Write unit tests for legacy code before you change it

Paste the full function or class and name any external dependencies you know of. Run the tests against the unmodified code: they must all pass before you start refactoring.

Language: Python 3.11 with pytest · Dependencies: datetime.now() and MySQL queries · Goal: split a 300-line function that calculates sales commissions.

Assuming the full function reads the customer's orders via PDO and uses date() to work out tenure…

2. Seams

• Database: add an optional ?PDO $pdo = null parameter that falls back to the current connection when null.
• Date: add ?DateTimeImmutable $now = null and replace date() with $now->format(...).

Both changes leave every existing call site untouched.

3. Characterization tests

<?php
use PHPUnit\Framework\TestCase;

final class CalculateDiscountTest extends TestCase
{
    private PDO $pdo;

    protected function setUp(): void
    {
        $this->pdo = new PDO('sqlite::memory:');
        // Minimal schema mirroring the real table: [FILL IN]
        $this->pdo->exec('CREATE TABLE orders (customer_id INT, total REAL, created_at TEXT)');
    }

    public function test_given_customer_without_orders_when_total_is_100_then_current_result(): void
    {
        $now = new DateTimeImmutable('2025-03-15 10:00:00');

        $result = calculateDiscount(['id' => 42], 100.0, $this->pdo, $now);

        $this->assertSame(0.0, $result); // [FILL IN: run and paste the real output]
    }

    public function test_given_negative_total_then_it_does_not_throw(): void
    {
        // POSSIBLE BUG: a negative total is not rejected; we lock in current behavior
        $result = calculateDiscount(['id' => 42], -50.0, $this->pdo, new DateTimeImmutable());
        $this->assertIsFloat($result);
    }
}

Run each test first, paste the real value into the assertion, and only then start refactoring…