Python - Files & I/O

Overview

Estimated time: 35–45 minutes

Open, read, and write files safely with context managers. Handle encodings correctly and use json/csv for simple data formats.

Learning Objectives

• Use with open(...) to manage file resources.
• Set encoding explicitly for text files (UTF-8).
• Read and write JSON/CSV with the standard library.

Prerequisites

• Python - Iterators & Generators

Text files

from pathlib import Path
p = Path("hello.txt")

with p.open("w", encoding="utf-8") as f:
    f.write("Hello, file!\n")

with p.open("r", encoding="utf-8") as f:
    content = f.read()
    print(content)

Expected Output: Hello, file!

JSON

import json

obj = {"name": "Ann", "age": 30}
with open("person.json", "w", encoding="utf-8") as f:
    json.dump(obj, f)

with open("person.json", "r", encoding="utf-8") as f:
    data = json.load(f)
    print(data["name"])  # Ann

CSV

import csv

rows = [("name", "age"), ("Ann", 30), ("Ben", 25)]
with open("people.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

with open("people.csv", "r", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

Common Pitfalls

• Not specifying encoding; default may vary by OS.
• Forgetting newline="" when writing CSV on Windows, causing blank lines.

Checks for Understanding

1. Why use with when opening files?
2. How do you write a dict to a JSON file?

Exercises

1. Write a list of dicts to JSON and read it back, printing one field from each.
2. Append a new row to an existing CSV file.