python - Finding Multiple Sequences in a File by Position after SpecifIcally Repeating Characters -

i trying write code, can sequences of different samples in file after line breaks position, output blank reason, can me?

import readline count = 0 brk = 0 open("file.txt") f:     while (count < 35):         l = f.readline()[brk + 2]         sp = raw_input ("starting position:")         sp = int(sp)         rl = sp + 6         print(l[sp:rl])         print(l[-30:0])         count = count + 1         brk = brk + 2      print ("done") 

in line l = f.readline()[brk + 2] program puts 1 character variable l. so, when trying print substring of l (in lines print(l[sp:rl]) , print(l[-30:0])), program prints empty lines. expected result.

to find add print l right after assigning of l.

it seems trying read 2-nd, 4-th, 6-th, etc lines of file. can this:

brk = 0 open("file.txt") f:     f.readline()     f.readline() #skip both first lines     while (count < 35):         l = f.readline()         f.readline() #skip next line         sp = raw_input ("starting position:")         sp = int(sp)         rl = sp + 6         print(l[sp:rl])         print(l[-30:0])         count = count + 1         brk = brk + 2 

also print(l[-30:0]) must print empty line. seems need print(l[-30:]) (last 30 characters of string l).