The auto-numbering takes the fields from left to right, so the first ``{...}`` specifier will use the first argument to :meth:`str.format`, the next specifier will use the next argument, and so on.You can't mix auto-numbering and explicit numbering -- either number all of your specifier fields or none of them -- but you can mix auto-numbering and named fields, as in the second example above.(Contributed by Eric Smith; :issue:`5237`.)