The problem is that in core.py do don't want to import `FixedWidth`. That would be a circular import. I did not think about the `_format_name`, but that's why I suggested the `hasattr('something_that_only_defined_in_fixedwidthtwoline')`. Your solution is better to read for anyone looking at the code, my solution would deal with the inheritance, since children would presumable also have those attributes.

In the end I think we can just go with either. This whole PR slightly hackish to make the UI do what most people expect. It's only the UI. It would be cleaner to build that into the classes, but that causes other complications (e.g. a class needs to have a state variable that remembers if `data_start` has be set already or if it's still the default value, because clearly you only want to set it to `header_start+x` if it has not be set manually already).

So, I suggest to just merge this now or to revert to my suggestion for this line and merge then.