard format for structured data on the web, and it maps directly onto Python dictionaries and lists:

import json

settings = {"theme": "dark", "font_size": 14, "recent": ["main.py", "notes.txt"]}

with open("settings.json", "w") as f:
    json.dump(settings, f, indent=2)

with open("settings.json") as f:
    print(f.read())

with open("settings.json") as f:
    loaded = json.load(f)

print(loaded["recent"][0])

Exercises

Exercise 1: Write and read

Write the three lines red, green and blue to colors.txt, then read the file back and print each colour in upper case.

# write colors.txt, then read it back

Exercise 2: Count lines and words

Create story.txt with the given text, then print how many lines and how many words it has. It should print Lines: 3 and Words: 15.

text = "Once upon a time\nthere was a small python\nwho loved to code all day\n"

Exercise 3: Visit log

Write a function log_visit(name) that appends a line Visited by <name> to log.txt. Call it three times, then print the file.

def log_visit(name):
    pass

Exercise 4: Save a high score

Save the dictionary to scores.json, load it back, and print the name with the highest score: Leo.

import json

high_scores = {"Mia": 1200, "Leo": 1850, "Ava": 990}

File tools at a glance

A quick reference for later. f is a file opened with with open(...) as f:.

Summary

• open(name, mode) opens a file: "r" reads, "w" writes (and erases), "a" appends.
• with open(...) as f: closes the file automatically. Always use it.
• f.read() reads everything, and for line in f: reads line by line. Use strip() to remove the \n.
• f.write() writes text. Add \n yourself, and convert numbers with str() or f-strings.
• A missing file raises FileNotFoundError, and os.path.exists() checks first.
• Use the csv module for spreadsheets and json for structured data.