Async fixtures (pytest-asyncio) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an async fixture in pytest-asyncio?

An async fixture is a special setup function defined with async def that allows asynchronous setup and teardown in pytest tests using pytest-asyncio.

beginner

How do you declare an async fixture in pytest-asyncio?

Use the @pytest_asyncio.fixture decorator above an async def function to declare an async fixture.

intermediate

Why use async fixtures instead of regular fixtures?

Async fixtures let you run asynchronous setup code like opening async database connections or starting async servers, which regular fixtures cannot do.

beginner

How do you use an async fixture in a test function?

Simply add the fixture name as a parameter to an async def test function. pytest-asyncio will await the fixture automatically.

intermediate

What happens if you try to use an async fixture in a regular (non-async) test function?

The test will fail because pytest cannot await the async fixture in a synchronous test function. The test must be async to use async fixtures.

Which decorator is used to create an async fixture in pytest-asyncio?

A @pytest.fixture

B @pytest_async.fixture

C @asyncio.fixture

D @pytest_asyncio.fixture

The correct decorator for async fixtures in pytest-asyncio is @pytest_asyncio.fixture.

What must a test function be to use an async fixture?

A A regular def function

B A static method

C An async def function

D A class method

The test function must be declared with async def so pytest-asyncio can await async fixtures.

What is a common use case for async fixtures?

A Setting up async database connections

B Running CPU-bound tasks

C Opening synchronous files

D Printing debug messages

Async fixtures are often used to set up asynchronous resources like database connections.

What happens if you forget to await an async fixture in your test?

A The test passes silently

B The test fails with an error

C The fixture runs synchronously

D The test is skipped

If an async fixture is not awaited properly, pytest will raise an error because it expects an awaitable.

Which pytest plugin enables async fixtures?

A pytest-asyncio

B pytest-mock

C pytest-cov

D pytest-xdist

The pytest-asyncio plugin provides support for async fixtures and async test functions.

Explain how to create and use an async fixture in pytest-asyncio.

Think about the decorator, async function, and how tests receive the fixture.

Describe why async fixtures are important in testing asynchronous code.

Consider what async fixtures help manage in tests.

Practice

1. What is the main purpose of using async def in pytest fixtures with pytest-asyncio?

easy

A. To allow the fixture to perform asynchronous setup and cleanup operations

B. To make the fixture run faster by using multiple threads

C. To automatically retry the fixture if it fails

D. To convert the fixture into a synchronous function

Solution

1. Step 1: Understand async def in pytest fixtures

   Using async def allows the fixture to run asynchronous code, which is necessary for async setup or cleanup tasks.

2. Step 2: Compare with other options

   Options A, B, and C describe unrelated behaviors: synchronous conversion, threading, retries, which are not the purpose of async def in fixtures.

3. Final Answer:

   To allow the fixture to perform asynchronous setup and cleanup operations -> Option A

4. Quick Check:

   async def in fixtures = async setup/cleanup [OK]

Hint: Async fixtures enable async setup and cleanup [OK]

Common Mistakes:

• Thinking async def makes tests run in parallel
• Confusing async with threading
• Assuming async def retries tests automatically

2. Which of the following is the correct way to define an async fixture using pytest-asyncio?

easy

A. async def my_fixture(): yield 'data'

B. def my_fixture(): yield 'data'

C. async def my_fixture(): return 'data'

D. def my_fixture(): return 'data'

Solution

1. Step 1: Identify async fixture syntax

   Async fixtures must be defined with async def and use yield to allow setup and cleanup.

2. Step 2: Evaluate options

   async def my_fixture(): yield 'data' correctly uses async def and yield. def my_fixture(): yield 'data' is synchronous. async def my_fixture(): return 'data' uses return which does not support cleanup. def my_fixture(): return 'data' is synchronous and uses return.

