This is what fails for instance: foo = StrictOrderingOnAppendList(['foo']) foo += ['bar'] bar = foo + ['qux']

Comment on attachment 8461413 [details] [diff] [review] Fix StrictOrderingOnAppendList.__add__ to not throw when the list content is not sorted Review of attachment 8461413 [details] [diff] [review]: ----------------------------------------------------------------- ::: python/mozbuild/mozbuild/util.py @@ +294,5 @@ > > + new_list = StrictOrderingOnAppendList() > + # Can't extend with self because it may already be the result of > + # several extensions and not be ordered. > + list.__iadd__(new_list, self) I think list.extend(new_list, self) would be more appropriate.