-
-            output_dir = output_file = None
-            if options.output_dir:
-                output_dir = options.output_dir
-            elif options.output_file:
-                output_file = options.output_file
+            if not (options.output_file or options.output_dir):
+                output_file = os.path.splitext(main_input)[0] + '.mobi'