gistfile1.py

$ cat n_line.py 
# -*- coding: utf-8 -*-
from itertools import islice
from cStringIO import StringIO

_content = """1st line
2nd line
3rd line
4th line
5th line"""

def nth_line(f, n):
    x = list(islice(f, n, n+1))
    return x[0] if x else None

f = StringIO(_content)
fourth_line = nth_line(f, 3)
print fourth_line