sprintf like functionality in Python

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 Python. Because of conditional statements, I can’t write them directly to the file.

e.g pseudo code:

sprintf(buf,"A = %d\n , B= %s\n",A,B)
/* some processing */
sprint(buf,"C=%d\n",c)
....
...
fprintf(file,buf)

So in the output file we have this kind of o/p:

A= foo B= bar
C= ded
etc...

Edit, to clarify my question:
buf is a big buffer contains all these strings which have formatted using sprintf. Going by your examples, buf will only contain current values, not older ones. e.g first in buf I wrote A= something ,B= something later C= something was appended in the same buf, but in your Python answers buf contains only last value, which is not I want - I want buf to have all the printfs I have done since the beginning, like in C.

Python has a % operator for this.

>>> a = 5
>>> b = "hello"
>>> buf = "A = %d\n , B = %s\n" % (a, b)
>>> print buf
A = 5
 , B = hello

>>> c = 10
>>> buf = "C = %d\n" % c
>>> print buf
C = 10

See this reference for all supported format specifiers.

You could as well use format:

>>> print "This is the {}th tome of {}".format(5, "knowledge")
This is the 5th tome of knowledge

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!