What is a “unit”?

A unit is the smallest testable piece of your code.

Usually:

• a function
• a method
• a small class

What unit tests should be

• fast (no DB, no network)
• deterministic (same input → same output)
• focused (one behavior per test)

What unit tests should avoid

• real HTTP calls
• real databases
• time-dependent behavior (unless controlled)

Example unit logic

def apply_discount(price: float, percent: float) -> float:
    if percent < 0 or percent > 100:
        raise ValueError("percent must be between 0 and 100")
    return round(price * (1 - percent / 100), 2)

def apply_discount(price: float, percent: float) -> float:
    if percent < 0 or percent > 100:
        raise ValueError("percent must be between 0 and 100")
    return round(price * (1 - percent / 100), 2)

A unit test for this function is small and has no dependencies.

Key takeaway

Unit tests are your fastest safety net for refactoring.

🧪 Try It Yourself

Exercise 1 – Write a unittest TestCase

Exercise 2 – assertRaises

Exercise 3 – setUp and tearDown