building an index for large text files for fast access

[Yi Xing]

Hi,

I need to read specific lines of huge text files. Each time, I know 
exactly which line(s) I want to read. readlines() or readline() in a 
loop is just too slow. Since different lines have different size, I 
cannot use seek(). So I am thinking of building an index for the file 
for fast access. Can anybody give me some tips on how to do this in 
Python? Thanks.

Yi