R. David Murray wrote:
> Putting nothing between the {}'s is an empty format string.

this is an empty replacement field

here:
http://docs.python.org/py3k/library/string.html#format-string-syntax

the definition of format string:
"Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output."

"The grammar for a replacement field is as follows:"
replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"