How to implement a pythonic equivalent of tail -F?

To my knowledge there’s no equivalent to “tail” in the Python function list. Solution would be to use tell() (get file size) and read() to work out the ending lines.

This blog post (not by me) has the function written out, looks appropriate to me!
http://www.manugarg.com/2007/04/real-tailing-in-python.html

If you can’t constraint the problem to work for a line-based read, you need to resort to blocks.

This should work:

import sys

needle = "needle"

blocks = []

inf = sys.stdin

if len(sys.argv) == 2:
    inf = open(sys.argv[1])

while True:
    block = inf.read()
    blocks.append(block)
    if len(blocks) >= 2:
        data = "".join((blocks[-2], blocks[-1]))
    else:
        data = blocks[-1]

    # attention, this needs to be changed if you are interested
    # in *all* matches separately, not if there was any match ata all
    if needle in data:
        print "found"
        blocks = []
    blocks[:-2] = []

    if block == "":
        break

The challenge lies in ensuring that you match needle even if it’s separated by two block-boundaries.

EDIT: as the comment below notes, O_NONBLOCK doesn’t work for files on disk. This will still help if anyone else comes along looking to tail data coming from a socket or named pipe or another process, but it doesn’t answer the actual question that was asked. Original answer remains below for posterity. (Calling out to tail and grep will work, but is a non-answer of sorts anyway.)

Either open the file with O_NONBLOCK and use select to poll for read availability and then read to read the new data and the string methods to filter lines on the end of a file…or just use the subprocess module and let tail and grep do the work for you just as you would in the shell.

You can use collections.deque to implement tail.

From http://docs.python.org/library/collections.html#deque-recipes …

def tail(filename, n=10):
    'Return the last n lines of a file'
    return deque(open(filename), n)

Of course, this reads the entire file contents, but it’s a neat and terse way of implementing tail.

def tail(f):
    f.seek(0, 2)

    while True:
        line = f.readline()

        if not line:
            time.sleep(0.1)
            continue

        yield line

def process_matches(matchtext):
    while True:
        line = (yield)  
        if matchtext in line:
            do_something_useful() # email alert, etc.


list_of_matches = ['ERROR', 'CRITICAL']
matches = [process_matches(string_match) for string_match in list_of_matches]    

for m in matches: # prime matches
    m.next()

while True:
    auditlog = tail( open(log_file_to_monitor) )
    for line in auditlog:
        for m in matches:
            m.send(line)

I use this to monitor log files. In the full implementation, I keep list_of_matches in a configuration file so it can be used for multiple purposes. On my list of enhancements is support for regex instead of a simple ‘in’ match.

You can use select to poll for new contents in a file.

def tail(filename, bufsize = 1024):
    fds = [ os.open(filename, os.O_RDONLY) ]
    while True:
        reads, _, _ = select.select(fds, [], [])
        if 0 < len(reads):
            yield os.read(reads[0], bufsize)

Looks like there’s a package for that: https://github.com/kasun/python-tail

you can use pytailf : Simple python tail -f wrapper

from tailf import tailf    

for line in tailf("myfile.log"):
    print line

If you just need a dead simple Python 3 solution for processing the lines of a text file as they’re written, and you don’t need Windows support, this worked well for me:

import subprocess
def tailf(filename):
    #returns lines from a file, starting from the beginning
    command = "tail -n +1 -F " + filename
    p = subprocess.Popen(command.split(), stdout=subprocess.PIPE, universal_newlines=True)
    for line in p.stdout:
        yield line
for line in tailf("logfile"):
    #do stuff

It blocks waiting for new lines to be written, so this isn’t suitable for asynchronous use without some modifications.