“How to test asynchronous applications using FastAPI and pytest” – Jeny Sadadia (PyCon AU 2023)

Testing asynchronous applications with FastAPI and pytest can be a challenging but rewarding experience. In this tutorial, I will guide you through the process of testing asynchronous code in FastAPI using pytest, a popular testing framework in the Python community.

First, let’s start by setting up our project. Make sure you have FastAPI and pytest installed in your Python environment. You can install these packages using pip:

pip install fastapi pytest

Next, create a new Python file for your FastAPI application. You can name it main.py. In this file, define a simple FastAPI application with an asynchronous route:

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello, World!"}

Now, let’s create a test file for our FastAPI application. You can name it test_main.py. In this file, write a test case using pytest that checks if the / endpoint returns the expected response:

import pytest
from fastapi.testclient import TestClient
from main import app

@pytest.fixture
def client():
    return TestClient(app)

# Asynchronous test case
@pytest.mark.asyncio
async def test_read_root(client):
    response = await client.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Hello, World!"}

In this test case, we use the pytest.mark.asyncio decorator to mark the test function as asynchronous. This allows us to use await to make asynchronous HTTP requests to our FastAPI application.

To run the tests, simply run pytest in the terminal:

pytest

If the test passes, you should see an output similar to the following:

=========================== test session starts ============================
collected 1 item

test_main.py .                                                       [100%]

============================= 1 passed in 0.15s =============================

Congratulations! You have successfully tested an asynchronous application with FastAPI and pytest. You can now expand your test suite by adding more test cases for other routes and endpoints in your FastAPI application.

In conclusion, testing asynchronous applications with FastAPI and pytest requires understanding how to write asynchronous test cases using the pytest.mark.asyncio decorator. By following the steps outlined in this tutorial, you can ensure the reliability and robustness of your asynchronous FastAPI application through comprehensive testing.