I would like to create a string buffer to do lots of processing, format and finally write the buffer in a text file using a C-style sprintf functionality in Pyt
If I understand your question correctly, format() is what you are looking for, along with its mini-language.
Silly example for python 2.7 and up:
>>> print "{} ...\r\n {}!".format("Hello", "world")
Hello ...
world!
For earlier python versions: (tested with 2.6.2)
>>> print "{0} ...\r\n {1}!".format("Hello", "world")
Hello ...
world!