Is the pytest fixtures + mocking lesson free?

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

How long does the pytest fixtures + mocking lesson take?

About 15 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 pytest fixtures + mocking 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 pytest fixtures + mocking lesson?

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

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Advanced·15 min·engineering · testing · pytest · mock

pytest fixtures + mocking

A fixture is a factory function pytest injects into any test that names it as an argument.

Why they matter

Without fixtures, every test starts with 5 lines of setup. With fixtures:

Setup lives in one place.
Each test gets a fresh instance (default scope="function").
Fixtures can depend on other fixtures — composition for free.

Scopes

function (default) — new per test.
class — shared across tests in one class.
module — shared across a file.
session — created once. Use for expensive stuff (real DB, browser).

Mocking with `monkeypatch`

The best mocking tool comes built into pytest:

def test_something(monkeypatch):
    monkeypatch.setattr(mymodule, "send_email", fake_send)
    # ... test uses fake_send until test ends, then auto-restored

Cleaner than unittest.mock.patch for most cases. Auto-undoes at test teardown.

`unittest.mock` when you need it

For call-tracking, MagicMock is unbeatable:

from unittest.mock import MagicMock
mock_client = MagicMock()
mock_client.fetch.return_value = {"ok": True}
service = MyService(mock_client)
service.do_thing()
mock_client.fetch.assert_called_once_with("some-arg")

Try it

Add a bob fixture that depends on repo.
Write a test that monkeypatches send_welcome_email to raise, and asserts signup still returns a valid uid (or handles the error — your choice, document it).

Practice

3 graded exercises

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

Progress0 / 3

Exercise 1
Define a pytest fixture alice that returns {'name': 'Alice', 'age': 30}. Then define test_alice(alice) that asserts alice['name'] == 'Alice'. Since Pyodide can't run the pytest runner, call test_alice manually by passing the dict directly, then print 'ok'.
Exercise 2
Fixture composition. Define a fixture repo returning an empty dict(). Then define alice_repo(repo) that adds {'alice': 30} to the passed repo and returns it. Manually chain: _r = repo(); result = alice_repo(_r); print the result.
Exercise 3
Use pytest.MonkeyPatch() directly (since we can't get the auto-injected fixture in Pyodide). Define read_env() returning os.environ.get('API_KEY', 'unset'). Create mp = pytest.MonkeyPatch(), call mp.setenv('API_KEY', 'secret123'), print read_env(). Then mp.undo() and print read_env() again. Expected output: secret123 then unset.

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pytest fixtures + mocking

A fixture is a factory function pytest injects into any test that names it as an argument.

Why they matter

Without fixtures, every test starts with 5 lines of setup. With fixtures:

Setup lives in one place.
Each test gets a fresh instance (default scope="function").
Fixtures can depend on other fixtures — composition for free.

Scopes

function (default) — new per test.
class — shared across tests in one class.
module — shared across a file.
session — created once. Use for expensive stuff (real DB, browser).

Mocking with `monkeypatch`

The best mocking tool comes built into pytest:

def test_something(monkeypatch):
    monkeypatch.setattr(mymodule, "send_email", fake_send)
    # ... test uses fake_send until test ends, then auto-restored

Cleaner than unittest.mock.patch for most cases. Auto-undoes at test teardown.

`unittest.mock` when you need it

For call-tracking, MagicMock is unbeatable:

from unittest.mock import MagicMock
mock_client = MagicMock()
mock_client.fetch.return_value = {"ok": True}
service = MyService(mock_client)
service.do_thing()
mock_client.fetch.assert_called_once_with("some-arg")

Try it

Add a bob fixture that depends on repo.
Write a test that monkeypatches send_welcome_email to raise, and asserts signup still returns a valid uid (or handles the error — your choice, document it).

Practice

3 graded exercises

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

Progress0 / 3

Exercise 1
Define a pytest fixture alice that returns {'name': 'Alice', 'age': 30}. Then define test_alice(alice) that asserts alice['name'] == 'Alice'. Since Pyodide can't run the pytest runner, call test_alice manually by passing the dict directly, then print 'ok'.
Exercise 2
Fixture composition. Define a fixture repo returning an empty dict(). Then define alice_repo(repo) that adds {'alice': 30} to the passed repo and returns it. Manually chain: _r = repo(); result = alice_repo(_r); print the result.
Exercise 3
Use pytest.MonkeyPatch() directly (since we can't get the auto-injected fixture in Pyodide). Define read_env() returning os.environ.get('API_KEY', 'unset'). Create mp = pytest.MonkeyPatch(), call mp.setenv('API_KEY', 'secret123'), print read_env(). Then mp.undo() and print read_env() again. Expected output: secret123 then unset.

pytest fixtures + mocking

Why they matter

Scopes

Mocking with monkeypatch

unittest.mock when you need it

Try it

pytest fixtures + mocking

Why they matter

Scopes

Mocking with monkeypatch

unittest.mock when you need it

Try it

Mocking with `monkeypatch`

`unittest.mock` when you need it

Mocking with `monkeypatch`

`unittest.mock` when you need it