#36916: add support for streaming with TaskGroups
-------------------------------------+-------------------------------------
Reporter: Thomas Grainger | Type:
| Uncategorized
Status: new | Component: HTTP
| handling
Version: dev | Severity: Normal
Keywords: structured- | Triage Stage:
concurrency, taskgroups | Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
-------------------------------------+-------------------------------------
https://forum.djangoproject.com/t/streamingresponse-driven-
by-a-taskgroup/40320/4
https://github.com/django/new-features/issues/117
### Feature Description
see https://forum.djangoproject.com/t/streamingresponse-driven-
by-a-taskgroup/40320/4
I'd like to be able to write code that combines multiple streams of data:
```py
async def news_and_weather(request: HttpRequest) -> StreamingHttpResponse:
async def gen() -> AsyncGenerator[bytes]:
async def push(ws_url: str, tx: MemoryObjectSendStream) -> None:
async with tx, connect_ws(ws_url) as conn:
async for msg in conn:
await tx.send(msg)
async with anyio.create_task_group() as tg:
tx, rx = anyio.create_memory_object_stream[bytes]()
with tx, rx:
tg.start_soon(push, "ws://example.com/news", tx.clone())
tg.start_soon(push, "ws://example.com/weather",
tx.clone())
tx.close()
async for msg in rx:
yield msg # yield in async generator!! illegal inside
TaskGroup!
return StreamingHttpResponse(gen())
```
### Problem
however this doesn’t work because I’m using a yield inside an async
generator that’s not a context manager, and calling aclosing() on that
async generator is not sufficient to allow a TaskGroup to cancel itself
and catch the cancel error.
```py
from useful_types import SupportsAnext
class AsyncIteratorBytesResource(Protocol):
"""
all the machinery needed to safely run an AsyncGenerator[Bytes]
(for django-stubs) this allows AsyncGenerator[bytes] but is less
strict
so would also allow a anyio MemoryObjectRecieveStream[bytes]]
"""
async def __aiter__(self) -> SupportsAnext[bytes]: ...
async def aclose(self) -> object: ...
async def news_and_weather(request: HttpRequest) ->
StreamingAcmgrHttpResponse:
@contextlib.asynccontextmanager
async def acmgr_gen() -> AsyncGenerator[AsyncIteratorBytesResource]:
async def push(ws_url: str, tx: MemoryObjectSendStream) -> None:
async with tx, connect_ws(ws_url) as conn:
async for msg in conn:
await tx.send(msg)
async with anyio.create_task_group() as tg:
tx, rx = anyio.create_memory_object_stream[bytes]()
with tx, rx:
tg.start_soon(push, "ws://example.com/news", tx.clone())
tg.start_soon(push, "ws://example.com/weather",
tx.clone())
tx.close()
yield rx # yield inside asynccontextmanager, permitted
inside TaskGroup
return StreamingAcmgrHttpResponse(acmgr_gen())
```
### Implementation Suggestions
https://github.com/django/django/pull/19364/changes
--
Ticket URL: <https://code.djangoproject.com/ticket/36916>
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