---
title: 'pytest-asyncio: pytest support for asyncio'
---
[](https://pypi.python.org/pypi/pytest-asyncio)
[](https://github.com/pytest-dev/pytest-asyncio/actions?workflow=CI)
[](https://codecov.io/gh/pytest-dev/pytest-asyncio)
[](https://github.com/pytest-dev/pytest-asyncio)
[](https://github.com/ambv/black)
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.
{.sourceCode .python}
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](https://github.com/eugeniy/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:
{.sourceCode .bash}
$ 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](https://docs.pytest.org/en/latest/reference/customize.html):
{.sourceCode .ini}
pytest.ini
[pytest]
asyncio_mode = auto
The value can be overridden by command-line option for `pytest`
invocation:
{.sourceCode .bash}
$ 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.
{.sourceCode .python}
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.
{.sourceCode .python}
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.
{.sourceCode .python}
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`.
{.sourceCode .python3}
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.
{.sourceCode .python}
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](https://docs.python.org/3/library/unittest.html) library are
not supported, users are recommended to use
[unitest.IsolatedAsyncioTestCase](https://docs.python.org/3/library/unittest.html#unittest.IsolatedAsyncioTestCase)
or an async framework such as
[asynctest](https://asynctest.readthedocs.io/en/latest).
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.