I want a meaningful name to appear in debugging output generated by repr() or str(), not just _ all over the place.  I just don't want to specifically come up with the meaningful name myself.

Right now I pass in the same generated name ('__'.join (field_names)) to the constructor, but this means I need to repeat that logic in any other similar application, and I would have to put in special handling if any of my attribute names required renaming.

I would rather be explicit that I'm not providing a specific name.  With your '_' suggestion it looks like a magic value - why '_'?  By specifying None, it's obvious at the call point that I'm explicitly declining to provide a name, and then the code generates a semi-meaningful name automatically.

Also, please note that I moved the place where typename is assigned to after the part where it handles the rename stuff, so the generated names automatically incorporate a suitable default and remain valid identifiers.

I'm having trouble seeing the downside here.  I'm adding one "is None" check and one line of code to the existing procedure.  I can't believe I'm the only person who has wanted to skip making up a type name but still wanted something vaguely meaningful in debug output.