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pythondjangoserver-sent-events

Django 3.1: StreamingHttpResponse with an async generator

Documentation for Django 3.1 says this about async views:

The main benefits are the ability to service hundreds of connections without using Python threads. This allows you to use slow streaming, long-polling, and other exciting response types.

I believe that "slow streaming" means we could implement an SSE view without monopolizing a thread per client, so I tried to sketch a simple view, like so:

async def stream(request):

    async def event_stream():
        while True:
            yield 'data: The server time is: %s\n\n' % datetime.datetime.now()
            await asyncio.sleep(1)

    return StreamingHttpResponse(event_stream(), content_type='text/event-stream')

(note: I adapted the code from this response)

Unfortunately, when this view is invoked, it raises the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/asgiref/sync.py", line 330, in thread_handler
    raise exc_info[1]
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/exception.py", line 38, in inner
    response = await get_response(request)
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 231, in _get_response_async
    response = await wrapped_callback(request, *callback_args, **callback_kwargs)
  File "./chat/views.py", line 144, in watch
    return StreamingHttpResponse(event_stream(), content_type='text/event-stream')
  File "/usr/local/lib/python3.7/site-packages/django/http/response.py", line 367, in __init__
    self.streaming_content = streaming_content
  File "/usr/local/lib/python3.7/site-packages/django/http/response.py", line 382, in streaming_content
    self._set_streaming_content(value)
  File "/usr/local/lib/python3.7/site-packages/django/http/response.py", line 386, in _set_streaming_content
    self._iterator = iter(value)
TypeError: 'async_generator' object is not iterable

To me, this shows that StreamingHttpResponse doesn't currently support async generators.

I tried to modify StreamingHttpResponse to use async for but I wasn't able to do much.

Any idea how I could do that?

Solution

  • This is an old question but it came up on a Google result since I was looking for a solution to the same issue. In the end I found this repo https://github.com/valberg/django-sse - which uses async views in Django 4.2 to stream via SSE (specifically see here).

    I understand this is a recent addition to Django so I hope it helps anyone else looking for an answer.