Fixed #20474 -- Proxied and deprecated django.db.backend

django/db/__init__.py

@@ -8,6 +8,7 @@ from django.db.utils import (DEFAULT_DB_ALIAS,
     ProgrammingError, NotSupportedError, DatabaseError,
     InterfaceError, Error,
     load_backend, ConnectionHandler, ConnectionRouter)
+from django.utils.functional import cached_property
 
 __all__ = ('backend', 'connection', 'connections', 'router', 'DatabaseError',
     'IntegrityError', 'DEFAULT_DB_ALIAS')
@@ -45,7 +46,28 @@ class DefaultConnectionProxy(object):
         return delattr(connections[DEFAULT_DB_ALIAS], name)
 
 connection = DefaultConnectionProxy()
-backend = load_backend(connection.settings_dict['ENGINE'])
+
+class DefaultBackendProxy(object):
+    """
+    Temporary proxy class used during deprecation period of the `backend` module
+    variable.
+    """
+    @cached_property
+    def _backend(self):
+        warnings.warn("Accessing django.db.backend is deprecated.",
+            PendingDeprecationWarning, stacklevel=2)
+        return load_backend(connections[DEFAULT_DB_ALIAS].settings_dict['ENGINE'])
+
+    def __getattr__(self, item):
+        return getattr(self._backend, item)
+
+    def __setattr__(self, name, value):
+        return setattr(self._backend, name, value)
+
+    def __delattr__(self, name):
+        return delattr(self._backend, name)
+
+backend = DefaultBackendProxy()
 
 def close_connection(**kwargs):
     warnings.warn(

docs/internals/deprecation.txt

@@ -373,6 +373,7 @@ these changes.
 
 * The following private APIs will be removed:
 
+  - ``django.db.backend``
   - ``django.db.close_connection()``
   - ``django.db.backends.creation.BaseDatabaseCreation.set_autocommit()``
   - ``django.db.transaction.is_managed()``

tests/backends/tests.py

@@ -8,7 +8,7 @@ import threading
 
 from django.conf import settings
 from django.core.management.color import no_style
-from django.db import (backend, connection, connections, DEFAULT_DB_ALIAS,
+from django.db import (connection, connections, DEFAULT_DB_ALIAS,
     DatabaseError, IntegrityError, transaction)
 from django.db.backends.signals import connection_created
 from django.db.backends.postgresql_psycopg2 import version as pg_version
@@ -50,7 +50,8 @@ class OracleChecks(unittest.TestCase):
     def test_dbms_session(self):
         # If the backend is Oracle, test that we can call a standard
         # stored procedure through our cursor wrapper.
-        convert_unicode = backend.convert_unicode
+        from django.db.backends.oracle.base import convert_unicode
+
         cursor = connection.cursor()
         cursor.callproc(convert_unicode('DBMS_SESSION.SET_IDENTIFIER'),
                         [convert_unicode('_django_testing!')])
@@ -60,8 +61,10 @@ class OracleChecks(unittest.TestCase):
     def test_cursor_var(self):
         # If the backend is Oracle, test that we can pass cursor variables
         # as query parameters.
+        from django.db.backends.oracle.base import Database
+
         cursor = connection.cursor()
-        var = cursor.var(backend.Database.STRING)
+        var = cursor.var(Database.STRING)
         cursor.execute("BEGIN %s := 'X'; END; ", [var])
         self.assertEqual(var.getvalue(), 'X')