Testing Core Features

Your application is working and now properly structured.

The next step is to verify that your core logic behaves correctly.

You will write tests for:

• user authentication
• task management
• expense calculations

Goal of This Lesson

By the end of this lesson, you will:

• set up testing
• write basic tests
• validate your application logic
• detect errors early

Step 1: Install pytest

In your terminal:

pip install pytest

Step 2: Create Test File

Create:

tests/test_app.py

Step 3: Import Functions to Test

import time

from app.services.database import create_tables
from app.services.user_service import register_user, login_user
from app.services.task_service import add_task, get_tasks
from app.services.expense_service import add_expense, get_total_expenses

# Make sure tables exist before any test runs.
create_tables()


def _unique(prefix):
    """Return a username unique to this test run."""
    return f"{prefix}_{int(time.time() * 1000)}"

The _unique helper prevents tests from failing on repeat runs because of duplicate usernames.

Step 4: Test User Registration

def test_register_user():
    username = _unique("test_user")
    result = register_user(username, "123")
    assert result is True

Step 5: Test Login

def test_login_user():
    username = _unique("test_login")
    register_user(username, "123")
    user_id = login_user(username, "123")
    assert user_id is not None

Step 6: Test Task Creation

def test_add_task():
    username = _unique("task_user")
    register_user(username, "123")
    user_id = login_user(username, "123")

    add_task(user_id, "Test Task")
    tasks = get_tasks(user_id)

    assert len(tasks) > 0

Step 7: Test Expense Calculation

def test_expense_total():
    username = _unique("expense_user")
    register_user(username, "123")
    user_id = login_user(username, "123")

    add_expense(user_id, 10, "Food")
    add_expense(user_id, 20, "Transport")

    total = get_total_expenses(user_id)

    assert total >= 30

Step 8: Run Tests

In terminal:

pytest

Expected Output

4 passed

Important Notes

• Tests should not depend on GUI
• Only test service logic
• Use simple assertions
• Use _unique to keep tests isolated when run repeatedly

Common Issues

Duplicate Users

If tests fail due to duplicate usernames, the _unique helper should prevent this — make sure you are using it.

Database State

Tests use the same database file.

In advanced setups, you would use a separate test database.

Why This Step Matters

Testing ensures your application:

• works correctly
• handles edge cases
• stays stable as you add features

What You Are Testing

Input → Function → Output

Summary

You added automated tests to verify your application logic, ensuring your system behaves correctly and reliably.