clean the environment from pre-existing PYTHONWARNINGS for test_warnings

Lib/unittest/test/test_runner.py

@@ -139,6 +139,18 @@ class TestCleanUp(unittest.TestCase):
 class Test_TextTestRunner(unittest.TestCase):
     """Tests for TextTestRunner."""
 
+    def setUp(self):
+        # clean the environment from pre-existing PYTHONWARNINGS to make
+        # test_warnings results consistent
+        self.pythonwarnings = os.environ.get('PYTHONWARNINGS')
+        if self.pythonwarnings:
+            del os.environ['PYTHONWARNINGS']
+
+    def tearDown(self):
+        # bring back pre-existing PYTHONWARNINGS if present
+        if self.pythonwarnings:
+            os.environ['PYTHONWARNINGS'] = self.pythonwarnings
+
     def test_init(self):
         runner = unittest.TextTestRunner()
         self.assertFalse(runner.failfast)