check mandatory args if **kwargs taken too

agent.py

@@ -369,10 +369,9 @@ class SerialLineReceiver(SerialLineReceiverBase):
                     "Command %s got unexpected keyword arguments: %s" % (
                         unicode(func), ", ".join(unexpected_kwargs)))
 
-            if argspec.defaults:
-                mandatory_args = argspec.args[0:-len(argspec.defaults)]
-            else:
         mandatory_args = argspec.args
+        if argspec.defaults:  # remove those with default value
+            mandatory_args = mandatory_args[0:-len(argspec.defaults)]
         missing_kwargs = set(mandatory_args) - set(args)
         if missing_kwargs:
             raise TypeError("Command %s missing arguments: %s" % (