> Is following test passed?

>         check((1,), '1')

That test succeeds.

> Is CPython crashes when change signature of testfunc?

--- a/Lib/test/test_tcl.py      Sat Jul 12 18:26:03 2014 +0300
+++ b/Lib/test/test_tcl.py      Sun Jul 13 19:21:55 2014 +0300
@@ -416,7 +416,7 @@
 
     def test_user_command(self):
         result = None
-        def testfunc(arg):
+        def testfunc(arg=None, *args):
             nonlocal result
             result = arg
             return arg

This change does not have any effect on the original failure. The assertion failure and crash continues to occur.