Python File Handling: Read, Write and Append Files (with JSON and CSV)

Opening files the right way

Use the with statement. It opens the file and guarantees it is closed afterwards, even if an error occurs:

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

with open("notes.txt", encoding="utf-8") as f:   # mode "r" is default
    content = f.read()
print(content)

File modes: "r" read (default), "w" write (erases existing content!), "a" append, "x" create-only, add "b" for binary ("rb", "wb"). Always pass encoding="utf-8" for text so the file behaves the same on every OS.

Inside the Coding Python app files are created in the app's private storage, so these examples run unchanged on Android.

Reading line by line

with open("notes.txt", encoding="utf-8") as f:
    for number, line in enumerate(f, start=1):
        print(number, line.rstrip())      # strip the trailing newline

with open("notes.txt", encoding="utf-8") as f:
    lines = f.readlines()                 # list of lines
print(len(lines), lines[0])

Iterating the file object directly reads one line at a time and works for files far larger than memory. read() loads the whole thing at once.

Appending and handling missing files

with open("log.txt", "a", encoding="utf-8") as f:
    f.write("app started\n")

try:
    with open("missing.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("No such file - creating a default")
    with open("missing.txt", "w") as f:
        f.write("default\n")

pathlib: modern path handling

from pathlib import Path

p = Path("data") / "scores.txt"      # joins with the right separator
p.parent.mkdir(exist_ok=True)
p.write_text("10\n20\n30\n")

print(p.exists(), p.suffix, p.stem, p.name)
total = sum(int(x) for x in p.read_text().split())
print("Total:", total)
for file in Path("data").glob("*.txt"):
    print("found", file)
p.unlink()                         # delete

Path.read_text() / write_text() cover the common cases in a single call and avoid the with block entirely.

JSON files

JSON is the universal format for config files and web APIs, and it maps directly onto Python dicts and lists:

import json

settings = {"theme": "dark", "font_size": 14, "recent": ["a.py", "b.py"]}
with open("settings.json", "w") as f:
    json.dump(settings, f, indent=2)

with open("settings.json") as f:
    loaded = json.load(f)
print(loaded["recent"][0], type(loaded))

text = json.dumps(settings)          # to a string
print(text)
print(json.loads('{"ok": true, "n": null}'))

Try the JSON example built into the app — the Coding Python app runs real Python 3 on your phone, with examples, quizzes and challenges built in.

Get it free

CSV files

import csv

rows = [["name", "score"], ["Ada", 93], ["Linus", 88]]
with open("scores.csv", "w", newline="") as f:
    csv.writer(f).writerows(rows)

with open("scores.csv", newline="") as f:
    for row in csv.DictReader(f):
        print(row["name"], int(row["score"]) + 1)

Always pass newline="" when opening CSV files, otherwise Windows inserts blank lines. For serious data work you would move to pandas, but csv handles most scripts.

Next lesson: modules and the standard library, where these json, csv and pathlib imports come from.

Frequently asked questions