Unlike Ruby Python doesn't have great facilities for writing making implicit APIs like this:

   with Table():
      header(...)
      row(...)

You can do something like this but I'd recommend against it as it involves a lot of magic and has trade-offs which make it an uncommon choice. Instead you'd do something like:

    with Table() as table:
        table.header(...)
        table.row(...)

This follows through to your template API as well. So you'd want to construct an object that represents your template, and have methods on that to add a table. For example:

    page = sierra.Page(...)
    with page.div(...), page.add_table() as table:
       table.header(...)
       table.row()
   
    print(page)  # __str__() should return the raw HTML response

This simplifies things, and you can use an in-memory StringIO object rather than a file. You can then return this string to flask as a text/html response and avoid Jinja2 entirely.

Also don't use CamelCase, please! It looks very unpythonic.

However you're really trying to create an API for producing a tree of nodes (some of which might logically map to individual tags, some of which might not) which are then serialized to HTML.

I'd recommend looking at how you can use BeautifulSoup for a lot of this. BS4 contains a lot of classes and functions for creating trees of HTML-like nodes, so you could use this rather than a StringIO object and directly writing "<div>" and having to support all of the attributes as method arguments. You could just use "soup.new_tag(element, *kwargs)" and only do something "special" when handling pandas dataframes or other stuff (like ol/ul tags).

Check out the BS4 documentation and methods for tree manipulation here: https://www.crummy.com/software/BeautifulSoup/bs4/doc/