Is the Testing with pytest lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Testing with pytest runs real Python in your browser via Pyodide.

How long does the Testing with pytest lesson take?

About 12 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Testing with pytest lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Testing with pytest lesson?

Advanced level. Assumes comfort with classes, functions, and standard library. Not for first-time Python learners.

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Advanced·12 min·engineering · testing · pytest

Testing with pytest

Python's stdlib has unittest, but nobody uses it if pytest is an option. pytest is:

Zero ceremony — a function named test_* in a file named test_*.py is a test.
Plain assert — no self.assertEqual verbosity.
Rich failure output — shows the actual value that broke.

The whole API

assert expected == actual — pytest rewrites this to show both sides on failure.
with pytest.raises(SomeError, match="..."): — assert exception + optional message.
@pytest.mark.parametrize("a,b,expected", [...]) — one test, many inputs.
@pytest.fixture — reusable setup (next lesson).
pytest -v verbose, pytest -k slugify filter by name, pytest -x stop on first failure.

Test organization

myapp/
  __init__.py
  slugify.py
tests/
  test_slugify.py

Then just run pytest. It discovers everything.

Coverage

pip install pytest-cov
pytest --cov=myapp --cov-report=term-missing

Shows which lines your tests miss. Aim for 80%+ on business logic; don't chase 100% on plumbing.

Try it

Add a test that slugify("Python 🐍") behaves how you'd want.
Add a parametrize case where b is negative.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Write a pytest-style test function named 'test_add' that asserts 1 + 1 equals 2. Then call test_add() so it runs. Pytest is auto-installed for you.
Exercise 2
Use pytest.raises to assert that int('abc') raises a ValueError. Put it inside a function named 'test_raises' and call it.
Exercise 3
Parametrize a test with @pytest.mark.parametrize. Decorate 'test_add(a, b, expected)' with cases (2, 3, 5) and (10, 20, 30), asserting a + b == expected. Then invoke it once with (2, 3, 5) so it runs.

← PreviousDatabase migrations with Alembic Next →pytest fixtures + mocking

Testing with pytest

Python's stdlib has unittest, but nobody uses it if pytest is an option. pytest is:

Zero ceremony — a function named test_* in a file named test_*.py is a test.
Plain assert — no self.assertEqual verbosity.
Rich failure output — shows the actual value that broke.

The whole API

assert expected == actual — pytest rewrites this to show both sides on failure.
with pytest.raises(SomeError, match="..."): — assert exception + optional message.
@pytest.mark.parametrize("a,b,expected", [...]) — one test, many inputs.
@pytest.fixture — reusable setup (next lesson).
pytest -v verbose, pytest -k slugify filter by name, pytest -x stop on first failure.

Test organization

myapp/
  __init__.py
  slugify.py
tests/
  test_slugify.py

Then just run pytest. It discovers everything.

Coverage

pip install pytest-cov
pytest --cov=myapp --cov-report=term-missing

Shows which lines your tests miss. Aim for 80%+ on business logic; don't chase 100% on plumbing.

Try it

Add a test that slugify("Python 🐍") behaves how you'd want.
Add a parametrize case where b is negative.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Write a pytest-style test function named 'test_add' that asserts 1 + 1 equals 2. Then call test_add() so it runs. Pytest is auto-installed for you.
Exercise 2
Use pytest.raises to assert that int('abc') raises a ValueError. Put it inside a function named 'test_raises' and call it.
Exercise 3
Parametrize a test with @pytest.mark.parametrize. Decorate 'test_add(a, b, expected)' with cases (2, 3, 5) and (10, 20, 30), asserting a + b == expected. Then invoke it once with (2, 3, 5) so it runs.