Files, Modules & Mini-Project 2

▶Today's videos

Watch each video, then work the matching sections below. Watching alone will not do it.

1with, and why

Every program you have written so far forgot everything the moment it ended.

# writing
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("first line\n")
    f.write("second line\n")

# reading, whole file
with open("notes.txt", encoding="utf-8") as f:
    content = f.read()

# reading, line by line — the memory-safe way
with open("notes.txt", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip("\n"))

Always pass encoding

encoding="utf-8" is not optional in practice. Without it Python uses whatever the operating system prefers, so a file that reads perfectly on your machine raises UnicodeDecodeError on someone else's. State it every time.

Missing files

import os

# ask first
if os.path.exists("config.txt"):
    with open("config.txt", encoding="utf-8") as f:
        settings = f.read()
else:
    settings = ""

# or try, and handle the failure — this is Day 15's territory
try:
    with open("config.txt", encoding="utf-8") as f:
        settings = f.read()
except FileNotFoundError:
    settings = ""

2Structured files: CSV

Do not write your own parser. Really.

A comma-separated file looks trivial to parse with .split(",") — and it is, until one field contains a comma, or a quote, or a newline. All three happen in real data. The standard library already solved this.

import csv

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

import csv

rows = [{"name": "Amin", "score": 88}, {"name": "Ada", "score": 95}]

with open("out.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.DictWriter(f, fieldnames=["name", "score"])
    writer.writeheader()
    writer.writerows(rows)

Note that everything DictReader gives you is a string — row["score"] is "88", not 88. Converting, and deciding what to do when the conversion fails, is your job. That is Mini-Project 2's real content.

3Modules

One file stops being enough at about 300 lines.

A module is just a .py file. Importing it runs it once and gives you its names.

The line everybody copies without understanding

def main():
    print("running as a program")


if __name__ == "__main__":
    main()

Python sets __name__ to "__main__" in the file you ran, and to the module's own name in anything you imported. So the guard means: run this only when I am the program, not when I am being imported.

4The standard library

Most of what you are about to write already exists.

from collections import Counter
from pathlib import Path
import json

# yesterday's counting pattern, in one line
print(Counter("abracadabra").most_common(3))

# paths that work on every operating system
p = Path("data") / "scores.csv"
print(p.suffix, p.stem, p.exists())

# structured data, saved and reloaded
record = {"name": "Amin", "scores": [88, 95]}
text = json.dumps(record)
print(text)
print(json.loads(text)["scores"])

5Mini-Project 2 — Contact Book

Your second graded deliverable · 100 points

Build a contact book that survives being closed. Everything in Weeks 1 and 2 comes together here: functions, lists, dictionaries, files and modules.

Required behaviour

1. A menu loop: add, list, search, delete, quit.
2. Each contact has a name, a phone number and an email.
3. Contacts persist to contacts.csv and reload on the next run.
4. Search matches part of a name, case-insensitively.
5. Adding a duplicate name asks whether to update the existing entry.
6. Bad input never crashes the program — including a missing or malformed CSV file.
7. The code is split across at least two modules, with an importable one containing no I/O.

Suggested build order

1. Hard-code three contacts in a list of dicts. Get list and search working with no files at all.
2. Add the menu loop around it.
3. Add saving. Check the CSV by opening it in a text editor.
4. Add loading. Now quit and restart — this is the moment the project becomes real.
5. Break it deliberately: delete the file, empty it, corrupt a row, add a stray blank line. Fix what breaks.
6. Split into modules. Run black and ruff. Read it once as a stranger.

Keep this file too. On Day 12 you will turn each contact from a dictionary into an object, and on Day 15 you will write tests for it. Having the earlier version to compare against is, as ever, the lesson.

>_Python playground

A real Python interpreter running inside your browser. Nothing is installed, nothing is uploaded, nothing can break.

✓Exercise set

Checked automatically the moment you submit. Work top to bottom — each one assumes the last. Your answers are saved in this browser.