Description: fixup for new argparse
Author: Johannes 'josch' Schauer <josch@debian.org>
Last-Update: 2018-11-22

--- a/tools/dose2html.py
+++ b/tools/dose2html.py
@@ -499,8 +499,8 @@ class _ExtendAction(argparse.Action):
             **kwargs)
 
     def __call__(self, parser, namespace, values, option_string=None):
-        items = argparse._copy.copy(argparse._ensure_value(namespace,
-                                                           self.dest, []))
+        items = getattr(namespace, self.dest, None)
+        items = argparse._copy_items(items)
         items.extend(values)
         setattr(namespace, self.dest, items)
 
