Chapter 1 · Introduction to Automated Testing and pytest

Chapter goals: understand what "automated testing" actually tests, install pytest, and write and run your first test.

1.1 What Is Automated Testing

Manual testing means "a human opens the page, clicks around, and eyeballs the result"; automated testing turns those checks into code so the machine re-runs them automatically on every change. Every test boils down to three steps:

1. Arrange — construct inputs and dependencies;
2. Act — invoke the code under test;
3. Assert — use assert to check whether the result matches expectations.

Why pytest

pytest is the de facto standard testing framework in the Python ecosystem: minimal syntax (plain assert works), a huge plugin ecosystem (400+ plugins), and adoption by mainstream projects such as FastAPI, pandas, and requests. This tutorial targets the current pytest 9.x.

1.2 Installation and Verification

python -m venv .venv && source .venv/bin/activate
pip install pytest

Verify the installation:

pytest --version
# pytest 9.x.x

1.3 Your First Test

pytest makes only two conventions about test files:

• The filename must be test_*.py or *_test.py;
• Test function names must start with test_.

Create test_demo.py:

# test_demo.py
def add(a, b):
    return a + b


def test_add_two_positive_numbers():
    assert add(2, 3) == 5


def test_add_negative_numbers():
    assert add(-1, -2) == -3

Run it:

$ pytest -v
================= test session starts =================
collected 2 items

test_demo.py::test_add_two_positive_numbers PASSED
test_demo.py::test_add_negative_numbers PASSED

================== 2 passed in 0.01s ==================

1.4 What pytest Tells You When Tests Fail

Deliberately write a failing test:

def test_failed_example():
    assert add(2, 2) == 5

pytest's killer feature is assertion rewriting: when something fails, the output doesn't just print a bare AssertionError — it prints every intermediate value in the expression:

>       assert add(2, 2) == 5
E       assert 4 == 5
E        +  where 4 = add(2, 2)

The line where 4 = add(2, 2) tells you exactly what the function returned — an experience unittest's self.assertEqual cannot offer.

1.5 What Should a Test Actually Verify?

Two common beginner mistakes:

• ❌ Cramming a dozen unrelated assertions into one test function (when it fails you lose track of which step broke);
• ❌ Testing the language itself just to chase coverage (assert isinstance([], list) has zero value).

A good unit test tests behavior, not implementation. For add, you care about "adding two numbers yields the correct sum", not whether it uses + or bit tricks internally.

1.6 Chapter Summary

• Automated testing = the Arrange / Act / Assert cycle;
• pytest discovers tests by naming conventions: test_*.py files plus test_-prefixed functions;
• Plain assert statements are all you need; assertion rewriting produces detailed failure output;
• Test behavior, not implementation.

🧪 随堂测验

点击你认为正确的选项。答错时会展示正确答案与原因解析。

1. Which filename will pytest collect by default?

2. What is the correct way to write assertions in pytest?

3. Test function test_add outputs "F" in the progress dots. What does it mean?

4. Regarding "good unit tests", which statement is correct?

🛠️ Hands-On Exercises

1. Add 3 more edge-case tests for this chapter's add function: zero, floats, and very large integers.
2. Create a strings.py implementing slugify(text) (replace spaces with -, lowercase everything), and write at least 3 tests for it.
3. Deliberately break a test, observe the where ... lines in pytest's output, then restore the correct version.

After finishing, run pytest -v and make sure everything passes before moving on to Chapter 2.