tail -f in python with no time.sleep

Question

I need to emulate "tail -f" in python, but I don't want to use time.sleep in the reading loop. I want something more elegant like some kind of blocking read, or select.select with timeout, but python 2.6 "select" documentation specifically says: "it cannot be used on regular files to determine whether a file has grown since it was last read." Any other way? In a few days if no solution is given I will read tail's C source code to try to figure it out. I hope they don't use sleep, hehe Thanks.

MarioR

Answer 1

Why don't you just use subprocess.call on tail itself?

subproces.call(['tail', '-f', filename])

Edit: Fixed to eliminate extra shell process.

Edit2: Fixed to eliminate deprecated os.popen and thus the need to interpolate parameters, escape espaces and other stuff, and then run a shell process.

Answer 2

When reading from a file, your only choice is sleep (see the source code). If you read from a pipe, you can simply read since the read will block until there is data ready.

The reason for this is that the OS doesn't support the notion "wait for someone to write to a file". Only recently, some filesystems added an API where you can listen for changes made to a file but tail is too old to use this API and it's also not available everywhere.

Answer 3

(update) Either use FS monitors tools

• For linux
• For Windows
• For Mac

Or a single sleep usage (which I would you consider as much more elegant).

import time
def follow(thefile):
    thefile.seek(0,2)      # Go to the end of the file
    while True:
         line = thefile.readline()
         if not line:
             time.sleep(0.1)    # Sleep briefly
             continue
         yield line

logfile = open("access-log")
loglines = follow(logfile)
for line in loglines:
    print line

Answer 4

IMO you should use sleep, it works on all platform and code will be simple

Otherwise you can use platform specific APIs which can tell you when file change e.g. on window use FindFirstChangeNotification on folder and watch for FILE_NOTIFY_CHANGE_LAST_WRITE events

On linux i think you can use i-notify

On Mac OSX use FSEvents

Answer 5

If you can use GLib on all platforms, you should use glib.io_add_watch; then you can use a normal GLib mainloop and process events as they happen, without any polling behavior.

http://library.gnome.org/devel/pygobject/stable/glib-functions.html#function-glib--io-add-watch

Answer 6

You can see here how to do a "tail -f" like using inotify.