Python IsolatedAsyncioTestCase: own async setup and cleanup per test

Operation contract

The fixture creates a queue in asyncSetUp, consumes one receipt in an async test and records cleanup in asyncTearDown. The runner uses an owned in-memory text stream so only stable outcome fields are printed. The event list proves setup, test and cleanup ran in that order. This checks one local queue operation, not network I/O or a production task scheduler.

Failure and ownership boundary

An asynchronous test can still leak a task whose result it never observes. Own created tasks, cancel or await them in cleanup and test the failure path as well as success. Python asyncio TaskGroup: cancel sibling work and retain failure evidence, Python asyncio.gather: a sibling can continue after the first failure and Python asyncio timeout: cancellation and cleanup ownership matter when tests create background work.

Working program

import asyncio
import io
import unittest

events = []
class QueueCase(unittest.IsolatedAsyncioTestCase):
    async def asyncSetUp(self):
        self.queue = asyncio.Queue()
        events.append("setup")
    async def test_receipt(self):
        await self.queue.put("R41")
        self.assertEqual(await self.queue.get(), "R41")
        events.append("test")
    async def asyncTearDown(self):
        self.assertTrue(self.queue.empty())
        events.append("cleanup")

result = unittest.TextTestRunner(stream=io.StringIO()).run(
    unittest.defaultTestLoader.loadTestsFromTestCase(QueueCase))
print("passed:", result.wasSuccessful())
print("events:", events)

Output

passed: True
events: ['setup', 'test', 'cleanup']

Costs and limits

The test pays event-loop setup/teardown cost in addition to one queue operation. It does not measure thread contention or real service time. The in-memory queue is local to this fixture and has no persistence.

Common Mistakes

• Observe every spawned task; loop cleanup is not proof that its work succeeded.
• An in-memory async test is not a deployed I/O test.

Connected lessons

Python asyncio TaskGroup: cancel sibling work and retain failure evidence, Python asyncio.gather: a sibling can continue after the first failure, Python asyncio timeout: cancellation and cleanup ownership.

Trace the next boundary

Python AsyncMock: assert that a collaborator was awaited.