gh-108927: Fix removing testing modules from sys.modules

Lib/test/libregrtest/main.py

@@ -293,7 +293,7 @@ def run_tests_sequentially(self, runtests):
         else:
             tracer = None
 
-        save_modules = sys.modules.keys()
+        save_modules = set(sys.modules)
 
         jobs = runtests.get_jobs()
         if jobs is not None:
@@ -318,9 +318,9 @@ def run_tests_sequentially(self, runtests):
             result = self.run_test(test_name, runtests, tracer)
 
             # Unload the newly imported modules (best effort finalization)
-            for module in sys.modules.keys():
+            for module in list(sys.modules):
                 if module not in save_modules and module.startswith("test."):
-                    support.unload(module)
+                    sys.modules.pop(module, None)
 
             if result.must_stop(self.fail_fast, self.fail_env_changed):
                 break

Lib/test/libregrtest/single.py

@@ -76,10 +76,6 @@ def _load_run_test(result: TestResult, runtests: RunTests) -> None:
     # Load the test module and run the tests.
     test_name = result.test_name
     module_name = abs_module_name(test_name, runtests.test_dir)
-
-    # Remove the module from sys.module to reload it if it was already imported
-    sys.modules.pop(module_name, None)
-
     test_mod = importlib.import_module(module_name)
 
     if hasattr(test_mod, "test_main"):

Lib/test/regrtestdata/import_from_tests/test_regrtest_a.py

@@ -0,0 +1,6 @@
+import unittest
+import test_regrtest_b.util
+
+class Test(unittest.TestCase):
+    def test(self):
+        test_regrtest_b.util

Lib/test/regrtestdata/import_from_tests/test_regrtest_b/__init__.py

@@ -0,0 +1,5 @@
+import unittest
+
+class Test(unittest.TestCase):
+    def test(self):
+        pass

Lib/test/regrtestdata/import_from_tests/test_regrtest_c.py

@@ -0,0 +1,6 @@
+import unittest
+import test_regrtest_b.util
+
+class Test(unittest.TestCase):
+    def test(self):
+        test_regrtest_b.util

Lib/test/test_regrtest.py

@@ -1866,6 +1866,14 @@ def test_random_seed(self):
     def test_random_seed_workers(self):
         self._check_random_seed(run_workers=True)
 
+    def test_import_from_tests(self):
+        testdir = os.path.join(os.path.dirname(__file__),
+                               'regrtestdata', 'import_from_tests')
+        tests = [f'test_regrtest_{name}' for name in ('a', 'b', 'c')]
+        args = ['-Wd', '-E', '-bb', '-m', 'test', '--testdir=%s' % testdir, *tests]
+        output = self.run_python(args)
+        self.check_executed_tests(output, tests, stats=3)
+
 
 class TestUtils(unittest.TestCase):
     def test_format_duration(self):

Misc/NEWS.d/next/Tests/2023-09-05-20-46-35.gh-issue-108927.TpwWav.rst

@@ -0,0 +1,4 @@
+Fixed order dendence in running tests in the same process
+when a test that has submodules (e.g. test_importlib) follows a test that
+imports its submodule (e.g. test_importlib.util) and precedes a test
+(e.g. test_unittest or test_compileall) that uses that submodule.