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Why unit tests and how to make them work for you

Why TDD?​

Test-driven development (TDD) is a software development approach where you:

  1. Write a failing test first - This ensures you understand the requirement
  2. Write the minimum code to make it pass - Keep things simple
  3. Refactor - Clean up the code while tests keep you safe

This cycle is often called "Red-Green-Refactor."

Benefits of TDD​

1. Design feedback​

When code is hard to test, it's often a sign that the design could be improved. TDD forces you to think about the interface of your code before the implementation.

2. Documentation​

Tests serve as living documentation. They show exactly how your code is meant to be used and what behavior is expected.

3. Confidence to refactor​

With a good test suite, you can refactor code with confidence. If tests pass after refactoring, you know the behavior hasn't changed.

4. Fewer bugs​

Writing tests first helps you think through edge cases before they become bugs in production.

5. Faster development​

While it might seem slower initially, TDD often speeds up development because:

  • You spend less time debugging
  • You catch issues early when they're cheap to fix
  • You have confidence when making changes

Common objections​

"Writing tests doubles my work"​

Actually, tests often save time because:

  • You debug less
  • You have confidence when refactoring
  • You catch bugs before they reach production

"I don't have time for tests"​

If you don't have time to test, you don't have time to debug. Testing saves time in the long run.

"My code is too simple to test"​

Simple code today becomes complex code tomorrow. Tests give you confidence when adding features.

Making TDD work for you​

1. Start small​

Don't try to test everything at once. Start with one simple function and grow from there.

2. Test behavior, not implementation​

Test what your code does, not how it does it. This makes tests more resilient to refactoring.

3. Keep tests fast​

Slow tests don't get run. Use mocking to isolate slow dependencies.

4. Write readable tests​

Tests are documentation. Make them clear and easy to understand.

5. One assert per test​

Each test should verify one thing. This makes failures easier to diagnose.

The testing pyramid​

  • Unit tests: Fast, test individual functions in isolation
  • Integration tests: Test how components work together
  • E2E tests: Test the whole system from user perspective

Most of your tests should be unit tests because they're fast and reliable.

Python testing tools​

pytest​

The most popular Python testing framework. Simple to use, powerful features.

def test_addition():
    assert 1 + 1 == 2

unittest​

Python's built-in testing framework. More verbose but no dependencies needed.

import unittest

class TestAddition(unittest.TestCase):
    def test_addition(self):
        self.assertEqual(1 + 1, 2)

Coverage​

Measures how much of your code is covered by tests.

pip install pytest-cov
pytest --cov=.

Hypothesis​

Property-based testing - generates test cases automatically.

from hypothesis import given
from hypothesis import strategies as st

@given(st.integers(), st.integers())
def test_addition_commutative(a, b):
    assert a + b == b + a

Summary​

TDD might feel uncomfortable at first, but it's a skill that pays dividends. Start small, practice regularly, and you'll find yourself writing better code with more confidence.