3. Final Answer:

   async def my_fixture(): yield 'data' -> Option A

4. Quick Check:

   Async fixture = async def + yield [OK]

Hint: Async fixtures use async def and yield, not return [OK]

Common Mistakes:

• Using return instead of yield in async fixtures
• Defining fixture without async def
• Mixing synchronous and asynchronous syntax

3. Given the following code, what will be printed when running the test?

import pytest
import asyncio

@pytest.fixture
async def async_resource():
    print('Setup')
    yield 'resource'
    print('Cleanup')

@pytest.mark.asyncio
def test_example(async_resource):
    print(f'Test using {async_resource}')

medium

A. Test using resource\nSetup\nCleanup

B. Setup\nTest using resource\nCleanup

C. Setup\nCleanup\nTest using resource

D. Test using resource only

Solution

1. Step 1: Understand async fixture execution order

   The fixture prints 'Setup' before yielding the resource, then the test runs, printing 'Test using resource', and finally the fixture prints 'Cleanup' after the test finishes.

2. Step 2: Match output sequence

   The output order is 'Setup', then 'Test using resource', then 'Cleanup', matching Setup\nTest using resource\nCleanup.

3. Final Answer:

   Setup\nTest using resource\nCleanup -> Option B

4. Quick Check:

   Fixture setup -> test -> fixture cleanup = Setup, Test, Cleanup [OK]

Hint: Fixture prints before yield, cleanup prints after yield [OK]

Common Mistakes:

• Assuming cleanup runs before test
• Confusing yield with return
• Ignoring async execution order

4. What is wrong with this async fixture code?

import pytest

@pytest.fixture
async def resource():
    data = await get_data()
    return data

Assuming get_data() is an async function.

medium

A. Fixture should not call async functions

B. Fixture must not be async if it uses await

C. Fixture must be decorated with @pytest.mark.asyncio

D. Async fixtures must use yield, not return, to allow cleanup

Solution

1. Step 1: Check async fixture structure

   Async fixtures that need cleanup must use yield to separate setup and teardown phases.

2. Step 2: Analyze the code

   This fixture uses return, so it cannot perform cleanup after the test. Using yield is required for cleanup.

3. Final Answer:

   Async fixtures must use yield, not return, to allow cleanup -> Option D

4. Quick Check:

   Async fixture cleanup requires yield, not return [OK]

Hint: Use yield in async fixtures for cleanup, not return [OK]

Common Mistakes:

• Using return instead of yield in async fixtures
• Thinking async fixtures can't await
• Adding @pytest.mark.asyncio to fixtures instead of tests

5. You want to write an async fixture that opens a database connection before tests and closes it after. Which code snippet correctly implements this using pytest-asyncio?

hard

A. @pytest.fixture
async def db_conn():
conn = open_db()
yield conn
conn.close()

B. async def db_conn():
conn = await open_db()
yield conn
await conn.close()

C. @pytest.fixture
async def db_conn():
conn = await open_db()
yield conn
await conn.close()

D. @pytest.fixture
async def db_conn():
conn = await open_db()
return conn

Solution

1. Step 1: Identify correct fixture decorator and async syntax

   The fixture must be decorated with @pytest.fixture and defined as async def to support async setup and cleanup.

2. Step 2: Check for proper use of yield and await

   @pytest.fixture async def db_conn(): conn = await open_db() yield conn await conn.close() correctly awaits open_db(), yields the connection, and awaits conn.close() after the test. async def db_conn(): conn = await open_db() yield conn await conn.close() misses the decorator. @pytest.fixture async def db_conn(): conn = open_db() yield conn conn.close() misses awaits. @pytest.fixture async def db_conn(): conn = await open_db() return conn uses return, so no cleanup.

3. Final Answer:

   @pytest.fixture async def db_conn(): conn = await open_db() yield conn await conn.close() -> Option C

4. Quick Check:

   Async fixture with @pytest.fixture + async def + yield + await cleanup [OK]

Hint: Always decorate async fixtures with @pytest.fixture and use yield [OK]

Common Mistakes:

• Forgetting @pytest.fixture decorator
• Using return instead of yield for cleanup
• Not awaiting async calls in fixture