Documentation: https://www.uvicorn.org

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 supports HTTP/1.1 and WebSockets.

Quickstart

Install using pip:

$ pip install uvicorn

This will install uvicorn with minimal (pure Python) dependencies.

$ pip install 'uvicorn[standard]'

This will install uvicorn with "Cython-based" dependencies (where possible) and other "optional extras".

In this context, "Cython-based" means the following:

• the event loop uvloop will be installed and used if possible.
• the http protocol will be handled by httptools if possible.

Moreover, "optional extras" means that:

• the websocket protocol will be handled by websockets (should you want to use wsproto you'd need to install it manually) if possible.
• the --reload flag in development mode will use watchfiles.
• windows users will have colorama installed for the colored logs.
• python-dotenv will be installed should you want to use the --env-file option.
• PyYAML will be installed to allow you to provide a .yaml file to --log-config, if desired.

Create an application, in example.py:

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'),
        ],
    })
    await send({
        'type': 'http.response.body',
        'body': b'Hello, world!',
    })

Run the server:

$ uvicorn example:app

Why ASGI?

Most well established Python Web frameworks started out as WSGI-based frameworks.

WSGI applications are a single, synchronous callable that takes a request and returns a response. This doesn’t allow for long-lived connections, like you get with long-poll HTTP or WebSocket connections, which WSGI doesn't support well.

Having an async concurrency model also allows for options such as lightweight background tasks, and can be less of a limiting factor for endpoints that have long periods being blocked on network I/O such as dealing with slow HTTP requests.

Alternative ASGI servers

A strength of the ASGI protocol is that it decouples the server implementation from the application framework. This allows for an ecosystem of interoperating webservers and application frameworks.

Daphne

The first ASGI server implementation, originally developed to power Django Channels, is the Daphne webserver.

It is run widely in production, and supports HTTP/1.1, HTTP/2, and WebSockets.

Any of the example applications given here can equally well be run using daphne instead.

$ pip install daphne
$ daphne app:App

Hypercorn

Hypercorn was initially part of the Quart web framework, before being separated out into a standalone ASGI server.

Hypercorn supports HTTP/1.1, HTTP/2, and WebSockets.

It also supports the excellent trio async framework, as an alternative to asyncio.

$ pip install hypercorn
$ hypercorn app:App

Mangum

Mangum is an adapter for using ASGI applications with AWS Lambda & API Gateway.

Granian

Granian is an ASGI compatible Rust HTTP server which supports HTTP/2, TLS and WebSockets.