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Pytest support for asyncio

Project description Supported Python versions

pytest-asyncio is an Apache2 licensed library, written in Python, for testing asyncio code with pytest.

asyncio code is usually written in the form of coroutines, which makes it slightly more difficult to test using normal testing tools. pytest-asyncio provides useful fixtures and markers to make testing easier.

async def test_some_asyncio_code():
    res = await library.do_something()
    assert b"expected result" == res

pytest-asyncio has been strongly influenced by pytest-tornado.


  • fixtures for creating and injecting versions of the asyncio event loop
  • fixtures for injecting unused tcp/udp ports
  • pytest markers for treating tests as asyncio coroutines
  • easy testing with non-default event loops
  • support for async def fixtures and async generator fixtures
  • support auto mode to handle all async fixtures and tests automatically by asyncio; provide strict mode if a test suite should work with different async frameworks simultaneously, e.g. asyncio and trio.


To install pytest-asyncio, simply:

$ pip install pytest-asyncio

This is enough for pytest to pick up pytest-asyncio.


Starting from pytest-asyncio>=0.17, three modes are provided: auto, strict and legacy (default).

The mode can be set by asyncio_mode configuration option in configuration file:

# pytest.ini
asyncio_mode = auto

The value can be overridden by command-line option for pytest invocation:

$ pytest tests --asyncio-mode=strict

Auto mode

When the mode is auto, all discovered async tests are considered asyncio-driven even if they have no @pytest.mark.asyncio marker.

All async fixtures are considered asyncio-driven as well, even if they are decorated with a regular @pytest.fixture decorator instead of dedicated @pytest_asyncio.fixture counterpart.

asyncio-driven means that tests and fixtures are executed by pytest-asyncio plugin.

This mode requires the simplest tests and fixtures configuration and is recommended for default usage unless the same project and its test suite should execute tests from different async frameworks, e.g. asyncio and trio. In this case, auto-handling can break tests designed for other framework; please use strict mode instead.

Strict mode

Strict mode enforces @pytest.mark.asyncio and @pytest_asyncio.fixture usage. Without these markers, tests and fixtures are not considered as asyncio-driven, other pytest plugin can handle them.

Please use this mode if multiple async frameworks should be combined in the same test suite.

Legacy mode

This mode follows rules used by pytest-asyncio<0.17: tests are not auto-marked but fixtures are.

This mode is used by default for the sake of backward compatibility, deprecation warnings are emitted with suggestion to either switching to auto mode or using strict mode with @pytest_asyncio.fixture decorators.

In future, the default will be changed.



Creates and injects a new instance of the default asyncio event loop. By default, the loop will be closed at the end of the test (i.e. the default fixture scope is function).

Note that just using the event_loop fixture won’t make your test function a coroutine. You’ll need to interact with the event loop directly, using methods like event_loop.run_until_complete. See the pytest.mark.asyncio marker for treating test functions like coroutines.

Simply using this fixture will not set the generated event loop as the default asyncio event loop, or change the asyncio event loop policy in any way. Use pytest.mark.asyncio for this purpose.

def test_http_client(event_loop):
    url = ""
    resp = event_loop.run_until_complete(http_client(url))
    assert b"HTTP/1.1 200 OK" in resp

This fixture can be easily overridden in any of the standard pytest locations (e.g. directly in the test file, or in to use a non-default event loop. This will take effect even if you’re using the pytest.mark.asyncio marker and not the event_loop fixture directly.

def event_loop():
    loop = MyCustomLoop()
    yield loop

If the pytest.mark.asyncio marker is applied, a pytest hook will ensure the produced loop is set as the default global loop. Fixtures depending on the event_loop fixture can expect the policy to be properly modified when they run.


Finds and yields a single unused TCP port on the localhost interface. Useful for binding temporary test servers.


A callable which returns a different unused TCP port each invocation. Useful when several unused TCP ports are required in a test.

def a_test(unused_tcp_port_factory):
    port1, port2 = unused_tcp_port_factory(), unused_tcp_port_factory()

unused_udp_port and unused_udp_port_factory

Work just like their TCP counterparts but return unused UDP ports.

Async fixtures

Asynchronous fixtures are defined just like ordinary pytest fixtures, except they should be decorated with @pytest_asyncio.fixture.

import pytest_asyncio

async def async_gen_fixture():
    await asyncio.sleep(0.1)
    yield "a value"

async def async_fixture():
    return await asyncio.sleep(0.1)

All scopes are supported, but if you use a non-function scope you will need to redefine the event_loop fixture to have the same or broader scope. Async fixtures need the event loop, and so must have the same or narrower scope than the event_loop fixture.

auto and legacy mode automatically converts async fixtures declared with the standard @pytest.fixture decorator to asyncio-driven versions.



Mark your test coroutine with this marker and pytest will execute it as an asyncio task using the event loop provided by the event_loop fixture. See the introductory section for an example.

The event loop used can be overridden by overriding the event_loop fixture (see above).

In order to make your test code a little more concise, the pytest pytestmark feature can be used to mark entire modules or classes with this marker. Only test coroutines will be affected (by default, coroutines prefixed by test_), so, for example, fixtures are safe to define.

import asyncio

import pytest

# All test coroutines will be treated as marked.
pytestmark = pytest.mark.asyncio

async def test_example(event_loop):
    """No marker!"""
    await asyncio.sleep(0, loop=event_loop)

In auto mode, the pytest.mark.asyncio marker can be omitted, the marker is added automatically to async test functions.

Note about unittest

Test classes subclassing the standard unittest library are not supported, users are recommended to use unitest.IsolatedAsyncioTestCase or an async framework such as asynctest.


Contributions are very welcome. Tests can be run with tox, please ensure the coverage at least stays the same before you submit a pull request.

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