uvicorn/docs/index.md

An ASGI web server, for Python.

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Documentation: https://uvicorn.dev

Source Code: https://www.github.com/Kludex/uvicorn

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Uvicorn is an ASGI web server implementation for Python.

Until recently Python has lacked a minimal low-level server/application interface for async frameworks. The ASGI specification fills this gap, and means we're now able to start building a common set of tooling usable across all async frameworks.

Uvicorn currently supports HTTP/1.1 and WebSockets.

Sponsorship

Help us keep Uvicorn maintained and sustainable by becoming a sponsor.

Current sponsors:

Quickstart

Uvicorn is available on PyPI so installation is as simple as:

=== "pip" bash pip install uvicorn

=== "uv" bash uv add uvicorn

See the installation documentation for more information.

---

Let's create a simple ASGI application to run with Uvicorn:

async def app(scope, receive, send):
    assert scope['type'] == 'http'

    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            (b'content-type', b'text/plain'),
            (b'content-length', b'13'),
        ],
    })
    await send({
        'type': 'http.response.body',
        'body': b'Hello, world!',
    })

Then we can run it with Uvicorn:

uvicorn main:app

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Usage

The uvicorn command line tool is the easiest way to run your application.

Command line options

{{ uvicorn_help }}

For more information, see the settings documentation.

Running programmatically

There are several ways to run uvicorn directly from your application.

uvicorn.run

If you're looking for a programmatic equivalent of the uvicorn command line interface, use uvicorn.run():

import uvicorn

async def app(scope, receive, send):
    ...

if __name__ == "__main__":
    uvicorn.run("main:app", port=5000, log_level="info")

Config and Server instances

For more control over configuration and server lifecycle, use uvicorn.Config and uvicorn.Server:

import uvicorn

async def app(scope, receive, send):
    ...

if __name__ == "__main__":
    config = uvicorn.Config("main:app", port=5000, log_level="info")
    server = uvicorn.Server(config)
    server.run()

If you'd like to run Uvicorn from an already running async environment, use uvicorn.Server.serve() instead:

import asyncio
import uvicorn

async def app(scope, receive, send):
    ...

async def main():
    config = uvicorn.Config("main:app", port=5000, log_level="info")
    server = uvicorn.Server(config)
    await server.serve()

if __name__ == "__main__":
    asyncio.run(main())

Running with Gunicorn

!!! warning The uvicorn.workers module is deprecated and will be removed in a future release.

You should use the [`uvicorn-worker`](https://github.com/Kludex/uvicorn-worker) package instead.

```bash
python -m pip install uvicorn-worker
```

Gunicorn is a mature, fully featured server and process manager.

Uvicorn includes a Gunicorn worker class allowing you to run ASGI applications, with all of Uvicorn's performance benefits, while also giving you Gunicorn's fully-featured process management.

This allows you to increase or decrease the number of worker processes on the fly, restart worker processes gracefully, or perform server upgrades without downtime.

For production deployments we recommend using gunicorn with the uvicorn worker class.

gunicorn example:app -w 4 -k uvicorn.workers.UvicornWorker

For a PyPy compatible configuration use uvicorn.workers.UvicornH11Worker.

For more information, see the deployment documentation.

Application factories

The --factory flag allows loading the application from a factory function, rather than an application instance directly. The factory will be called with no arguments and should return an ASGI application.

def create_app():
    app = ...
    return app

uvicorn --factory main:create_app