Fixed #35402 -- Fixed crash in DatabaseFeatures.django_test_skips when running a subset of tests.

Thanks Tim Graham for the report and the review.

Author: jacobtylerwalls

django/db/backends/base/creation.py

@@ -358,27 +358,39 @@ def mark_expected_failures_and_skips(self):
         Mark tests in Django's test suite which are expected failures on this
         database and test which should be skipped on this database.
         """
-        # Only load unittest if we're actually testing.
-        from unittest import expectedFailure, skip
-
         for test_name in self.connection.features.django_test_expected_failures:
-            test_case_name, _, test_method_name = test_name.rpartition(".")
-            test_app = test_name.split(".")[0]
-            # Importing a test app that isn't installed raises RuntimeError.
-            if test_app in settings.INSTALLED_APPS:
-                test_case = import_string(test_case_name)
-                test_method = getattr(test_case, test_method_name)
-                setattr(test_case, test_method_name, expectedFailure(test_method))
+            self._mark_test(test_name)
         for reason, tests in self.connection.features.django_test_skips.items():
             for test_name in tests:
-                test_case_name, _, test_method_name = test_name.rpartition(".")
-                test_app = test_name.split(".")[0]
-                # Importing a test app that isn't installed raises
-                # RuntimeError.
-                if test_app in settings.INSTALLED_APPS:
-                    test_case = import_string(test_case_name)
-                    test_method = getattr(test_case, test_method_name)
-                    setattr(test_case, test_method_name, skip(reason)(test_method))
+                self._mark_test(test_name, reason)
+
+    def _mark_test(self, test_name, skip_reason=None):
+        # Only load unittest during testing.
+        from unittest import expectedFailure, skip
+
+        module_or_class_name, _, name_to_mark = test_name.rpartition(".")
+        test_app = test_name.split(".")[0]
+        # Importing a test app that isn't installed raises RuntimeError.
+        if test_app in settings.INSTALLED_APPS:
+            try:
+                test_frame = import_string(module_or_class_name)
+            except ImportError:
+                # import_string() can raise ImportError if a submodule's parent
+                # module hasn't already been imported during test discovery.
+                # This can happen in at least two cases:
+                # 1. When running a subset of tests in a module, the test
+                #    runner won't import tests in that module's other
+                #    submodules.
+                # 2. When the parallel test runner spawns workers with an empty
+                #    import cache.
+                test_to_mark = import_string(test_name)
+                test_frame = sys.modules.get(test_to_mark.__module__)
+            else:
+                test_to_mark = getattr(test_frame, name_to_mark)
+            if skip_reason:
+                setattr(test_frame, name_to_mark, skip(skip_reason)(test_to_mark))
+            else:
+                setattr(test_frame, name_to_mark, expectedFailure(test_to_mark))
 
     def sql_table_creation_suffix(self):
         """

tests/backends/base/test_creation.py

@@ -1,6 +1,7 @@
 import copy
 import datetime
 import os
+import sys
 from unittest import mock
 
 from django.db import DEFAULT_DB_ALIAS, connection, connections
@@ -333,6 +334,10 @@ def test_mark_expected_failures_and_skips(self):
                 "backends.base.test_creation.skip_test_function",
             },
         }
+        # Emulate the scenario where the parent module for
+        # backends.base.test_creation has not been imported yet.
+        popped_module = sys.modules.pop("backends.base")
+        self.addCleanup(sys.modules.__setitem__, "backends.base", popped_module)
         creation.mark_expected_failures_and_skips()
         self.assertIs(
             expected_failure_test_function.__unittest_expecting_failure__,