pytest-asyncio
Pytest plugin for testing async Python code — enables writing test functions as async def with await calls. Without pytest-asyncio, async def test functions are silently treated as sync and coroutines never execute. pytest-asyncio provides the event loop machinery, @pytest.mark.asyncio decorator (or asyncio_mode='auto' for auto-detection), async fixtures, and integration with anyio for trio/asyncio compatibility. Essential for testing FastAPI, aiohttp, async SQLAlchemy, and any other async Python agent services. Supports session-scoped event loops, asyncio modes (strict, auto, legacy), and Python 3.12+ asyncio improvements.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Testing plugin — no production security concerns. Async agent test code should use test-specific credentials via environment variables or fixtures, never hardcoded production keys.
⚡ Reliability
Best When
You're testing async Python agent services (FastAPI, aiohttp, async SQLAlchemy) with pytest — pytest-asyncio makes async tests first-class pytest citizens with proper event loop management.
Avoid When
Your agent code is synchronous, you use Trio (use pytest-trio), or you're wrapping everything in asyncio.run() without async fixtures.
Use Cases
- • Test async FastAPI agent endpoints with pytest-asyncio + httpx.AsyncClient — async def test_agent_endpoint() with await async_client.post('/agent/run') for async agent API integration tests
- • Test async agent LLM client calls — @pytest.mark.asyncio async def test_agent_llm() with await agent.generate_response() for unit testing async agent LLM interactions
- • Async fixtures for agent test setup — @pytest_fixture async def agent_db(): async with AsyncSession() as session: yield session for async database fixture in agent integration tests
- • Session-scoped event loop for agent integration tests — @pytest.fixture(scope='session') event_loop with asyncio_mode='auto' for sharing expensive async agent setup across test session
- • Test agent streaming responses — async def test_agent_stream() with async for chunk in agent.stream_response(): ... for testing async generator agent output patterns
Not For
- • Sync Python tests — pytest handles sync test functions natively; pytest-asyncio is only needed for async def test functions in agent services using asyncio
- • Trio-based async agent code — pytest-asyncio handles asyncio; use pytest-trio for Trio-based agent services (or use anyio with pytest-asyncio's anyio mode)
- • Simple asyncio.run() wrappers — if agent tests wrap async code in asyncio.run() in sync test functions, they work without pytest-asyncio; but async fixtures and event loop control require the plugin
Interface
Authentication
Testing plugin — no auth concepts.
Pricing
pytest-asyncio is Apache 2.0 licensed. Free for all use.
Agent Metadata
Known Gotchas
- ⚠ asyncio_mode='auto' vs strict mode — 0.21+ defaults to strict mode requiring explicit @pytest.mark.asyncio; add asyncio_mode = 'auto' to pytest.ini or pyproject.toml [tool.pytest.ini_options] to auto-detect async agent test functions without decorating each
- ⚠ Event loop scope mismatch with async fixtures — fixture with scope='session' and default function-scoped event loop causes ScopeMismatch error; set event_loop_policy fixture or use asyncio_mode='auto' with loop_scope='session' for session-scoped agent database fixtures
- ⚠ Deprecation warnings about event_loop fixture — pytest-asyncio 0.21+ deprecates overriding event_loop fixture for scope changes; use @pytest.fixture(loop_scope='session') syntax or pytest_configure hook; old event_loop override patterns cause warnings in CI
- ⚠ Async generator fixtures require async for cleanup — @pytest_fixture async def agent_client(): ... yield client; await client.aclose() — missing cleanup in async fixture causes ResourceWarning and potential connection leak in agent integration tests
- ⚠ pytest-anyio vs pytest-asyncio interaction — projects using anyio.from_thread or anyio in agent code may need pytest-anyio integration; mixing pytest-asyncio and anyio backends causes RuntimeError about multiple event loops in some scenarios
- ⚠ ScopeMismatch with multiple async fixtures — mixing function-scope and session-scope async fixtures that share resources requires explicit event_loop_policy configuration; agent tests with shared async database connections must declare consistent fixture scopes
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for pytest-asyncio.
Scores are editorial opinions as of 2026-03-06.