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Description

Starlette added support for the lifespan context function in 0.13.5. It's not well documented, but the code does suggest:

        # The lifespan context function is a newer style that replaces
        # on_startup / on_shutdown handlers. Use one or the other, not both.

For my purposes, it's much more convenient to use a (async)contextmanager for the startup and shutdown events of my application. It would be nice to have the option

The solution you would like

I'd like an easier way, and one that's officially supported, to use the lifespan context function.

from fastapi import FastAPI

async def lifespan(app):
    print("startup")
    async with SomeResource():
        yield
    print("shutdown")

app = FastAPI(lifespan=lifespan)

or

from fastapi import FastAPI

app = FastAPI()

@app.lifespan
async def lifespan(app):
    print("startup")
    async with SomeResource():
        yield
    print("shutdown")

Describe alternatives you've considered

I can already accomplish this simply by doing:

from fastapi import FastAPI

async def lifespan(app):
    print("startup")
    async with SomeResource():
        yield
    print("shutdown")

app = FastAPI()
app.router.lifespan_context = lifespan

however this is not officially supported and would likely break if accidentally using app.on_event in addition.

One could also do nasty stuff with __aenter__ and __aexit__:

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def startup()
    print("startup")
    app.state.resource = SomeResource()
    await app.state.resource.__aenter__()

@app.on_event("shutdown")
async def startup()
    print("shutdown")
    await app.state.resource.__aexit__(None, None, None)

but that seems quite ugly to me.

Environment

• OS: Linux
• FastAPI Version: 0.63.0
• Python version: 3.8.5