Position-less argument specifiers were not added until python 2.7. 

Specify the position to fix support for older versions of python (2.5, 2.6).

> Changed in version 2.7: The positional argument specifiers can be omitted, so '{} {}' is equivalent to '{0} {1}'.

[source](http://docs.python.org/2/library/string.html#format-string-syntax)