Fix obscure failures of datetime-related tests due to the datetime tests failing…

Fix obscure failures of datetime-related tests due to the datetime tests failing to restore the system state completely after testing the pure-Python versions.

Lib/test/datetimetester.py

@@ -1780,8 +1780,6 @@ class TestDateTime(TestDate):
         self.assertTrue(abs(from_timestamp - from_now) <= tolerance)
 
     def test_strptime(self):
-        import _strptime
-
         string = '2004-12-01 13:02:47.197'
         format = '%Y-%m-%d %H:%M:%S.%f'
         expected = _strptime._strptime_datetime(self.theclass, string, format)

Lib/test/test_datetime.py

 import unittest
 import sys
 from test.support import import_fresh_module, run_unittest
+
 TESTS = 'test.datetimetester'
+
 # XXX: import_fresh_module() is supposed to leave sys.module cache untouched,
 # XXX: but it does not, so we have to save and restore it ourselves.
 save_sys_modules = sys.modules.copy()
@@ -15,28 +17,32 @@ finally:
     sys.modules.update(save_sys_modules)
 test_modules = [pure_tests, fast_tests]
 test_suffixes = ["_Pure", "_Fast"]
+# XXX(gb) First run all the _Pure tests, then all the _Fast tests.  You might
+# not believe this, but in spite of all the sys.modules trickery running a _Pure
+# test last will leave a mix of pure and native datetime stuff lying around.
+test_classes = []
 
 for module, suffix in zip(test_modules, test_suffixes):
     for name, cls in module.__dict__.items():
-        if isinstance(cls, type) and issubclass(cls, unittest.TestCase):
-            name += suffix
-            cls.__name__ = name
-            globals()[name] = cls
-            def setUp(self, module=module, setup=cls.setUp):
-                self._save_sys_modules = sys.modules.copy()
-                sys.modules[TESTS] = module
-                sys.modules['datetime'] = module.datetime_module
-                sys.modules['_strptime'] = module._strptime
-                setup(self)
-            def tearDown(self, teardown=cls.tearDown):
-                teardown(self)
-                sys.modules.clear()
-                sys.modules.update(self._save_sys_modules)
-            cls.setUp = setUp
-            cls.tearDown = tearDown
+        if not (isinstance(cls, type) and issubclass(cls, unittest.TestCase)):
+            continue
+        cls.__name__ = name + suffix
+        @classmethod
+        def setUpClass(cls_, module=module):
+            cls_._save_sys_modules = sys.modules.copy()
+            sys.modules[TESTS] = module
+            sys.modules['datetime'] = module.datetime_module
+            sys.modules['_strptime'] = module._strptime
+        @classmethod
+        def tearDownClass(cls_):
+            sys.modules.clear()
+            sys.modules.update(cls_._save_sys_modules)
+        cls.setUpClass = setUpClass
+        cls.tearDownClass = tearDownClass
+        test_classes.append(cls)
 
 def test_main():
-    run_unittest(__name__)
+    run_unittest(*test_classes)
 
 if __name__ == "__main__":
     test_main()