It might be a little awkward to be changing the "default" fields when someone is asking for details. Since it's called default fields, it is probably more natural to make it a constant (which Python doesn't have, but a module level DEFAULT_FIELDS probably makes sense). Then you construct a "fields" variable or something like that, copy the defaults into it, and then append the user input.

This is all semantics, and me being nitpicky (sorry).