Python 2.6 and before (as well as Python 3.0) require that you number the placeholders:
'{0} {1}\n'.format(numb, foo)
The numbering, if omitted in Python 2.7 and Python 3.1 and up, is implicit, see the documentation:
Changed in version 2.7: The positional argument specifiers can be omitted, so
'{} {}'
is equivalent to'{0} {1}'
.
The implicit numbering is popular; a lot of examples here on Stack Overflow use it as it is easier to whip up a quick format string that way. I have forgotten to include them more than once when working on projects that must support 2.6 still.