Alternative Pytest Asyncio

This plugin allows you to have async pytest fixtures and tests.

This plugin only supports python 3.11 and above.

The code here is influenced by pytest-asyncio but with some differences:

• Error tracebacks from are from your tests, rather than asyncio internals

• There is only one loop for all of the tests

• You can manage the lifecycle of the loop yourself outside of pytest by using this plugin with your own loop

• No need to explicitly mark your tests as async. (pytest-asyncio requires you mark your async tests because it also supports other event loops like curio and trio)

Like pytest-asyncio it supports async tests, coroutine fixtures and async generator fixtures.

Changelog

0.9.5 - 19 February 2026

• Ensure the Loop class can get current loop without failing when there is no current loop

0.9.4 - 27 January 2026

• Make the plugin aware of PEP 669 (#33)

0.9.3 - 14 April 2025

• Fixed a bug where fixture errors would persist across multiple tests https://github.com/delfick/alt-pytest-asyncio/issues/26

0.9.2 - 23 February 2025

• Fixed a memory leak for large test suites

0.9.1 - 5 November 2024

• Made the plugin take into account pytest.ini for setting the default async timeout

0.9.0 - 20 October 2024

• Enabling the plugin must now be done by adding alt_pytest_asyncio.enable to the pytest list of enabled plugins if it’s not being manually enabled.

• Removed pytest.mark.async_timeout and replaced the functionality with a fixture.

• Changed the exported api of the plugin

  • alt_pytest_asyncio.plugin.OverrideLoop is now alt_pytest_asyncio.Loop

  • alt_pytest_asyncio.plugin.AltPytestAsyncioPlugin now takes managed_loop as a keyword argument instead of the first positional argument with the name loop.

  • The new Loop that replaces OverrideLoop now has an attributed controlled_loop instead of loop.

0.8.2 - 12 October 2024

• Added type annotations

• CI now tests against python 3.13

0.8.1 - 3 June 2024

• Remove a namespace conflict that restricted what names could be used as parametrize arguments.

0.8.0 - 1 June 2024

• Provide simple support for tests being aware of asyncio.Context

• Remove support for python less than 3.11

• Added support for asyncio ContextVars

0.7.2 - 1 October 2023

• Timeouts don’t take affect if the debugger is active

0.7.1 - 23 June 2023

• No functional changes, only fixing how hatchling understands the license field in the pyproject.toml with thanks to @piotrm-nvidia

0.7.0 - 12 April 2023

• Changed the pytest dependency to be greater than pytest version 7

• Using isort now

• Went from setuptools to hatch

• CI now runs against python 3.11

0.6.0 - 23 October 2021

• Fix bug where it was possible for an async generator fixture to be cleaned up even if it was never started.

• This library is now 3.7+ only

• Added an equivalent shutdown_asyncgen to the OverrideLoop helper

0.5.4 - 26 January 2021

• Added a --default-async-timeout option from the CLI. With many thanks to @andredias.

• Renamed existing pytest.ini option from default_alt_async_timeout to be default_async_timeout.

0.5.3 - 25 July 2020

• Make sure a KeyboardInterrupt on running tests still shows errors from failed tests

0.5.2 - 6 February 2020

• Added ability to make a different event loop for some tests

0.5.1 - 15 December 2019

• Added an ini option default_alt_async_timeout for the default async timeout for fixtures and tests. The default is now 5 seconds. So say you wanted the default to be 3.5 seconds, you would set default_alt_async_timeout to be 3.5

0.5 - 16 August 2019

• I made this functionality in a work project where I needed to run pytest.main from an existing event loop. I decided to make this it’s own module so I can have tests for this code.

Installation

Most users of this plugin won’t need to manually construct the plugin as that’s only required if you’re doing funky things where you want to manually call pytest.main (see next section).

For this majority case, enabling the plugin requires:

• The plugin be installed in the python environment

• Adding alt_pytest_asyncio.enable to the list of pytest plugins that are enabled.

Running from your own event loop

If you want to run pytest.main from with an existing event loop then you can do something like:

import alt_pytest_asyncio
import nest_asyncio
import asyncio
import pytest

async def my_tests():
   await do_some_setup_before_pytest()

   loop: asyncio.AbstractEventLoop = ...

   plugins = [
     alt_pytest_asyncio.plugin.AltPytestAsyncioPlugin(
         managed_loop=loop
     ),
   ]

   try:
       code = pytest.main([], plugins=plugins)
   finally:
       # Note that alt_pytest_asyncio will make sure all your async tests
       # have been finalized by this point, even if you KeyboardInterrupt
       # the pytest.main
       await do_any_teardown_after_pytest()

   if code != 0:
      raise Exception(repr(code))

if __name__ == '__main__':
   # Nest asyncio is required so that we can do run_until_complete in an
   # existing event loop - https://github.com/erdewit/nest_asyncio
   loop = asyncio.get_event_loop()
   nest_asyncio.apply(loop)

   alt_pytest_asyncio.run_coro_as_main(loop, my_tests())

Note that if you don’t need to run pytest from an existing event loop, you don’t need to do anything other than have alt_pytest_asyncio installed in your environment and alt_pytest_asyncio.enable in your pytest plugins list and you’ll be able to just use async keywords on your fixtures and tests.

