I have this assignment where I write certain lines from an input file. These lines I'm looking for in this case is only identified by its following line ("EOL"). It goes like (shortened):

while True:
     line = infile.readlines()
     if not line:
           break

     linesplit = line.split(",")
     line = line[:-1]

     if line startswith("Line"):
           write stuff
     if line startswith("$GPGGA"):
          calculate and write stuff
     if line startswith("EOL")
          outfile.write(i really need the line previous to this one here somehow)
          outfile.write(line + "\n")

the closest i've come to anything is to get out at which byte the "EOL" line is, but i can't from that tell it to print from byte x to byte y. I do know the length of each line.

You're making it too difficult. Store the previous record in memory.

prev_line = ""
for line in infile:
     linesplit = line.split(",")
     line = line[:-1]

     if line startswith("Line"):
           write stuff
     if line startswith("$GPGGA"):
          calculate and write stuff
     if line startswith("EOL")
          outfile.write("%s\n" % (prev_line))   <==========
     ##  copying = last thing in this loop
     prev_line = line                              <==========
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