Index: Lib/unittest/test/test_runner.py
===================================================================
--- Lib/unittest/test/test_runner.py	(revision 87568)
+++ Lib/unittest/test/test_runner.py	(working copy)
@@ -1,5 +1,6 @@
 import unittest
 
+import sys
 from cStringIO import StringIO
 import pickle
 
@@ -248,6 +249,20 @@
         expectedresult = (runner.stream, DESCRIPTIONS, VERBOSITY)
         self.assertEqual(runner._makeResult(), expectedresult)
 
+    def testStdErrLookedUpAtInstantiationTime(self):
+        # see issue 10786
+        old_stderr = sys.stderr
+        f = StringIO()
+        sys.stderr = f
+        runner = unittest.TextTestRunner()
+        self.assertTrue(runner.stream.stream is f)
+        sys.stderr = old_stderr
 
+    def testSpecifiedStreamUsed(self):
+        # see issue 10786
+        f = StringIO()
+        runner = unittest.TextTestRunner(f)
+        self.assertTrue(runner.stream.stream is f)
+
 if __name__ == '__main__':
     unittest.main()
Index: Lib/unittest/runner.py
===================================================================
--- Lib/unittest/runner.py	(revision 87568)
+++ Lib/unittest/runner.py	(working copy)
@@ -124,8 +124,10 @@
     """
     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):
+        if stream is None:
+            stream = sys.stderr
         self.stream = _WritelnDecorator(stream)
         self.descriptions = descriptions
         self.verbosity = verbosity