Introduction

Starlette is a lightweight ASGI framework/toolkit, which is ideal for building high performance asyncio services.

It is production-ready, and gives you the following:

Seriously impressive performance.
WebSocket support.
GraphQL support.
In-process background tasks.
Startup and shutdown events.
Test client built on requests.
CORS, GZip, Static Files, Streaming responses.
Session and Cookie support.
100% test coverage.
100% type annotated codebase.
Zero hard dependencies.

Requirements

Python 3.6+

Installation

$ pip3 install starlette

You'll also want to install an ASGI server, such as uvicorn, daphne, or hypercorn.

$ pip3 install uvicorn

Example

from starlette.applications import Starlette
from starlette.responses import JSONResponse
import uvicorn

app = Starlette()

@app.route('/')
async def homepage(request):
    return JSONResponse({'hello': 'world'})

if __name__ == '__main__':
    uvicorn.run(app, host='0.0.0.0', port=8000)

Dependencies

Starlette does not have any hard dependencies, but the following are optional:

requests - Required if you want to use the TestClient.
aiofiles - Required if you want to use FileResponse or StaticFiles.
python-multipart - Required if you want to support form parsing, with request.form().
graphene - Required for GraphQLApp support.
itsdangerous - Required for SessionMiddleware support.
pyyaml - Required for SchemaGenerator support.
ujson - Required if you want to use UJSONResponse.

You can install all of these with pip3 install starlette[full].

Framework or Toolkit

Starlette is designed to be used either as a complete framework, or as an ASGI toolkit. You can use any of its components independently.

from starlette.responses import PlainTextResponse


class App:
    def __init__(self, scope):
        self.scope = scope

    async def __call__(self, receive, send):
        response = PlainTextResponse('Hello, world!')
        await response(receive, send)

Run the App application in example.py:

$ uvicorn example:App
INFO: Started server process [11509]
INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

Modularity

The modularity that Starlette is designed on promotes building re-usable components that can be shared between any ASGI framework. This should enable an ecosystem of shared middleware and mountable applications.

The clean API separation also means it's easier to understand each component in isolation.

Performance

Independent TechEmpower benchmarks show Starlette applications running under Uvicorn as one of the fastest Python frameworks available. (*)

For high throughput loads you should:

Make sure to install ujson and use UJSONResponse.
Run using Gunicorn using the uvicorn worker class.
Use one or two workers per-CPU core. (You might need to experiment with this.)
Disable access logging.

For example:

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

Several of the ASGI servers also have pure Python implementations available, so you can also run under PyPy if your application code has parts that are CPU constrained.

Either programatically:

uvicorn.run(..., http='h11', loop='asyncio')

Or using Gunicorn:

gunicorn -k uvicorn.workers.UvicornH11Worker ...

— ⭐️ —

Starlette is BSD licensed code. Designed & built in Brighton, England.

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