Python File Handling: Read, Write and Append Files (with open and pathlib)

Programs often need to keep data after they stop running: notes, settings, logs, reports. Reading and writing files is how they do it. Python makes this straightforward with the built-in open() function and the pathlib module.

Writing a file#

with open("notes.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")

• open(filename, mode) opens a file. Mode "w" means write.
• with ... as f: gives you the open file as f, and closes it automatically when the block ends, even if an error occurs.
• write() does not add new lines for you; include \n yourself.

Reading a whole file#

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

print(content)

First line
Second line

The default mode is "r" (read).

Reading line by line#

Looping over a file gives one line at a time, which works even for files too large to fit in memory:

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

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

apple
banana
cherry

Each line ends with \n; strip() removes it (and any surrounding spaces).

Appending#

with open("log.txt", "w") as f:
    f.write("started\n")

with open("log.txt", "a") as f:
    f.write("finished\n")

with open("log.txt") as f:
    print(f.readlines())

['started\n', 'finished\n']

readlines() returns a list with one string per line.

File modes#

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

Encoding#

Always pass encoding="utf-8" for text files. Without it, Python uses the operating system's default, which on Windows may not handle characters like ₹ or Hindi text:

names = ["Asha", "Ravi", "Meera"]

with open("names.txt", "w", encoding="utf-8") as f:
    f.write("\n".join(names))

with open("names.txt", encoding="utf-8") as f:
    loaded = f.read().splitlines()

print(loaded)

['Asha', 'Ravi', 'Meera']

Handling a missing file#

open("missing.txt")

FileNotFoundError: [Errno 2] No such file or directory: 'missing.txt'

Catch it with try/except (see Exceptions), or check first with Path("missing.txt").exists().

pathlib: modern paths#

The pathlib module treats paths as objects, joins them with / on every operating system, and has handy shortcuts:

from pathlib import Path

p = Path("data") / "report.txt"
p.parent.mkdir(exist_ok=True)               # create the data folder
p.write_text("Total: 370\n", encoding="utf-8")

print(p.exists(), p.name, p.suffix, p.read_text(encoding="utf-8").strip())
print(sorted(x.name for x in Path(".").iterdir()))

True report.txt .txt Total: 370
['data']

Path.glob("*.txt") finds files by pattern, which is very useful for automation.

CSV files#

The csv module reads and writes spreadsheet-style data correctly, including commas inside quoted values:

import csv

rows = [["name", "city", "score"], ["Asha", "Pune", 92], ["Ravi", "Delhi", 85]]

with open("scores.csv", "w", newline="", encoding="utf-8") as f:
    csv.writer(f).writerows(rows)

with open("scores.csv", newline="", encoding="utf-8") as f:
    for row in csv.DictReader(f):
        print(row["name"], int(row["score"]) + 1)

Asha 93
Ravi 86

DictReader uses the header row as keys. Every value comes back as a string, so convert numbers yourself. Pass newline="" when opening CSV files, as the csv documentation recommends.

Common mistakes#

Practice#

250
120

560
90

total = 0
count = 0

with open("sales.txt") as f:
    for line in f:
        line = line.strip()
        if not line:
            continue
        total += int(line)
        count += 1

print(f"{count} sales, total {total}")

4 sales, total 1020

Key takeaway#