Reading from a frequently updated file

"An interactive session is worth 1000 words"

>>> f1 = open("bla.txt", "wt")
>>> f2 = open("bla.txt", "rt")
>>> f1.write("bleh")
>>> f2.read()
''
>>> f1.flush()
>>> f2.read()
'bleh'
>>> f1.write("blargh")
>>> f1.flush()
>>> f2.read()
'blargh'

In other words - yes, a single "open" will do.

Here is a slightly modified version of Jeff Bauer answer which is resistant to file truncation. Very useful if your file is being processed by logrotate.

import os
import time

def follow(name):
    current = open(name, "r")
    curino = os.fstat(current.fileno()).st_ino
    while True:
        while True:
            line = current.readline()
            if not line:
                break
            yield line

        try:
            if os.stat(name).st_ino != curino:
                new = open(name, "r")
                current.close()
                current = new
                curino = os.fstat(current.fileno()).st_ino
                continue
        except IOError:
            pass
        time.sleep(1)


if __name__ == '__main__':
    fname = "test.log"
    for l in follow(fname):
        print "LINE: {}".format(l)

I would recommend looking at David Beazley's Generator Tricks for Python, especially Part 5: Processing Infinite Data. It will handle the Python equivalent of a tail -f logfile command in real-time.

# follow.py
#
# Follow a file like tail -f.

import time
def follow(thefile):
    thefile.seek(0,2)
    while True:
        line = thefile.readline()
        if not line:
            time.sleep(0.1)
            continue
        yield line

if __name__ == '__main__':
    logfile = open("run/foo/access-log","r")
    loglines = follow(logfile)
    for line in loglines:
        print line,