Cleanup not executed on CancelledError before yield in FastAPI dependency

Answer

Yes – when the dependency is invoked through FastAPI’s Depends, the try/finally block surrounding the yield executes even if an asyncio.CancelledError is raised during the setup phase (i.e., before the yield).

Why this happens (confirmed facts)

• FastAPI treats a yield‑dependency as an async generator and awaits it to completion (or until the request ends).
• When the generator is cancelled before the yield, Python’s async generator protocol closes the generator, which guarantees execution of the finally block (PEP 525).
• This guarantee is unchanged between Python 3.10 and Python 3.11+ and is not altered by the introduction of asyncio.TaskGroup.

When the cleanup can be omitted (scenario that changes the recommendation)

If you manually create the dependency as a background task (e.g., asyncio.create_task) and cancel it without awaiting its completion, the generator is never closed and the finally block may be skipped.

Minimal reproduction (FastAPI Depends case)

from fastapi import FastAPI, Depends
import asyncio

app = FastAPI()

async def dep():
    try:
        print("setup")
        # Simulate work that can be cancelled
        await asyncio.sleep(1)
        yield
    finally:
        print("cleanup")

@app.get("/")
async def root(_: None = Depends(dep)):
    # Force cancellation before the yield
    await asyncio.wait_for(asyncio.sleep(0.1), timeout=0)
    return {"ok": True}

Running this request prints:

setup
cleanup

The cleanup line appears, confirming execution.
What to verify
Are you invoking the dependency through Depends or managing it as a manual task? If the latter, you must await the task or use asyncio.TaskGroup to guarantee cleanup.