Python - How can I open a file and specify the offset in bytes?

Question

I'm writing a program that will parse an Apache log file periodically to log it's visitors, bandwidth usage, etc..

The problem is, I don't want to open the log and parse data I've already parsed. For example:

line1
line2
line3

If I parse that file, I'll save all the lines then save that offset. That way, when I parse it again, I get:

line1
line2
line3 - The log will open from this point
line4
line5

Second time round, I'll get line4 and line5. Hopefully this makes sense...

What I need to know is, how do I accomplish this? Python has the seek() function to specify the offset... So do I just get the filesize of the log (in bytes) after parsing it then use that as the offset (in seek()) the second time I log it?

I can't seem to think of a way to code this >.<

Answer 1

You can manage the position in the file thanks to the seek and tell methods of the file class see https://docs.python.org/2/tutorial/inputoutput.html

The tell method will tell you where to seek next time you open

Answer 2

log = open('myfile.log')
pos = open('pos.dat','w')
print log.readline()
pos.write(str(f.tell())
log.close()
pos.close()

log = open('myfile.log')
pos = open('pos.dat')
log.seek(int(pos.readline()))
print log.readline()

Of course you shouldn't use it like that - you should wrap the operations up in functions like save_position(myfile) and load_position(myfile), but the functionality is all there.