FastAPI dependencies (yield): how to call them manually?

Question

FastAPI uses Depends() to inject variables either returned or yielded. Eg, FastAPI/SQL:

# Dependency
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()
...
def create_user(db: Session = Depends(get_db)):
...

If I wanted to use that get_db() somewhere else (outside a FastAPI route), how would I do that? I know it's Python core knowledge, but I can't seem to figure it out. My initial thought was db = yield from get_db(), but I can't call yield from in async functions (and don't know if it would work besides). Then I tried:

with get_db() as db:
   pass

Which fails as the original get_db() isn't wrapped as a @contextmanager. (Note, I don't want to decorate this - I'm using get_db as an example, I need to work with more complicated dependencies). Finally, I tried db = next(get_db()) - which works, but I don't think that's the correct solution. When/how will finally be invoked - when my method returns? And in some other dependencies, there's post-yield code that needs to execute; would I need to call next() again to ensure that code executes? Seems like next() isn't the right way. Any ideas?

alex_noname · Accepted Answer · 2021-03-10 09:00:12Z

21

You can use contextmanager not as a decorator but as a function returning context manager:

from contextlib import contextmanager

# Dependency
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()


# synchronously
with contextmanager(get_db)() as session:  # execute until yield. Session is yielded value
    pass
# execute finally on exit from with

But keep in mind that the code will execute synchronously. If you want to execute it in a thread, then you can use the FastAPI tools:

import asyncio
from contextlib import contextmanager

from fastapi.concurrency import contextmanager_in_threadpool


async def some_coro():
    async with contextmanager_in_threadpool(contextmanager(get_db)()) as session:
        pass

answered Mar 10, 2021 at 9:00

alex_noname

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3 Comments

lefnire Over a year ago

Oi, I'd never have figured that out - glad I asked! Thanks

saeedghadiri Over a year ago

Why don't we use context manager as a decorator for get_db function and then use with get_db() as session

Pratik K. Over a year ago

What if the get_db is async as well?

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