title: 'pytest-asyncio: pytest support for asyncio'

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.

@pytest.mark.asyncio
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.

Features

• 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]{.title-ref} 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.

Installation

To install pytest-asyncio, simply:

$ pip install pytest-asyncio

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

Modes

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
[pytest]
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.

Fixtures

event_loop

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 = "http://httpbin.org/get"
    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 conftest.py) 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.

@pytest.fixture
def event_loop():
    loop = MyCustomLoop()
    yield loop
    loop.close()

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.

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

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


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

Markers

pytest.mark.asyncio

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.

Contributing

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.