Project 7 — Contacts Manager · Project 8 — BMI Tracker · Project 9 — Budget Tracker · Project 10 — Library Manager

What you'll learn

• 1Model a Contact with a dataclass
• 2Implement ContactBook CRUD operations
• 3Full-text search across all fields
• 4Export to CSV and JSON formats
• 5Calculate BMI and classify the result
• 6Store daily measurements in JSON
• 7Plot weight history as an ASCII chart
• 8Compute 7-day and 30-day trends
• 9Model transactions with a dataclass
• 10Categorize income vs. expense transactions
• 11Compute monthly totals, savings rate, and top expenses
• 12Load and save all data to CSV
• 13Design a multi-class system: Book, Member, Library
• 14Implement borrow/return with due dates
• 15Detect overdue books and calculate fines
• 16Full-text search by title, author, or ISBN

Project 7 — Contacts Manager

Build an object-oriented contacts manager with name, phone, email, and tags. Support search, sort, and CSV/JSON export.

Features

• Contact: name, phones (list), emails (list), tags (list), notes
• Commands: add, show, edit, delete, search, list, export csv/json, quit
• Search: case-insensitive full-text across name, email, tags
• Sort: by name (default), by date added

Sample Session

> add
  Name: Alice Johnson
  Phone: 010-1234-5678
  Email: alice@example.com
  Tags (comma-sep): work, python
  Contact saved.

> search python
  [1] Alice Johnson  010-1234-5678  alice@example.com  [work, python]

> export csv contacts.csv
  Exported 1 contacts to contacts.csv

Project 8 — BMI Tracker

Track BMI over time with a history log, trend analysis, and an ASCII chart of weight progression.

BMI Categories

Sample Output

Date: 2024-05-01
Weight (kg): 72.5
Height (cm): 175
BMI: 23.67 — Normal

Weight History (last 7 days):
2024-04-25 |████████████████████ 74.0
2024-04-28 |███████████████████  73.2
2024-05-01 |██████████████████   72.5

Trend: ↓ 1.5 kg over 6 days

Common Mistakes (FAQ)

Q. My BMI comes out as ~7700 — what's wrong?

You used height in centimeters. BMI = kg / m², so convert first: 175 cm → 1.75 m, then bmi = weight / (1.75 ** 2). Dividing by 175² instead of 1.75² is the classic bug.

Q. The program crashes when I type the weight.

float(input()) raises ValueError on non-numeric input. Wrap it in try/except ValueError and re-prompt, so a stray letter or empty line doesn't kill the session.

Q. How do I avoid a ZeroDivisionError?

Guard the height before dividing: if height_m <= 0, ask again. A height of 0 (or a blank that you coerced to 0) is the only way to hit it.

Q. Which category does exactly 25.0 fall into?

Decide your boundaries explicitly: < 18.5 Underweight, 18.5–24.9 Normal, 25–29.9 Overweight, ≥ 30 Obese. Use >= for the lower edge of each band so 25.0 is Overweight, and document it — boundary handling is the most common grading miss.

Q. How do I scale the ASCII bar chart?

Scale each bar to the max weight in the window: bar_len = round(w / max_w * WIDTH), then print '█' * bar_len. Don't scale to a fixed number or short series look identical.

Project 9 — Budget Tracker

Track income and expenses by category. Generate monthly summaries, spending breakdowns, and savings rate reports — all stored in CSV.

Transaction Model

@dataclass
class Transaction:
    date: str       # YYYY-MM-DD
    type: str       # income | expense
    amount: float
    category: str   # food, transport, salary, ...
    note: str = 

Commands

• add <income|expense> <amount> <category> [note]
• list [--month YYYY-MM]
• summary [--month YYYY-MM]
• categories — show all categories with totals
• export <filename.csv>

Common Mistakes (FAQ)

Q. My totals are off by a cent (e.g. 0.1 + 0.2 = 0.30000000000000004).

Floating-point can't represent money exactly. For display, round(total, 2); for exact arithmetic, store amounts as integer cents (7250 = $72.50) or use decimal.Decimal. Pick one and stay consistent.

Q. Should expenses be negative or positive?

Cleanest: store a positive amount plus a type field ('income'/'expense'), then balance = sum(income) − sum(expense). Mixing signed amounts and a type field double-counts — choose one convention.

Q. How do I filter by month?

With ISO dates (YYYY-MM-DD), a string prefix compare is enough: tx.date[:7] == '2024-05'. No datetime parsing needed for month filtering.

Q. KeyError when totaling a new category.

Use collections.defaultdict(float) (or totals.get(cat, 0)) so a never-seen category starts at 0 instead of raising.

Q. After reloading the CSV, amounts won't add up.

The csv module returns every field as a string — '50' + '20' is '5020'. Convert on load: amount = float(row['amount']). Also write with newline='' on Windows to avoid blank rows.

Project 10 — Library Manager

Manage a book library with borrow/return tracking, member accounts, overdue detection, and full-text search.

Classes

• Book: isbn, title, author, category, copies_total, copies_available
• Member: id, name, email, borrowed_books (list of {isbn, due_date})
• Library: books (dict), members (dict), borrow/return/search methods

Commands

• book add/list/search <query>
• member add/list
• borrow <member_id> <isbn>
• return <member_id> <isbn>
• overdue — list all overdue borrowings with fines

Common Mistakes (FAQ)

Q. Should books/members be a list or a dict?

Use a dict keyed by ISBN / member id — lookup is O(1). A list forces an O(n) scan on every borrow/return/search and makes duplicate-key bugs easy to miss.

Q. A book gets borrowed even when no copies are left.

Check copies_available > 0 before decrementing, and return a clear message otherwise. Decrement on borrow, increment on return — and guard return so copies_available never exceeds copies_total.

Q. How do I compute overdue days correctly?

Parse the stored ISO date with date.fromisoformat(due) and compare to date.today(): overdue_days = (date.today() - due).days, overdue when > 0. Don't string-compare dates for arithmetic.

Q. Adding a member/book with an existing id silently overwrites.

dict assignment replaces without warning. Check `if isbn in self.books:` first and reject (or merge copies) so you don't lose the existing record.

Q. json.dump fails on my Book/Member objects.

Objects (and dataclasses) aren't JSON-serializable directly. Convert to plain dicts on save (dataclasses.asdict, or a to_dict()), and reconstruct the objects when loading.

📝 Exercises

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