Phase 6: Testing & Code Quality · ~35 minutes · Python · uv · pytest

Writing Your First Real Tests

assert 3 == 2.5 — pytest doesn't just tell you a test failed. It tells you exactly what your code actually returned, right in the failure message.

Hiring signal: Writes small, focused tests with self-explanatory names instead of one giant test that only tells you something broke

What you will learn

Write and run real pytest tests using plain assert statements
Read a real pytest failure output and identify exactly what was expected vs. what happened
Explain why many small, focused tests are more useful than one large test checking everything
Name tests so a failure is self-explanatory without reading the test body

Introduction

Type: Learn Languages: Python Prerequisites: Lesson 01 (Why Untested Code Lies to You) Time: ~35 minutes

Objective

Learning objectives

Write and run real pytest tests using plain assert statements
Read a real pytest failure output and identify exactly what was expected vs. what happened
Explain why many small, focused tests are more useful than one large test checking everything
Name tests so a failure is self-explanatory without reading the test body

What you're building

A script test_your_function.py for a real function you wrote in an earlier phase (Phase 04 or 05 is a good source — a sorting function, a recursive function, a class method):

At least 4 separate test_* functions, each testing exactly one behavior, with names specific enough that a failure alone tells you what broke
At least one test that specifically targets an edge case (empty input, a boundary value, or similar — per Lesson 01's discipline)
Run with pytest -v and include, as a comment at the top of the file, the real terminal output showing all tests passing
If you can find or construct a genuine bug in the function under test, include a second comment showing the real FAILED output before you fixed it

A test file has one single test function, test_everything(), containing 12 different assert statements checking 12 different behaviors of a function. It fails. What's the most accurate description of what you'll learn from the failure report?

A test function is one unit as far as pytest is concerned — it runs until the first assert that fails, then stops and reports that one function as FAILED, exactly as this lesson's test_even_length_list example did. With 12 unrelated checks crammed into one function, a failure tells you 'something in here is wrong' and stops before even attempting the other 11 — you'd have to comment out or reorder assertions by hand to find out about the rest. This is the precise, concrete cost of not splitting behaviors into separate, individually-named test functions.

Unlock the full lesson

You've read the first 2 sections. The rest of this lesson covers The Problem, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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