FastAPI "RuntimeError: There is no current event loop" in CI

By Kaveh Alemi·Latchkey

An async test or fixture called asyncio.get_event_loop() when no loop was running. On recent Python versions that no longer auto-creates a loop, and without pytest-asyncio driving the test, the coroutine has nothing to run on.

What this error means

An async FastAPI test fails with "RuntimeError: There is no current event loop in thread 'MainThread'" or a coroutine "was never awaited" warning and no assertions run.

RuntimeError: There is no current event loop in thread 'MainThread'.

  loop = asyncio.get_event_loop()

Diagnose it: what did pytest actually collect?

Most pytest failures that only happen in CI are collection or import-path problems rather than test failures. The runner has a different working directory, a different sys.path, and usually no editable install, so a test module that imports your package locally may not resolve at all.

# what would run, without running it
pytest --collect-only -q | tail -20

# where pytest thinks the root is (drives conftest and import mode)
pytest --collect-only 2>&1 | grep -i rootdir

# is the package importable at all from the runner cwd?
python -c "import yourpackage, sys; print(yourpackage.__file__)"
python -c "import sys; print(sys.path)"

Common causes

pytest-asyncio is not driving the async test

Without pytest-asyncio (and its mode), an async def test is collected but never awaited, so loop-dependent code fails.

get_event_loop with no running loop on newer Python

Recent Python deprecated auto-creating a loop in get_event_loop(), so code that assumed one exists now raises.

How to fix it

Install and enable pytest-asyncio

1. Install pytest-asyncio in the test environment.
2. Set an asyncio mode so async tests are awaited.
3. Use an async client such as httpx.AsyncClient with the app.

import pytest, httpx
from app.main import app

@pytest.mark.asyncio
async def test_health():
    transport = httpx.ASGITransport(app=app)
    async with httpx.AsyncClient(transport=transport, base_url="http://test") as ac:
        r = await ac.get("/health")
    assert r.status_code == 200

Get the running loop instead of a current one

Inside async code, use asyncio.get_running_loop() rather than get_event_loop().

import asyncio
loop = asyncio.get_running_loop()

Make a zero-test run fail the build

# fail explicitly when nothing is collected
pytest --strict-markers -q
test "${PIPESTATUS[0]}" -ne 5 || { echo "pytest collected no tests"; exit 1; }

How to prevent it

• Drive async tests with pytest-asyncio and an explicit mode.
• Use get_running_loop() inside coroutines.
• Prefer async clients for async endpoints instead of manual loops.

Frequently asked questions

Related guides

References

• Python documentation
• pip documentation
• GitHub Actions documentation