How to Test FastAPI with PyTest: Start Uvicorn in Background (2026)

Testing your FastAPI applications efficiently is crucial for ensuring high-quality software. In this tutorial, we will explore how to start a Uvicorn server in the background when testing with PyTest. This approach allows you to run your FastAPI app during tests and shut it down afterward, ensuring a clean testing environment.

Key Takeaways

• Learn how to set up a FastAPI application for testing using PyTest.
• Understand how to start and stop Uvicorn server in the background.
• Implement PyTest fixtures to manage server lifecycle during tests.
• Discover methods to ensure tests are isolated and repeatable.
• Troubleshoot common issues encountered during testing.

FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.7+ based on standard Python type hints. It's an excellent choice for creating RESTful APIs with automatic interactive documentation. When combined with Uvicorn, an ASGI server, it allows your app to handle a large number of requests efficiently.

Testing these applications with PyTest, a powerful testing tool for Python, ensures that your endpoints behave as expected. However, starting the Uvicorn server in the background while running tests can be challenging. This tutorial will guide you through the process of configuring PyTest fixtures to manage the server's lifecycle, providing a robust testing setup.

Prerequisites

• Python 3.7 or newer installed on your system.
• Basic knowledge of FastAPI and PyTest.
• Uvicorn installed; you can install it using pip install uvicorn.
• PyTest installed; you can install it using pip install pytest.

Step 1: Setting Up Your FastAPI Application

We'll start by creating a simple FastAPI application. This will serve as the basis for our tests.

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_main():
    return {"msg": "Hello World"}

This basic application will respond with a JSON message when the root endpoint is accessed.

Step 2: Installing Required Packages

Ensure you have the necessary packages installed. You can do this by running:

pip install fastapi uvicorn pytest

These packages will allow you to develop, serve, and test your FastAPI application.

Step 3: Writing a PyTest Fixture to Run Uvicorn

To start the Uvicorn server in the background, we need to create a PyTest fixture. This fixture will manage the server's lifecycle, ensuring it starts before tests and stops afterward.

import pytest
import subprocess
import time
import requests

@pytest.fixture(scope="module")
def uvicorn_server():
    # Start the server
    proc = subprocess.Popen([
        "uvicorn",
        "myapp:app",  # Adjust this to your app's import path
        "--host",
        "127.0.0.1",
        "--port",
        "8000",
    ])
    time.sleep(1)  # Give the server time to start up
    yield
    # Teardown
    proc.terminate()
    proc.wait()

This fixture uses Python's subprocess to start the server and time.sleep to allow the server some time to initialize before running the tests. Finally, it ensures the server is terminated after tests are complete.

Step 4: Writing Your Tests

With the server running in the background, we can now write our tests. PyTest will use the fixture to ensure the server is available during testing.

def test_read_main(uvicorn_server):
    response = requests.get("http://127.0.0.1:8000")
    assert response.status_code == 200
    assert response.json() == {"msg": "Hello World"}

Here, we use the requests library to send a GET request to our running server and assert that the response is as expected.

Step 5: Running Your Tests

With everything set up, you can now run your tests using PyTest.

pytest test_myapp.py

This command will execute the tests in test_myapp.py, starting and stopping the Uvicorn server as necessary.

Common Errors/Troubleshooting

• Server not starting: Ensure the path to your FastAPI app in the fixture is correct and that all dependencies are installed.
• Port already in use: Make sure port 8000 is free, or change it in the fixture and test accordingly.
• Test failing due to timing issues: Increase the sleep duration in the fixture to ensure the server has started before tests run.

By following these steps, you can effectively manage a FastAPI application's lifecycle during tests, ensuring reliable and repeatable test execution.

Frequently Asked Questions

Why use a fixture for starting Uvicorn?

Using a fixture ensures that the server starts before tests and stops afterward, maintaining a clean test environment.

Can I use a different server instead of Uvicorn?

Yes, but Uvicorn is recommended for FastAPI due to its high performance and compatibility.

What if my tests need a database connection?

You can extend the fixture to include database setup and teardown steps to ensure a complete test environment.