R. David Murray wrote:
> it is made clear in various places that every object has an str

here:
http://docs.python.org/py3k/library/string.html#format-specification-mini-language

3rd paragraph:
"A general convention is that an empty format string ("") produces the same result as if you had called str() on the value. A non-empty format string typically modifies the result."

"an empty format string ("")" what does it mean ?

"".format(value) or "{}".format(value) or "{0}".format(value) ?