#35174: The newly introduced Signal.asend returns TypeError: unhashable type:
'list' if all receivers are asynchronous functions
-----------------------------------------+------------------------
Reporter: VaĊĦek Dohnal | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: 5.0
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-----------------------------------------+------------------------
== About this issue
Django 5 added support for asynchronous signals using the `asend` and
`asend_robust` methods
([https://docs.djangoproject.com/en/5.0/releases/5.0/#signals]). If a
signal is created with **only asynchronous receivers**, the `asend` (or
`asend_robust`) function call will crash on this error:
{{{
Traceback (most recent call last):
File "C:\Users\***-py3.11\Lib\site-packages\asgiref\sync.py", line 534,
in thread_handler
raise exc_info[1]
File "C:\Users\***-py3.11\Lib\site-
packages\django\core\handlers\exception.py", line 42, in inner
response = await get_response(request)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\***-py3.11\Lib\site-packages\asgiref\sync.py", line 534,
in thread_handler
raise exc_info[1]
File "C:\Users\***-py3.11\Lib\site-
packages\django\core\handlers\base.py", line 253, in _get_response_async
response = await wrapped_callback(
^^^^^^^^^^^^^^^^^^^^^^^
File "C:\work\contrib\django-asgi-lifespan\signals.py", line 47, in root
await my_signal.asend_robust(sender=None)
File "C:\Users\***-py3.11\Lib\site-
packages\django\dispatch\dispatcher.py", line 393, in asend_robust
responses, async_responses = await asyncio.gather(
^^^^^^^^^^^^^^^
File
"C:\Users\***\AppData\Local\Programs\Python\Python311\Lib\asyncio\tasks.py",
line 826, in gather
if arg not in arg_to_fut:
^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'list'
}}}
== Sample project demonstrating the error
Create file `signals.py` with this content:
{{{#!python
import asyncio
import logging
from django import conf, http, urls
from django.core.handlers.asgi import ASGIHandler
from django.dispatch import Signal, receiver
logging.basicConfig(level=logging.DEBUG)
conf.settings.configure(
ALLOWED_HOSTS="*",
ROOT_URLCONF=__name__,
LOGGING=None,
)
app = ASGIHandler()
my_signal = Signal()
@receiver(my_signal)
async def my_async_receiver(sender, **kwargs):
logging.info("my_async_receiver::started")
await asyncio.sleep(1)
logging.info("my_async_receiver::finished")
async def root(request):
logging.info("root::started")
await my_signal.asend_robust(sender=None)
logging.info("root::ended")
return http.JsonResponse({"message": "Hello World"})
urlpatterns = [urls.path("", root)]
}}}
Run this file:
{{{
uvicorn signals:app --log-level=DEBUG --reload
}}}
Execute the view:
{{{
curl -v http://127.0.0.1:8000
}}}
See the error.
If we modify the above code and add at least one synchronous receiver,
everything will work fine.
{{{#!python
import asyncio
import logging
import time
from django import conf, http, urls
from django.core.handlers.asgi import ASGIHandler
from django.dispatch import Signal, receiver
logging.basicConfig(level=logging.DEBUG)
conf.settings.configure(
ALLOWED_HOSTS="*",
ROOT_URLCONF=__name__,
LOGGING=None,
)
app = ASGIHandler()
my_signal = Signal()
@receiver(my_signal)
async def my_async_receiver(sender, **kwargs):
logging.info("my_async_receiver::started")
await asyncio.sleep(1)
logging.info("my_async_receiver::finished")
@receiver(my_signal)
def my_standard_receiver(sender, **kwargs):
logging.info("my_standard_receiver::started")
time.sleep(1)
logging.info("my_standard_receiver::finished")
async def root(request):
logging.info("root::started")
await my_signal.asend_robust(sender=None)
logging.info("root::ended")
return http.JsonResponse({"message": "Hello World"})
urlpatterns = [urls.path("", root)]
}}}
Output of uvicorn in this case:
{{{
INFO: Started server process [80144]
INFO: Waiting for application startup.
INFO: ASGI 'lifespan' protocol appears unsupported.
INFO: Application startup complete.
INFO:root:root::started
INFO:root:my_async_receiver::started
INFO:root:my_standard_receiver::started
INFO:root:my_standard_receiver::finished
INFO:root:my_async_receiver::finished
INFO:root:root::ended
INFO: 127.0.0.1:60295 - "GET / HTTP/1.1" 200 OK
}}}
== Proposed solution
In my opinion, the error is located here:
[https://github.com/django/django/blob/main/django/dispatch/dispatcher.py#L205-L259].
The method groups receivers into synchronous and asynchronous (which is
documented behavior). If there are no synchronous receivers, the type is
assigned to the variable: `sync_send = list`, and I think this is where
the problem lies.
Here is my proposed fix to the method mentioned. If there are no
synchronous receivers, an empty asynchronous function is used as
placeholder. With this change, everything works as it should.
{{{#!python
async def asend(self, sender, **named):
"""
Send signal from sender to all connected receivers in async mode.
All sync receivers will be wrapped by sync_to_async()
If any receiver raises an error, the error propagates back through
send, terminating the dispatch loop. So it's possible that all
receivers won't be called if an error is raised.
If any receivers are synchronous, they are grouped and called
behind a
sync_to_async() adaption before executing any asynchronous
receivers.
If any receivers are asynchronous, they are grouped and executed
concurrently with asyncio.gather().
Arguments:
sender
The sender of the signal. Either a specific object or
None.
named
Named arguments which will be passed to receivers.
Return a list of tuple pairs [(receiver, response), ...].
"""
if (
not self.receivers
or self.sender_receivers_cache.get(sender) is NO_RECEIVERS
):
return []
sync_receivers, async_receivers = self._live_receivers(sender)
if sync_receivers:
@sync_to_async
def sync_send():
responses = []
for receiver in sync_receivers:
response = receiver(signal=self, sender=sender,
**named)
responses.append((receiver, response))
return responses
else:
async def sync_send():
return []
responses, async_responses = await asyncio.gather(
sync_send(),
asyncio.gather(
*(
receiver(signal=self, sender=sender, **named)
for receiver in async_receivers
)
),
)
responses.extend(zip(async_receivers, async_responses))
return responses
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/35174>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
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