SynchronousOnlyOperation exception while running django on gevent workers

[peekuh]

I'm having trouble getting the gevent worker type to serve my django application. Here's what I've done so far:

in my wsgi.py file, called the patch_all method.

import os

from django.core.wsgi import get_wsgi_application

if os.environ.get('GEVENT_ENABLED', 'false').lower() == 'true':
    from gevent import monkey
    monkey.patch_all() 

os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'myapp.settings')

application = get_wsgi_application()

this is the command I use to deploy the application:

.venv/bin/python3.13 -m gunicorn --workers 4 --worker-class gevent --bind 0.0.0.0:8000 myapp.wsgi:application

here's the View I see the error on (just one of many endpoints that are failing with the same error)

from django.contrib.auth import login, authenticate
from rest_framework.views import APIView
from rest_framework.permissions import AllowAny, IsAuthenticated
from rest_framework.authtoken.models import Token

class LoginView(UserSerializerMixin, APIView):
    permission_classes = [AllowAny]
    serializer_class = LoginSerializer
    throttle_scope = 'login'
    
    def post(self, request):
        serializer = self.serializer_class(data=request.data)
        serializer.is_valid(raise_exception=True)
        
        email = serializer.validated_data['email']
        password = serializer.validated_data['password']
        
        user = authenticate(request, username=email, password=password)
        
        if user is not None:            
            token, created = Token.objects.get_or_create(user=user)
            
            response = JsonResponse({
                'token': token.key,  
                'user':  self.serialize_user_data(user)
            })

            return response
        
        return Response({"error": "Invalid credentials"}, status=status.HTTP_400_BAD_REQUEST)

