Adds missing celery.backends.default_backend

celery/backends/__init__.py

@@ -1,4 +1,5 @@
-from celery.app import app_or_default
+from celery import current_app
+from celery.local import LocalProxy
 from celery.utils import get_cls_by_name
 
 BACKEND_ALIASES = {
@@ -16,8 +17,12 @@ _backend_cache = {}
 
 def get_backend_cls(backend, loader=None):
     """Get backend class by name/alias"""
-    loader = loader or app_or_default().loader
+    loader = loader or current_app.loader
     if backend not in _backend_cache:
         aliases = dict(BACKEND_ALIASES, **loader.override_backends)
         _backend_cache[backend] = get_cls_by_name(backend, aliases)
     return _backend_cache[backend]
+
+
+# deprecate this.
+default_backend = LocalProxy(lambda: current_app.backend)