[Python] ContextVar set and reset in the same function fails - created in a different context

I have this function:

async_session = contextvars.ContextVar("async_session")

async def get_async_session() -> AsyncGenerator[AsyncSession, None]:
async with async_session_maker() as session:
try:
_token = async_session.set(session)
yield session
finally:
async_session.reset(_token)


This fails with:

ValueError: <Token var=<ContextVar name='async_session' at 0x7e1470e00e00> at 0x7e14706d4680> was created in a different Context


How can this happen? AFAICT the only way for the Context to be changed is for a whole function call. So how can the context change during a yield?

This function is being used as a FastAPI Depends in case that makes a difference - but I can't see how it does. It's running under Python 3.8 and the version of FastAPI is equally ancient - 0.54.

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