Full traceback of the error

 Exception ignored in: <function _after_fork at 0x706e6bf4d120>
 Traceback (most recent call last):
   File "/python/cpython-3.13-linux-x86_64-gnu/lib/python3.13/threading.py", line 1588, in _after_fork
     thread._after_fork(new_ident=ident)
   File "/python/cpython-3.13-linux-x86_64-gnu/lib/python3.13/threading.py", line 1424, in _after_fork
     Thread._after_fork(self, new_ident=new_ident)
   File "/python/cpython-3.13-linux-x86_64-gnu/lib/python3.13/threading.py", line 934, in _after_fork
     assert self._handle.ident == new_ident
 AttributeError: '_DummyThread' object has no attribute '_handle'
 Exception ignored in: <bound method _ForkHooks.after_fork_in_child of <gevent.threading._ForkHooks object at 0x706e6b6f4c20>>
 Traceback (most recent call last):
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/gevent/threading.py", line 376, in after_fork_in_child
     assert len(active) == 1
 AssertionError:
 Exception ignored in: <function _after_fork at 0x706e6bf4d120>
 Traceback (most recent call last):
   File "/python/cpython-3.13-linux-x86_64-gnu/lib/python3.13/threading.py", line 1588, in _after_fork
     thread._after_fork(new_ident=ident)
   File "/python/cpython-3.13-linux-x86_64-gnu/lib/python3.13/threading.py", line 1424, in _after_fork
     Thread._after_fork(self, new_ident=new_ident)
   File "/python/cpython-3.13-linux-x86_64-gnu/lib/python3.13/threading.py", line 934, in _after_fork             
     assert self._handle.ident == new_ident
 AttributeError: '_DummyThread' object has no attribute '_handle'
 Exception ignored in: <bound method _ForkHooks.after_fork_in_child of <gevent.threading._ForkHooks object at 0x706e6b6f4c20>>
 Traceback (most recent call last):
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/gevent/threading.py", line 376, in after_fork_in_child                                                                                                                             
     assert len(active) == 1
 AssertionError: 
 [2025-10-20 19:20:14,162] ERROR [django.request] Internal Server Error: /api/v1/auth/login/                      
 Traceback (most recent call last):
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/core/handlers/exception.py", line 55, in inner
     response = get_response(request)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/core/handlers/base.py", line 197, in _get_response
     response = wrapped_callback(request, *callback_args, **callback_kwargs)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/views/decorators/csrf.py", line 65, in _view_wrapper
     return view_func(request, *args, **kwargs)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/views/generic/base.py", line 104, in view    
     return self.dispatch(request, *args, **kwargs)
            ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 509, in dispatch     
     response = self.handle_exception(exc)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 469, in handle_exception
     self.raise_uncaught_exception(exc)
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
     raise exc
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 506, in dispatch     
     response = handler(request, *args, **kwargs)
   File "/opt/app-root/src/users/views.py", line 59, in post
     user = authenticate(request, username=email, password=password)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/views/decorators/debug.py", line 75, in sensitive_variables_wrapper
     return func(*func_args, **func_kwargs)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/contrib/auth/__init__.py", line 79, in authenticate
     user = backend.authenticate(request, **credentials)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/contrib/auth/backends.py", line 42, in authenticate
     user = UserModel._default_manager.get_by_natural_key(username)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/contrib/auth/base_user.py", line 37, in get_by_natural_key
     return self.get(**{self.model.USERNAME_FIELD: username})
            ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/manager.py", line 87, in manager_method
     return getattr(self.get_queryset(), name)(*args, **kwargs)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 645, in get        
     num = len(clone)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 382, in __len__    
 [2025-10-20 19:20:14,162] ERROR [django.request] Internal Server Error: /api/v1/auth/login/
     self._fetch_all()
     ~~~~~~~~~~~~~~~^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 1928, in _fetch_all
     self._result_cache = list(self._iterable_class(self))
                          ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 91, in __iter__    
     results = compiler.execute_sql(
         chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size
     )
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/sql/compiler.py", line 1572, in execute_sql
     cursor = self.connection.cursor()
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/utils/asyncio.py", line 24, in inner
     raise SynchronousOnlyOperation(message)
 django.core.exceptions.SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async.
 Traceback (most recent call last):
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/core/handlers/exception.py", line 55, in inner                                                                                                                              
     response = get_response(request)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/core/handlers/base.py", line 197, in _get_response                                                                                                                          
     response = wrapped_callback(request, *callback_args, **callback_kwargs)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/views/decorators/csrf.py", line 65, in _view_wrapper
     return view_func(request, *args, **kwargs)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/views/generic/base.py", line 104, in view    
     return self.dispatch(request, *args, **kwargs)
            ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 509, in dispatch     
     response = self.handle_exception(exc)                                                                        
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 469, in handle_exception                                                                                                                            
     self.raise_uncaught_exception(exc)
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^                                                                           
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception                                                                                                                    
     raise exc
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 506, in dispatch
     response = handler(request, *args, **kwargs)                                                                 
   File "/opt/app-root/src/users/views.py", line 59, in post                                                      
     user = authenticate(request, username=email, password=password)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/views/decorators/debug.py", line 75, in sensitive_variables_wrapper                                                                                                         
     return func(*func_args, **func_kwargs)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/contrib/auth/__init__.py", line 79, in authenticate                                                                                                                         
     user = backend.authenticate(request, **credentials)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/contrib/auth/backends.py", line 42, in authenticate                                                                                                                         
     user = UserModel._default_manager.get_by_natural_key(username)
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/contrib/auth/base_user.py", line 37, in get_by_natural_key                                                                                                                  
     return self.get(**{self.model.USERNAME_FIELD: username})
            ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                     
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/manager.py", line 87, in manager_method                                                                                                                           
     return getattr(self.get_queryset(), name)(*args, **kwargs)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 645, in get        
     num = len(clone)                                                                                             
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 382, in __len__
     self._fetch_all()                                                                                            
     ~~~~~~~~~~~~~~~^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 1928, in _fetch_all
     self._result_cache = list(self._iterable_class(self))                                                        
                          ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/query.py", line 91, in __iter__    
     results = compiler.execute_sql(
         chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size                                             
     )                                                                                                            
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/db/models/sql/compiler.py", line 1572, in execute_sql                                                                                                                       
     cursor = self.connection.cursor()
   File "/opt/app-root/src/.venv/lib/python3.13/site-packages/django/utils/asyncio.py", line 24, in inner         
     raise SynchronousOnlyOperation(message)                                                                      
 django.core.exceptions.SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async.

[pajod]

AttributeError: '_DummyThread' object has no attribute '_handle'

AFAIR you can only hit this with a few combinations of gevent 25 and cPython 3.13 versions. Which versions are you using, and how do those compare to what gevent upstream is testing?

[peekuh]

AttributeError: '_DummyThread' object has no attribute '_handle'

AFAIR you can only hit this with a few combinations of gevent 25 and cPython 3.13 versions. Which versions are you using, and how do those compare to what gevent upstream is testing?

It does seem like I'm using python 3.13! Gevent version 25.5.1.
what would be right version combination to use? I'll experiment with python 3.12 and see if that does the trick.

[pajod]

Use gevent 25.8.1 or later if you want to use Python 3.13.4 or later. (explanation inside the relevant gevent commit)