Timeouts

Note

The pytest.mark.async_timeout(seconds) that existed before version 0.9.0 no longer has an effect and has been replaced with the fixtures as mentioned below

This plugin can configure the timeout for any async fixture or test using the async_timeout fixture or by creating a default_async_timeout fixture.

For example:

import pytest
import alt_pytest_asyncio

AsyncTimeout = alt_pytest_asyncio.protocols.AsyncTimeout

async def test_something(async_timeout: AsyncTimeout) -> None:
   async_timeout.set_timeout_seconds(10)
   await something_that_may_take_a_while()

This test will be cancelled after 10 seconds and raise an assertion error saying the test took too long and the file and line number where the test is.

Note

The async_timeout passed into a fixture or a test is a new instance specific to that fixture or test. Setting it in a fixture only affects that fixture and setting it in a test only affects that test.

You can also set a default_async_timeout fixture to change the default:

import pytest
import alt_pytest_asyncio

AsyncTimeout = alt_pytest_asyncio.protocols.AsyncTimeout


@pytest.fixture()
def default_async_timeout() -> float:
    return 0.5

@pytest.fixture
async def my_amazing_fixture() -> int:
   # Will timeout because of our default 0.5
   await asyncio.sleep(1)
   return 1

@pytest.fixture
async def my_amazing_fixture(async_timeout: AsyncTimeout) -> int:
   # Change timeout for just this fixture
   async_timeout.set_timeout_seconds(2)
   await asyncio.sleep(1)
   return 1

For fixtures that have a non function scope, they require a {scope}_default_async_timeout fixture:

import pytest


@pytest.fixture(scope="session")
def session_default_async_timeout() -> float:
    return 5

@pytest.fixture(scope="session")
async def some_fixture() -> None:
    # timeout here is 5
    pass

class TestStuff:
    @pytest.fixture(scope="class")
    async def some_fixture() -> None:
        # timeout here is 5
        pass

    class TestMore:
        @pytest.fixture(scope="class")
        async def class_default_async_timeout() -> int:
            return 8

        @pytest.fixture(scope="class")
        async def some_fixture() -> None:
            # timeout here is 8
            pass

The plugin knows about the scopes function, class, module, package and session. So say a scope="class" async fixture is executed, the closest class_default_async_timeout fixture is used unless that doesn’t exist, in which case module_default_async_timeout is used, otherwise package_default_async_timeout, otherwise session_default_async_timeout.

There is a default session_default_async_timeout available which returns the value set by the default_async_timeout pytest option the plugin provides.

And you can have a timeout on generator fixtures:

import pytest
from collections.abc import Iterator
import alt_pytest_asyncio

AsyncTimeout = alt_pytest_asyncio.protocols.AsyncTimeout

@pytest.fixture()
async def my_amazing_fixture(async_timeout: AsyncTimeout) -> Iterator[int]:
   async_timeout.set_timeout_seconds(0.5)

   try:
      await asyncio.sleep(1)
      yield 1
   finally:
      await asyncio.sleep(1)

Note that for generator fixtures, the timeout is applied in whole to both the setup and finalization of the fixture. As in the real timeout for the entire fixture is essentially double the single timeout specified.

The default timeout is 5 seconds. You can change this default by setting the default_async_timeout option to the number of seconds you want.

This setting is also available from the CLI using the --default-async-timeout option.

Note that if the timeout fires whilst you have the debugger active then the timeout will not cancel the current test. This is determined by checking if sys.gettrace() returns a non-None value.

The object that is provided when the fixture/test asks for async_timeout can be modified by overriding the async_timeout session scope’d fixture and returning an object that inherits from and implements alt_pytest_asyncio.base.AsyncTimeoutProvider. This is a python “abc” class with a single method load which is called to return the object given to the fixture or test. This object must implement alt_pytest_asyncio.base.AsyncTimeout. The default implementation can be found at alt_pytest_asyncio.plugin.LoadedAsyncTimeout.

Overriding the loop

Sometimes it may be necessary to close the current loop in a test. For this to not then break the rest of your tests, you will need to set a new event loop for your test and then restore the old loop afterwards.

For this, we have a context manager that will install a new asyncio loop and then restore the original loop on exit.

Usage looks like:

import alt_pytest_asyncio

class TestThing:
    @pytest.fixture(autouse=True)
    def custom_loop(self) -> alt_pytest_asyncio.protocols.Loop:
        with alt_pytest_asyncio.Loop() as custom_loop:
            yield custom_loop

    def test_thing(self, custom_loop: alt_pytest_asyncio.protocols.Loop):
        custom_loop.run_until_complete(my_thing())

By putting the loop into an autouse fixture, all fixtures used by the test will have the custom loop. If you want to include module level fixtures too then use the OverrideLoop in a module level fixture too.

If the Loop is instantiated with new_loop=True then it will create and manage a new event loop whilst it’s being used as a context manager. This new loop will be available on the object as .controlled_loop.

The run_until_complete on the custom_loop in the above example will do a run_until_complete on the new loop, but in a way that means you won’t get unhandled exception during shutdown errors when the context manager closes the new loop.

When the context manager exits and closes the new loop, it will first cancel all tasks to ensure finally blocks are run.