diff -r c3cec0f77eff Lib/unittest/loader.py
--- a/Lib/unittest/loader.py	Tue Oct 20 18:22:36 2015 +0300
+++ b/Lib/unittest/loader.py	Tue Oct 20 21:52:28 2015 +0200
@@ -479,6 +479,8 @@
                     return tests, True
                 finally:
                     self._loading_packages.discard(name)
+        else:
+            return None, False
 
 
 defaultTestLoader = TestLoader()
diff -r c3cec0f77eff Lib/unittest/test/test_discovery.py
--- a/Lib/unittest/test/test_discovery.py	Tue Oct 20 18:22:36 2015 +0300
+++ b/Lib/unittest/test/test_discovery.py	Tue Oct 20 21:52:28 2015 +0200
@@ -53,7 +53,8 @@
             os.path.isdir = original_isdir
 
         path_lists = [['test2.py', 'test1.py', 'not_a_test.py', 'test_dir',
-                       'test.foo', 'test-not-a-module.py', 'another_dir'],
+                       'test.foo', 'test-not-a-module.py', 'another_dir',
+                       'socket'],
                       ['test4.py', 'test3.py', ]]
         os.listdir = lambda path: path_lists.pop(0)
         self.addCleanup(restore_listdir)
@@ -64,8 +65,15 @@
         self.addCleanup(restore_isdir)
 
         def isfile(path):
+            if isdir(path):
+                return False
             # another_dir is not a package and so shouldn't be recursed into
-            return not path.endswith('dir') and not 'another_dir' in path
+            if 'another_dir' in path:
+                return False
+            # a socket is neither a directory nor a regular file (issue 22457)
+            if path.endswith('socket'):
+                return False
+            return True
         os.path.isfile = isfile
         self.addCleanup(restore_isfile)