async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
        yield

    @asynccontextmanager
    async def router_lifespan(app: FastAPI) -> AsyncGenerator[dict[str, bool], None]:
        yield {"router": True}

    @asynccontextmanager
    async def sub_router_lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
        yield

    sub_router = APIRouter(lifespan=sub_router_lifespan)

    for line in message.splitlines():
        yield line


@app.get("/story/stream-no-async", response_class=StreamingResponse)
def stream_story_no_async() -> Iterable[str]:
    for line in message.splitlines():
        yield line


@app.get("/story/stream-no-annotation", response_class=StreamingResponse)
async def stream_story_no_annotation():
    for line in message.splitlines():
        yield line

And when using `Depends(scope="request")` (the default), the exit code after `yield` is executed after the response is sent.

You can read more about it in the docs for [Dependencies with `yield` - Early exit and `scope`](../tutorial/dependencies/dependencies-with-yield.md#early-exit-and-scope).

### Dependencies with `yield` and `StreamingResponse`, Technical Details { #dependencies-with-yield-and-streamingresponse-technical-details }

The first thing to notice, is that we are defining an async function with `yield`. This is very similar to Dependencies with `yield`.

{* ../../docs_src/events/tutorial003_py310.py hl[14:19] *}

The first part of the function, before the `yield`, will be executed **before** the application starts.

And the part after the `yield` will be executed **after** the application has finished.

### Async Context Manager { #async-context-manager }

            async for item in iterate_in_threadpool(func(*args, **kwargs)):
                yield item

        return gen_wrapper

    elif inspect.isasyncgenfunction(func):

        @wraps(func)
        async def async_gen_wrapper(*args, **kwargs):
            async for item in func(*args, **kwargs):
                yield item

        return async_gen_wrapper

    @wraps(func)

    for item in items:
        yield item


@app.get("/items/stream-no-async", response_class=EventSourceResponse)
def sse_items_no_async() -> Iterable[Item]:
    for item in items:
        yield item


@app.get("/items/stream-no-annotation", response_class=EventSourceResponse)
async def sse_items_no_annotation():
    for item in items:
        yield item

            if self.open:
                yield item
            else:
                raise ValueError("Session closed")


@contextmanager
def acquire_session() -> Generator[Session, None, None]:
    session = Session()
    try:
        yield session
    finally:
        session.open = False


def dep_session() -> Any:
    with acquire_session() as s:
        yield s


def broken_dep_session() -> Any:

            if self.open:
                yield item
            else:
                raise ValueError("Session closed")


@contextmanager
def acquire_session() -> Generator[Session, None, None]:
    session = Session()
    try:
        yield session
    finally:
        session.open = False


def dep_session() -> Any:
    with acquire_session() as s:
        yield s


def broken_dep_session() -> Any:

///

### `yield from` { #yield-from }

When you are iterating over something, like a file-like object, and then you are doing `yield` for each item, you could also use `yield from` to yield each item directly and skip the `for` loop.

This is not particular to FastAPI, it's just Python, but it's a nice trick to know. 😎

def gen_dependency(value: str) -> Generator[str, None, None]:
    yield value


async def async_gen_dependency(value: str) -> AsyncGenerator[str, None]:
    yield value


class CallableDependency:
    def __call__(self, value: str) -> str:
        return value


class CallableGenDependency:
    def __call__(self, value: str) -> Generator[str, None, None]:
        yield value


class AsyncCallableDependency: