iterating over a file with two pointers

[nikhil Pandey]

hi,
I want to iterate over the lines of a file and when i find certain lines, i need another loop starting from the next of that "CERTAIN" line till a few (say 20) lines later.
so, basically i need two pointers to lines (one for outer loop(for each line in file)) and one for inner loop. How can i do that in python?
please help. I am stuck up on this.