Python - Not reading from file?

Question

I'm playing with the generator for processing infinite data, located here , which is a pretty simple thing for returning the tail from a file:

(Source code from here )

# 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

# Example use
# Note : This example requires the use of an apache log simulator.
# 
# Go to the directory run/foo and run the program 'logsim.py' from
# that directory.   Run this program as a background process and
# leave it running in a separate window.  We'll write program
# that read the output file being generated
# 

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

My issue is that using a test file, '/var/log/test.log' when I write to it, it doesn't print anything to the console I have my generator working on.

I can print the file from other terminals and see that the new lines are added, but the python code doesn't seem to read the new data?

I thought that maybe it was an issue reading from /var/log/ so I created another file in /Users/Me/Python/ and it still won't read the data.

I feel like I'm missing something really simple :(

Edit:

(I'm on OSX, Python 2.7.5 btw) I tried it out line by line, like so:

FID = open(fileName, 'r')
FID.seek(0,2)
FID.tell() # For example says 125
FID.readlines() # returns []
# Now I edit the log file, add in a couple of new lines (in TextMate if that helps)
FID.readlines() # returns []
FID.seek(0,2)
FID.tell() # returns 125
os.stat(fileName) # says size is 142

Is my file descriptor not reading the updated file or something?

Answer 1

Got the same problem on OSX Yosemite with Brew's Python 2.7.8

Fixed it by adding the following just before trying to read freshly added data: FID.seek(0, os.SEEK_CUR)

This does not really change the filepointer, but somehow invalidates some cached state and sees new data.

Let me know if this works for you as well.