Async Tests

You have already seen how to test your FastAPI applications using the provided TestClient. Up to now, you have only seen how to write synchronous tests, without using async functions.

Being able to use asynchronous functions in your tests could be useful, for example, when you're querying your database asynchronously. Imagine you want to test sending requests to your FastAPI application and then verify that your backend successfully wrote the correct data in the database, while using an async database library.

Let's look at how we can make that work.

If we want to call asynchronous functions in our tests, our test functions have to be asynchronous. AnyIO provides a neat plugin for this, that allows us to specify that some test functions are to be called asynchronously.

Even if your FastAPI application uses normal def functions instead of async def, it is still an async application underneath.

The TestClient does some magic inside to call the asynchronous FastAPI application in your normal def test functions, using standard pytest. But that magic doesn't work anymore when we're using it inside asynchronous functions. By running our tests asynchronously, we can no longer use the TestClient inside our test functions.

The TestClient is based on HTTPX, and luckily, we can use it directly to test the API.

For a simple example, let's consider a file structure similar to the one described in Bigger Applications and Testing:

.
├── app
│   ├── __init__.py
│   ├── main.py
│   └── test_main.py

The file main.py would have:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": "Tomato"}

The file test_main.py would have the tests for main.py, it could look like this now:

import pytest
from httpx import ASGITransport, AsyncClient

from .main import app


@pytest.mark.anyio
async def test_root():
    async with AsyncClient(
        transport=ASGITransport(app=app), base_url="http://test"
    ) as ac:
        response = await ac.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Tomato"}

You can run your tests as usual via:

$ uv run pytest

---> 100%

The marker @pytest.mark.anyio tells pytest that this test function should be called asynchronously:

import pytest
from httpx import ASGITransport, AsyncClient

from .main import app


@pytest.mark.anyio
async def test_root():
    async with AsyncClient(
        transport=ASGITransport(app=app), base_url="http://test"
    ) as ac:
        response = await ac.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Tomato"}

Then we can create an AsyncClient with the app, and send async requests to it, using await.

import pytest
from httpx import ASGITransport, AsyncClient

from .main import app


@pytest.mark.anyio
async def test_root():
    async with AsyncClient(
        transport=ASGITransport(app=app), base_url="http://test"
    ) as ac:
        response = await ac.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Tomato"}

This is the equivalent to:

response = client.get('/')

...that we used to make our requests with the TestClient.

As the testing function is now asynchronous, you can now also call (and await) other async functions apart from sending requests to your FastAPI application in your tests, exactly as you would call them anywhere else in your code.