Reference

Configuration

The pytest-asyncio mode can be set by the asyncio_mode configuration option in the configuration file:

# pytest.ini
[pytest]
asyncio_mode = auto

The value can also be set via the --asyncio-mode command-line option:

$ pytest tests --asyncio-mode=strict

If the asyncio mode is set in both the pytest configuration file and the command-line option, the command-line option takes precedence. If no asyncio mode is specified, the mode defaults to strict.

Fixtures

event_loop

Creates a new asyncio event loop based on the current event loop policy. The new loop is available as the return value of this fixture or via asyncio.get_running_loop. The event loop is closed when the fixture scope ends. The fixture scope defaults to function scope.

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

Note that, when using the event_loop fixture, you need to interact with the event loop using methods like event_loop.run_until_complete. If you want to await code inside your test function, you need to write a coroutine and use it as a test function. The asyncio marker is used to mark coroutines that should be treated as test functions.

The event_loop fixture can be overridden in any of the standard pytest locations, e.g. directly in the test file, or in conftest.py. This allows redefining the fixture scope, for example:

@pytest.fixture(scope="session")
def event_loop():
    policy = asyncio.get_event_loop_policy()
    loop = policy.new_event_loop()
    yield loop
    loop.close()

If you need to change the type of the event loop, prefer setting a custom event loop policy over redefining the event_loop fixture.

If the pytest.mark.asyncio decorator is applied to a test function, the event_loop fixture will be requested automatically by the test function.

unused_tcp_port

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

unused_tcp_port_factory

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

Works just like their TCP counterparts but returns unused UDP ports.

Markers

pytest.mark.asyncio

A coroutine or async generator with this marker will be treated as a test function by pytest. The marked function will be executed as an asyncio task in the event loop provided by the event_loop fixture.

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.

Decorators

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

import pytest_asyncio


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


@pytest_asyncio.fixture(scope="module")
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 mode automatically converts async fixtures declared with the standard @pytest.fixture decorator to asyncio-driven versions.