Issue 10786: unittest.TextTestRunner default stream no longer bound at import time

Lib/unittest/runner.py

@@ -125,8 +125,10 @@ class TextTestRunner(object):
     """
     resultclass = TextTestResult
 
-    def __init__(self, stream=sys.stderr, descriptions=True, verbosity=1,
+    def __init__(self, stream=None, descriptions=True, verbosity=1,
                  failfast=False, buffer=False, resultclass=None, warnings=None):
+        if stream is None:
+            stream = sys.stderr
         self.stream = _WritelnDecorator(stream)
         self.descriptions = descriptions
         self.verbosity = verbosity

Lib/unittest/test/test_runner.py

@@ -299,3 +299,20 @@ def get_parse_out_err(p):
             self.assertEqual(out.count(msg), 3)
         for msg in [ae_msg, at_msg]:
             self.assertEqual(out.count(msg), 1)
+
+    def testStdErrLookedUpAtInstantiationTime(self):
+        # see issue 10786
+        old_stderr = sys.stderr
+        f = io.StringIO()
+        sys.stderr = f
+        try:
+            runner = unittest.TextTestRunner()
+            self.assertTrue(runner.stream.stream is f)
+        finally:
+            sys.stderr = old_stderr
+
+    def testSpecifiedStreamUsed(self):
+        # see issue 10786
+        f = io.StringIO()
+        runner = unittest.TextTestRunner(f)
+        self.assertTrue(runner.stream.stream is f)

Misc/NEWS

@@ -20,6 +20,10 @@ Core and Builtins
 Library
 -------
 
+- Issue 10786: unittest.TextTestRunner default stream no longer bound at
+  import time. `sys.stderr` now looked up at instantiation time. Fix contributed
+  by Mark Roddy.
+
 - Issue 10753 - Characters ';','=' and ',' in the PATH_INFO environment
   variable won't be quoted when the URI is constructed by the wsgiref.util 's
   request_uri method. According to RFC 3986, these characters can be a part of