The example config file says...
<!-- Exclude URLs that point to default index.html files.
URLs for directories get included, so these files are
<filter action="drop" type="wildcard" pattern="*index.
However with regard to lastmod those index files are not
redundant at all.
A sitemap created by walking the tree produces lastmod
dates of the date the directory was created, not the date
the index file was actually changed.
Everything runs if I include the filter above, but the
lastmod dates are all wrong.
If I remove the filter above I always get the response
I assume I need to change something in sitemap_gen.py
to allo the inclusion of so called "redundant" index files.