nonobie — Real dev skills, explained like a friend would

Warming up the neural circuits...

What you'll learn

By the end of this chapter you will:

Read and write text files with with open() — never leaking handles
Build cross-platform paths with pathlib instead of string concatenation
Round-trip JSON and CSV data with the json and csv modules
Choose encoding and newline behavior deliberately, not accidentally

The why

Programs that can't persist are toys. Real programs read config files, process CSV data, save results to disk, and log errors. File I/O is the plumbing that connects your code to the outside world.

Core concepts: File operations

Reading FilesOpen section

Let's learn

python

# Read entire file
with open("data.txt", "r") as f:
    content = f.read()
 
# Read lines
with open("data.txt") as f:
    lines = f.readlines()  # list of strings
 
# Read line by line (memory efficient)
with open("data.txt") as f:
    for line in f:
        print(line.strip())

Mode	Description
`'r'`	Read (default)
`'w'`	Write (overwrites!)
`'a'`	Append
`'x'`	Create (fails if exists)
`'b'`	Binary (add to other modes)

Key: Always use with — it closes the file automatically, even if an error occurs.

Writing FilesOpen section

Let's learn

python

# Write string
with open("output.txt", "w") as f:
    f.write("Hello, World!\n")
 
# Write lines
lines = ["Line 1\n", "Line 2\n"

pathlib: Cross-Platform PathsOpen section

Let's learn

python

from pathlib import Path
 
# Build paths
data_dir = Path("data")
file_path = data_dir / "users.json"
# Windows: data\users.json
# Unix: data/users.json
 
# Check existence
print(file_path.exists

JSON: Structured DataOpen section

Let's learn

python

import json
 
# Read JSON from file
with open("config.json") as f:
    config = json.load(f)
 
# Write JSON to file
data = {"name": "Alice", "age"

CSV: Tabular DataOpen section

Let's learn

python

import csv
 
# Read CSV
with open("data.csv") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)  # ['Alice', '30', 'Engineer']
 
# Read as dicts

Mental model: File operations

plaintext

Open → Read/Write → Close
  ↑        ↑          ↑
 with    f.read()   automatic
         f.write()

Common mistakes

Mistake	Why it's wrong	Fix
`open("file")` without `with`	File handle leaks	Always use `with open()`
`open("file", "w")` accidentally	Overwrites file!	Double-check mode
`path = "dir" + "/" + "file"`	Not cross-platform	Use `Path("dir") / "file"`
CSV extra blank lines (Windows)	Missing `newline=""`	`open("f.csv", "w", newline="")`

Practice: Prove your Mastery

beginner Challenge

3 min

Read File

Read a file and print its contents using with open().

Input— given data

data

Read "data.txt"

Expected Output— your code should produce this

expected

File contents printed

Write your solutionCtrl+Enter to run

Ctrl+Enter

beginner Challenge

3 min

pathlib Path

Build a cross-platform path using pathlib.

Input— given data

data

Path("data") / "users.json"

Expected Output— your code should produce this

expected

data/users.json (Unix) or data\users.json (Windows)

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

JSON Round-trip

Write a dict to JSON file, then read it back.

Input— given data

data

{"name": "Alice", "age": 30}

Expected Output— your code should produce this

expected

Same dict read back

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

CSV Reader

Read a CSV file using csv.DictReader.

Input— given data

data

CSV with name, age columns

Expected Output— your code should produce this

expected

List of dicts

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

File Cleanup

Why is with open() better than f = open()?

Input— given data

data

Explain resource management

Expected Output— your code should produce this

expected

with ensures file is closed automatically

Write your solutionCtrl+Enter to run

Ctrl+Enter

Interview Questions

Beginner:

"What does with open() do?"

with open() is a context manager that automatically closes the file when the block exits, even if an error occurs. It prevents file handle leaks.
"What's the difference between json.load() and json.loads()?"

json.load(f) reads JSON from a file object. json.loads(s) parses a JSON string. Use load for files, loads for strings.

Senior:

"Why use pathlib instead of os.path?"

pathlib provides an object-oriented for paths. It's more readable (Path("dir") / "file" vs ), cross-platform, and has useful methods (exists, is_file, glob). It's the modern way to handle paths in Python.

Summary

with open("file") as f: — safe reading, automatic close
pathlib.Path("dir") / "file" — cross-platform paths
json.load(f) / json.dump(data, f) — JSON files
csv.reader(f) / csv.DictReader(f) — CSV files
Always use with to prevent file handle leaks

Quick recall

text

I want to…
├── Read file → with open("f") as f: f.read()
├── Write file → with open("f", "w") as f: f.write(s)
├── Build path → Path("dir") / "file"
├── Read JSON → json.load(f)
├── Write JSON → json.dump(data, f, indent=2)
├── Read CSV → csv.DictReader(f)
└── Write CSV → csv.writer(f)

Self-Check Quiz

Q1 / 3

MCQ

Why use 'with open()' instead of 'f = open()'?

Operation	Code
Build path	`Path("dir") / "file"`
Check exists	`path.exists()`
Read text	`path.read_text()`
Write text	`path.write_text(s)`
List dir	`path.iterdir()`
Glob	`path.glob("*.txt")`

Chapter 9 — File I/O & pathlib

The why

Core concepts: File operations

Common mistakes

Practice: Prove your Mastery

Read File

pathlib Path

JSON Round-trip

CSV Reader

File Cleanup

Interview Questions

Summary

Quick recall

Self-Check Quiz