Week 06 · Intermediate Track

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.

Intermediate Track2 min readFree lesson

Level: Intermediate | Prerequisite: Week 5 (Exception Handling)

Learning Goals

Handle text files safely with with open
Read and write CSV files using the csv module
Save and load structured data in JSON

1. Text Files

with Statement (Recommended)

Automatically closes the file.

#Write
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("First line")
    f.writelines(["A\n", "B\n", "C\n"])

#Read
with open("notes.txt", encoding="utf-8") as f:
    content = f.read()      #All at once
    #Or
    for line in f:           #One line at a time
        print(line.rstrip())

Modes

Mode	Meaning
`r`	Read (default)
`w`	Write (overwrite existing content)
`a`	Append
`r+`	Read/Write
`b`	Binary (e.g., `rb`, `wb`)

⚠️ Always specify encoding="utf-8" (safe for Korean).

2. CSV

Comma-separated table format.

import csv

#Write
with open("scores.csv", "w", encoding="utf-8", newline="") as f:
    w = csv.writer(f)
    w.writerow(["name", "score"])
    w.writerows([["Alice", 92], ["Bob", 78]])

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

⚠️ Missing newline="" causes blank lines (Windows).

3. JSON

Serialize structured data to text.

import json

data = {
    "name": "Hong Gil-dong",
    "skills": ["Python", "SQL"],
    "active": True,
}

#Save
with open("profile.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

#Load
with open("profile.json", encoding="utf-8") as f:
    loaded = json.load(f)
print(loaded["skills"])

Options:

ensure_ascii=False — Keep Korean characters as is
indent=2 — Indent for human readability

Key Examples (examples/)

File	Content
`01_text.py`	Text File Read/Write
`02_csv_write.py`	Write CSV
`03_csv_read.py`	Read CSV (DictReader)
`04_json.py`	JSON dump/load

⚠️ Common Mistakes

Omitting close after open — Automatically handled by using with.
encoding missing — Korean characters break on Windows.
json.dump vs json.dumps — dump is for files, dumps is for strings.
CSV newline="" missing — Empty lines are added on Windows.
Types that cannot be saved as JSON — datetime, user-defined classes. Conversion is needed.

❓ FAQ

Q1. Which should you use, CSV or JSON? A. Tabular data uses CSV, nested/structured data uses JSON.

Q2. Can I read large files all at once? A. They are loaded into memory, so GB-sized files are risky. Iterate line by line or use generators.

Q3. What is pickle? A. Stores the Python object as it is. However, it cannot be read in other languages and poses a security risk. JSON is recommended.

📝 Exercises (exercises/)

exercise1.md — Notepad (text addition)
exercise2.md — CSV score statistics
exercise3.md — Permanent To-Do JSON storage

Next Week

Week 7. Object-Oriented (1)

Example files

The runnable files used in this lesson. Copy them into your own project and experiment.

examples/01_text.pyPython

#Write
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("First line")
    f.write("Second line")
    f.writelines(["A\n", "B\n", "C\n"])

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

print("---")

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

#Append mode
with open("notes.txt", "a", encoding="utf-8") as f:
    f.write("Appended line")

import os
os.remove("notes.txt")

examples/02_csv_write.pyPython

import csv

with open("scores.csv", "w", encoding="utf-8", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "score", "dept"])
    writer.writerows([
        ["Alice", 92, "A"],
        ["Bob", 78, "B"],
        ["Charlie", 85, "A"],
    ])

print("scores.csv created")

#More clearly with DictWriter
with open("scores2.csv", "w", encoding="utf-8", newline="") as f:
    writer = csv.DictWriter(f, fieldnames=["name", "score"])
    writer.writeheader()
    writer.writerow({"name": "Alice", "score": 92})
    writer.writerow({"name": "Bob", "score": 78})

print("scores2.csv created")

examples/03_csv_read.pyPython

import csv

#Read the file created with the above example
with open("scores.csv", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row)
        print(f"{row['name']}: {row['score']} points ({row['dept']} class)")

