python: read file continuously, even after it has been logrotated

Question

I have a simple python script, where I read logfile continuosly (same as tail -f)

while True:
    line = f.readline()
    if line:
        print lin         


        

Answer 1

Thanks to @tdelaney and @Dolda2000's answers, I ended up with what follows. It should work on both Linux and Windows, and also handle logrotate's copytruncate or create options (respectively copy then truncate size to 0 and move then recreate file).

file_name = 'my_log_file'
seek_end = True
while True:  # handle moved/truncated files by allowing to reopen
    with open(file_name) as f:
        if seek_end:  # reopened files must not seek end
            f.seek(0, 2)
        while True:  # line reading loop
            line = f.readline()
            if not line:
                try:
                    if f.tell() > os.path.getsize(file_name):
                        # rotation occurred (copytruncate/create)
                        f.close()
                        seek_end = False
                        break
                except FileNotFoundError:
                    # rotation occurred but new file still not created
                    pass  # wait 1 second and retry
                time.sleep(1)
            do_stuff_with(line)

A limitation when using copytruncate option is that if lines are appended to the file while time-sleeping, and rotation occurs before wake-up, the last lines will be "lost" (they will still be in the now "old" log file, but I cannot see a decent way to "follow" that file to finish reading it). This limitation is not relevant with "move and create" create option because f descriptor will still point to the renamed file and therefore last lines will be read before the descriptor is closed and opened again.