Pytest

Pytest is the standard testing framework for modern Python. Master professional testing strategies used by real engineering teams.

Learn Python, JavaScript, Java and more with free interactive lessons, real projects and AI-powered help. Beginner-friendly.

Part of the free Python course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

This lesson takes you from basic testing → professional test suite architecture.

🔥 1. Why pytest?

🧪 2. Basic Test Structure

⚙️ 3. What Is a Fixture?

A fixture is reusable setup logic that you can "inject" into tests via function arguments.

🧱 4. Fixture Scopes

By default, fixtures are function-scoped. You can control lifespan with scope:

🔁 5. Setup & Teardown with yield

🧪 6. Parametrised Tests

Instead of writing multiple duplicate tests, use parametrisation:

🧪 7. Basic Mocking

🔧 Factory Fixtures

🧠 Combining Fixtures + Parametrisation

🔐 Monkeypatching

🛰 Mocking External APIs

🧪 Spy Objects

🧪 Test Suite Structure

🧱 Integration Testing

🧱 Mocking Time and Randomness

🎓 Summary

You've learned professional pytest strategies:

📋 Quick Reference — pytest

You can now write fixtures, parametrised tests, and mocks — the full professional pytest toolkit used at tech companies worldwide.

Up next: CLI Tools — build polished command-line applications with argparse and Typer.

Practice quiz

How do pytest tests check conditions?

self.assertEqual(x, y)
A plain assert statement, e.g. assert add(2, 3) == 5
expect(x).toBe(y)
check(x == y)

Answer: A plain assert statement, e.g. assert add(2, 3) == 5. pytest uses Python's built-in assert — no special methods or boilerplate classes required.

By naming convention, pytest discovers test functions that...

start with test_
end with _test
are inside a Test class only
have a @test decorator

Answer: start with test_. Files are test_*.py and functions start with test_ — that's how pytest finds them.

What is a pytest fixture?

A failed test
Reusable setup logic injected into tests via function arguments
A mocking library
A configuration file

Answer: Reusable setup logic injected into tests via function arguments. Fixtures provide reusable setup/teardown that tests receive as arguments.

What is the DEFAULT fixture scope?

session
module
class
function

Answer: function. Fixtures are function-scoped by default — fresh for each test, the safest and most isolated option.

Which scope runs a fixture only ONCE per entire test run?

function
class
module
session

Answer: session. session scope creates one instance for the whole run — ideal for global resources like app config.

In a fixture, what does the code AFTER a yield statement do?

Provides the value to the test
Runs as teardown after the test finishes
Skips the test
Nothing

Answer: Runs as teardown after the test finishes. yield returns the value; the lines after yield run as teardown once the test completes.

Why use @pytest.mark.parametrize?

To mock external systems
To run one test function with many input/output cases
To set fixture scope
To skip slow tests

Answer: To run one test function with many input/output cases. Parametrisation runs the same test across multiple inputs instead of duplicating test functions.

After fake_api.get.return_value = {'name': 'Boopie'}, what does fetch_user(fake_api) return for result['name']?

None
Boopie
An error
'name'

Answer: Boopie. A Mock's return_value is what the call yields, so api.get('/user') returns that dict.

What does fake_api.get.assert_called_once_with('/user') verify?

That get returned '/user'
That get was called exactly once with that argument
That get raised an exception
That get is a real API

Answer: That get was called exactly once with that argument. It asserts the mock was called exactly one time with the given argument — a key way to verify behavior.

Which pytest construct asserts that a block raises a specific exception?

pytest.raises(ValueError)
pytest.expect(ValueError)
assert ValueError
pytest.catch(ValueError)

Answer: pytest.raises(ValueError). with pytest.raises(ValueError): ... passes only if that exception is raised inside the block.