Issue 10470:  'python -m unittest' launches test discovery by default.(If you need to pass options to test discovery the discover subcommand must still be specified explicitly.)
diff --git a/Lib/unittest/main.py b/Lib/unittest/main.py
index 6acff8f..8473a33 100644
--- a/Lib/unittest/main.py
+++ b/Lib/unittest/main.py
@@ -109,7 +109,8 @@
         sys.exit(2)
 
     def parseArgs(self, argv):
-        if len(argv) > 1 and argv[1].lower() == 'discover':
+        if ((len(argv) > 1 and argv[1].lower() == 'discover') or
+            (len(argv) == 1 and self.module is None)):
             self._do_discovery(argv[2:])
             return