Expense Tracker API

A small FastAPI service for logging expenses in multiple currencies and computing their total converted into a single base currency using live exchange rates.

Built as a clean, self-contained REST API demonstrating request validation, SQLite persistence, external API integration, and proper error handling.

Features

• Log expenses with amount, currency, category, and date
• Automatic input validation (rejects non-positive amounts, malformed dates)
• Persistent storage via SQLite (survives restarts)
• Convert and total all expenses into any base currency using live rates from the Frankfurter API
• Gracefully skips unknown/unsupported currencies and reports them
• Delete expenses by ID with proper 404 handling

Tech stack

• FastAPI — web framework
• Pydantic — request validation
• SQLite (sqlite3) — persistence
• httpx — async HTTP client for the exchange-rate API

Setup

python3 -m venv venv
source venv/bin/activate        # Windows: venv\Scripts\activate
pip install -r requirements.txt
uvicorn main:app --reload

The API runs at http://localhost:8000. Interactive docs at http://localhost:8000/docs.

Endpoints

Method	Path	Description
GET	/	Health check
POST	/expenses	Log a new expense
GET	/expenses	List all expenses
GET	/expenses/total	Total all expenses in a base currency (default USD)
DELETE	/expenses/{expense_id}	Delete an expense by ID

Example: log an expense

curl -X POST http://localhost:8000/expenses \
  -H "Content-Type: application/json" \
  -d '{"amount": 45.50, "currency": "USD", "category": "food", "date": "2026-07-25"}'

Example: get the total in EUR

curl "http://localhost:8000/expenses/total?base_currency=EUR"

Response:

{
  "base_currency": "EUR",
  "total": 123.45,
  "expenses_count": 7,
  "skipped": []
}

Running tests

The test suite uses pytest, with an isolated temporary database per test and a mocked exchange-rate API (so tests run offline and produce deterministic totals).

pip install -r requirements.txt
python -m pytest tests/ -v

Notes

• Currency codes are normalized to uppercase on input.
• The exchange-rate lookup makes a single API call per total request, regardless of how many expenses or currencies are involved.
• The SQLite database file (expenses.db) is created automatically on first run and is excluded from version control.