Week 6 — File I/O

Persist data to disk with Python's file I/O. Read and write plain text, CSV, and JSON files using context managers, the csv module, and json module.

fileopencsvjsonpathlibcontext manager

Duration

⏱ 2.5 hours

Level

📊 Intermediate

Prerequisite

🎯 Week 5

OUTCOME

Build a student grade manager that loads/saves CSV and exports JSON reports

What you'll learn

1Open, read, write, and append files with open()
2Use context managers (with statement) for safe file handling
3Parse and write CSV files with the csv module
4Serialize and deserialize JSON with the json module
5Navigate paths with pathlib

1. Text Files

python

# Write
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("Line 1\n")
    f.write("Line 2\n")

# Read all at once
with open("notes.txt", encoding="utf-8") as f:
    text = f.read()

# Read line by line (memory-efficient)
with open("notes.txt", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip())

💡

Always specify encoding="utf-8" to avoid platform-dependent encoding surprises.

2. CSV Files

python

import csv

# Write CSV
with open("students.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.DictWriter(f, fieldnames=["name","score"])
    writer.writeheader()
    writer.writerows([{"name":"Alice","score":90},{"name":"Bob","score":82}])

# Read CSV
with open("students.csv", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["score"])

3. JSON Files

python

import json

data = {"name": "Alice", "scores": [90, 85, 92], "active": True}

# Serialize to file
with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

# Deserialize from file
with open("data.json") as f:
    loaded = json.load(f)
print(loaded["scores"])  # [90, 85, 92]

4. Common Mistakes

Opening without "with" — file may not be closed on error. Always use "with open(...) as f:".
Writing CSV without newline="" → blank rows on Windows.
json.dumps() returns a string; json.dump() writes to a file. Easy to mix up.

💻 Examples

Run these examples and check the output yourself.

01_text_rw.py— Read and write a log file

CODE

from datetime import datetime
LOGFILE = "app.log"

def log(msg):
    ts = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open(LOGFILE, "a", encoding="utf-8") as f:
        f.write(f"[{ts}] {msg}\n")

def read_log():
    with open(LOGFILE, encoding="utf-8") as f:
        return f.read()

log("Application started")
log("User logged in: alice")
print(read_log())

02_grade_csv.py— Grade manager with CSV load/save

CODE

import csv
from pathlib import Path

FILE = Path("grades.csv")

def load():
    if not FILE.exists(): return []
    with FILE.open(encoding="utf-8") as f:
        return list(csv.DictReader(f))

def save(records):
    with FILE.open("w", newline="", encoding="utf-8") as f:
        w = csv.DictWriter(f, fieldnames=["name","score"])
        w.writeheader(); w.writerows(records)

records = load()
records.append({"name": "Diana", "score": "95"})
save(records)
print(load())

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

Config File Manager

Goal: Read and write a JSON config file with default fallback.

Requirements

load_config(path) → dict (returns {} if file missing)
save_config(path, data) → writes JSON with indent=2
update_config(path, key, value) → load, update key, save

▶Toggle solution

SOLUTION

import json
from pathlib import Path

def load_config(path):
    p = Path(path)
    if not p.exists(): return {}
    return json.loads(p.read_text())

def save_config(path, data):
    Path(path).write_text(json.dumps(data, indent=2))

def update_config(path, key, value):
    cfg = load_config(path)
    cfg[key] = value
    save_config(path, cfg)

update_config("config.json", "theme", "dark")
print(load_config("config.json"))

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