Async fixtures (pytest-asyncio)

Introduction

Async fixtures help prepare things that need to wait for some time or work with async code before tests run.

When you need to set up a database connection that uses async calls.

When you want to prepare a web server or client that works asynchronously.

When you have to load data or resources using async functions before tests.

When cleaning up resources after async operations in tests.

When testing code that uses async functions and you want to share setup steps.

Syntax

PyTest

import pytest

@pytest.fixture
async def my_async_fixture():
    # setup code here
    yield resource
    # teardown code here

Use async def to define async fixtures.

Use yield to provide the fixture value and then run cleanup after the test.

Examples

This fixture waits for setup, gives 'ready' to the test, then cleans up asynchronously.

PyTest

import pytest

@pytest.fixture
async def sample_fixture():
    await some_async_setup()
    yield 'ready'
    await some_async_cleanup()

This fixture gets a number asynchronously and provides it to tests.

PyTest

import pytest

@pytest.fixture
async def number_fixture():
    number = await get_async_number()
    yield number

Sample Program

This test uses an async fixture that waits a bit before and after the test. It prints messages to show the order.

PyTest

import pytest
import asyncio

@pytest.fixture
async def async_resource():
    print('Setup start')
    await asyncio.sleep(0.1)  # simulate async setup
    resource = 'resource ready'
    yield resource
    print('Teardown start')
    await asyncio.sleep(0.1)  # simulate async cleanup

@pytest.mark.asyncio
async def test_async_fixture(async_resource):
    print(f'Test received: {async_resource}')
    assert async_resource == 'resource ready'

Important Notes

Remember to mark async test functions with @pytest.mark.asyncio to run them properly.

Async fixtures allow you to prepare and clean up resources that need waiting without blocking.

Summary

Async fixtures use async def and yield to manage setup and cleanup.

They are useful when your tests need async resources or operations.

Mark async tests with @pytest.mark.asyncio to run them correctly.

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