#Organize
import os
for fn in ("scores.csv", "scores2.csv"):
    if os.path.exists(fn):
        os.remove(fn)

examples/04_json.pyPython

import json

data = {
    "name": "Hong Gil-dong",
    "age": 20,
    "skills": ["Python", "SQL", "Git"],
    "active": True,
    "address": {"city": "Seoul", "zip": "12345"},
}

#Save
with open("profile.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

#Load
with open("profile.json", encoding="utf-8") as f:
    loaded = json.load(f)

print(loaded["name"])
print(loaded["skills"])
print(loaded["address"]["city"])

#Directly as a string
s = json.dumps(data, ensure_ascii=False)
print(s[:50], "...")

import os
os.remove("profile.json")

Assignment

Goal

Take notes in a text file using Add/View CLI.

Commands

add — Enter note → append to the end of the file
show — Print all contents
clear — Clear the file
quit

Requirements

File: notes.txt
When adding, format as [YYYY-MM-DD HH:MM] Memo content

Task 2. CSV Score Statistics

Goal

Read students.csv and print Average/Maximum/Minimum scores.

Input File Format

name,score
Alice,92
Bob,78
Charlie,85

Output Example

Number of students: 3
Average: 85.00
Highest: Alice (92)
Lowest: Bob (78)

Task 3. To-Do JSON Persistent Storage

Goal

Add JSON storage to the Week 10 basic To-Do List program.

Requirements

Load if todos.json exists at start
Save after every change (or once at exit)
Keep data even after restarting

Data Structure

[
  {"task": "Buy milk", "done": false},
  {"task": "Read a book", "done": true}
]

Show the reference solution

Try the assignment yourself first — comparing your attempt with a working solution is where most of the learning happens.

solution1.pyPython

from datetime import datetime
from pathlib import Path

NOTES = Path("notes.txt")

while True:
    cmd = input("Command (add/show/clear/quit):").strip()
    if cmd == "quit":
        break
    elif cmd == "add":
        memo = input("Memo:")
        ts = datetime.now().strftime("%Y-%m-%d %H:%M")
        with NOTES.open("a", encoding="utf-8") as f:
            f.write(f"[{ts}] {memo}\n")
        print("Added")
    elif cmd == "show":
        if NOTES.exists():
            print(NOTES.read_text(encoding="utf-8"))
        else:
            print("(No memo)")
    elif cmd == "clear":
        NOTES.write_text("", encoding="utf-8")
        print("Cleared")

solution2.pyPython

import csv
from pathlib import Path

#Create demo input file
Path("students.csv").write_text(
    "name,score\nAlice,92\nBob,78\nCharlie,85\nDave,67\n",
    encoding="utf-8"
)

with open("students.csv", encoding="utf-8") as f:
    rows = list(csv.DictReader(f))

scores = [(r["name"], int(r["score"])) for r in rows]
total = sum(s for _, s in scores)
avg = total / len(scores)
top = max(scores, key=lambda x: x[1])
bot = min(scores, key=lambda x: x[1])

print(f"Number of students: {len(scores)}")
print(f"Average: {avg:.2f}")
print(f"Highest: {top[0]} ({top[1]})")
print(f"Lowest: {bot[0]} ({bot[1]})")

Path("students.csv").unlink()

solution3.pyPython

import json
from pathlib import Path

DB = Path("todos.json")

def load():
    if DB.exists():
        return json.loads(DB.read_text(encoding="utf-8"))
    return []

def save(todos):
    DB.write_text(json.dumps(todos, ensure_ascii=False, indent=2), encoding="utf-8")

todos = load()
print(f"Loaded {len(todos)} to-dos")

while True:
    cmd = input("Command (add/list/done <n>/quit):").strip()
    if cmd == "quit":
        save(todos)
        break
    if cmd == "add":
        task = input("To-do:")
        todos.append({"task": task, "done": False})
        save(todos)
    elif cmd == "list":
        for i, t in enumerate(todos, 1):
            mark = "[x]" if t["done"] else "[ ]"
            print(f"{i}. {mark} {t['task']}")
    elif cmd.startswith("done "):
        idx = int(cmd.split()[1]) - 1
        if 0 <= idx < len(todos):
            todos[idx]["done"] = True
            save(todos)

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