In http://docs.python.org/library/string.html#format-specification-mini-language, it says "The fill character can be any character other than ‘{‘ or ‘}’."

But that's not actually true. It's only true when using str.format, as it does the parsing and the { and } mean special things for that parser. 

But for format itself there is no restriction:

>>> format(3, '{>10')
'{{{{{{{{{3'

As this section should be documenting the format specifiers themselves, I think it should not concern itself with restrictions imposed by str.format.