Being forced to number the arguments when using positional arguments in
a format string is difficult to maintain.  Adding an argument to the
format string either requires renumbering all subsequent arguments, or
using an out of sequence number such that the order of the format()
arguments no longer matches the order of the "{...}" arguments.  Making
the integer optional would solve this.  Thus, "{}" would be like the old
"%s" where it was far easier to add %s arguments in the middle of the
format string.

Python 3.0a3