fastapi-lifespan-dependencies 0.1.0

Allows for the creation of lifespan dependencies - FastAPI dependencies that are only run once per app.

FastAPI Lifespan Dependencies

Allows for the creation of lifespan dependencies - FastAPI dependencies that are only run once per app, instead of once per path.

This library is inspired by fastapi-lifespan-manager and by this suggestion on the FastAPI GitHub page.

Usage

To get started with lifespan dependencies, create a Lifespan object and pass it to the FastAPI constructor:

from fastapi import FastAPI
from fastapi_lifespan_dependencies import Lifespan

lifespan = Lifespan()

app = FastAPI(lifespan=lifespan)

Any function that yields a value can be turned into a lifespan dependency by using the Lifespan.register decorator method (both def and async def functions are supported):

from redis.asyncio import redis

@lifespan.register
async def get_db():
    db = Redis(host="localhost", port=6379)

    yield db

    await db.aclose()

That's it!

You can now use fastapi.Depends to access the value yielded by the lifespan dependency:

from typing import Annotated
from fastapi import Depends

@app.get("/example")
async def get_example(db: Annotated[Redis, Depends(get_db)]):
    return await db.get('example')


@app.delete("/example")
async def delete_example(db: Annotated[Redis, Depends(get_db)]):
    return await db.delete('example')

Usage with Other Dependencies

Similarly to normal FastAPI dependencies, lifespan dependencies can depend on each other:

from pydantic_settings import BaseSettings

class Config(BaseSettings):
    redis_host: str
    redis_port: int


@lifespan.register
def get_config():
    yield Config()


@lifespan.register
async def get_db(config: Annotated[Config, Depends(get_config)]):
    db = Redis(host=config.redis_host, port=config.redis_port)

    yield db

    await db.aclose()

Lifespan dependencies can also depend on normal dependencies, as long as none of them require path-related dependencies (like Query() or Body()).

When Are Lifespan Dependencies Run?

Every function decorated with Lifespan.register will be run when the application starts, up until a yield statement is encountered. Everything past the yield will only be executed right before the application shuts down.

This means that, unlike normal FastAPI dependencies, using the same dependency in multiple path functions won't run the dependency multiple times, and every function will share the same instance of the dependency.

Examples

More examples can be found here.