How can I tail a log file in Python

Question

I d like to make the output of tail -F or something similar available to me in Python without blocking or locking  I ve found some really old code to do that here  but I m thinking there must be a better way or a library to do the same thing by now  Anyone know of one   Ideally  I d have something like tail getNewData   that I could call every time I wanted more data

User · Answer

You could use the  tailer  library  https   pypi python org pypi tailer   It has an option to get the last few lines     Get the last 3 lines of the file tailer tail open  test txt    3      Line 9    Line 10    Line 11     And it can also follow a file     Follow the file as it grows for line in tailer follow open  test txt         print line   If one wants tail-like behaviour  that one seems to be a good option

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So  this is coming quite late  but I ran into the same problem again  and there s a much better solution now  Just use pygtail      Pygtail reads log file lines that have not been read  It will even   handle log files that have been rotated  Based on logcheck s logtail2    http   logcheck org

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The only portable way to tail -f a file appears to be  in fact  to read from it and retry  after a sleep  if the read returns 0  The tail utilities on various platforms use platform-specific tricks  e g  kqueue on BSD  to efficiently tail a file forever without needing sleep   Therefore  implementing a good tail -f purely in Python is probably not a good idea  since you would have to use the least-common-denominator implementation  without resorting to platform-specific hacks   Using a simple subprocess to open tail -f and iterating through the lines in a separate thread  you can easily implement a non-blocking tail operation in Python   Example implementation   import threading  Queue  subprocess tailq   Queue Queue maxsize 10    buffer at most 100 lines  def tail forever fn       p   subprocess Popen   tail    -f   fn   stdout subprocess PIPE      while 1          line   p stdout readline           tailq put line          if not line              break  threading Thread target tail forever  args  fn    start    print tailq get     blocks print tailq get nowait     throws Queue Empty if there are no lines to read

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Ideally  I d have something like tail getNewData   that I could call every time I wanted more data      We ve already got one and itsa very nice   Just call f read   whenever you want more data   It will start reading where the previous read left off and it will read through the end of the data stream   f   open  somefile log   p   0 while True      f seek p      latest data   f read       p   f tell       if latest data          print latest data         print str p  center 10  center 80         For reading line-by-line  use f readline     Sometimes  the file being read will end with a partially read line   Handle that case with f tell   finding the current file position and using f seek   for moving the file pointer back to the beginning of the incomplete line   See this ActiveState recipe for working code

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If you are on linux you implement a non-blocking implementation in python in the following way   import subprocess subprocess call  xterm -title log -hold -e   tail -f filename   amp    shell True  executable   bin csh   print  Done

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You can also use  AWK  command  See more at  http   www unix com shell-programming-scripting 41734-how-print-specific-lines-awk html awk can be used to tail last line  last few lines or any line in a file  This can be called from python

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Python is  batteries included  - it has a nice solution for it  https   pypi python org pypi pygtail  Reads log file lines that have not been read  Remembers where it finished last time  and continues from there   import sys from pygtail import Pygtail  for line in Pygtail  some log        sys stdout write line

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Using the sh module  pip install sh    from sh import tail   runs forever for line in tail  -f     var log some log file log    iter True       print line     update   Since sh tail with  iter True is a generator  you can   import sh tail   sh tail  -f     var log some log file log    iter True    Then you can  getNewData  with   new data   tail next     Note that if the tail buffer is empty  it will block until there is more data  from your question it is not clear what you want to do in this case     update      This works if you replace -f with -F  but in Python it would be locking  I d be more interested in having a function I could call to get new data when I want it  if that s possible      Eli   A container generator placing the tail call inside a while True loop and catching eventual I O exceptions will have almost the same effect of -F   def tail F some file       while True          try              for line in sh tail  -f   some file   iter True                   yield line         except sh ErrorReturnCode 1              yield None   If the file becomes inaccessible  the generator will return None  However it still blocks until there is new data if the file is accessible  It remains unclear for me what you want to do in this case   Raymond Hettinger approach seems pretty good   def tail F some file       first call   True     while True          try              with open some file  as input                  if first call                      input seek 0  2                      first call   False                 latest data   input read                   while True                      if   n  not in latest data                          latest data    input read                           if   n  not in latest data                              yield                                if not os path isfile some file                                   break                             continue                     latest lines   latest data split   n                       if latest data -1       n                           latest data   latest lines -1                      else                          latest data   input read                       for line in latest lines  -1                           yield line     n          except IOError              yield      This generator will return    if the file becomes inaccessible or if there is no new data    update      The second to last answer circles around to the top of the file it seems whenever it runs out of data      Eli    I think the second will output the last ten lines whenever the tail process ends  which with -f is whenever there is an I O error  The tail --follow --retry behavior is not far from this for most cases I can think of in unix-like environments   Perhaps if you update your question to explain what is your real goal  the reason why you want to mimic tail --retry   you will get a better answer      The last answer does not actually follow the tail and merely reads what s available at run time      Eli   Of course  tail will display the last 10 lines by default    You can position the file pointer at the end of the file using file seek  I will left a proper implementation as an exercise to the reader   IMHO the file read   approach is far more elegant than a subprocess based solution

User · Answer

Another option is the tailhead library that provides both Python versions of of tail and head utilities and API that can be used in your own module   Originally based on the tailer module  its main advantage is the ability to follow files by path i e  it can handle situation when file is recreated  Besides  it has some bug fixes for various edge cases

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Adapting Ijaz Ahmad Khan s answer to only yield lines when they are completely written  lines end with a newline char  gives a pythonic solution with no external dependencies   def follow file  - gt  Iterator str           Yield each line from a file as they are written          line          while True          tmp   file readline           if tmp is not None              line    tmp             if line endswith   n                    yield line                 line              else              time sleep 0 1    if   name         main         for line in follow open  test txt    r             print line  end

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All the answers that use tail -f are not pythonic   Here is the pythonic way    using no external tool or library   def follow thefile        while True          line   thefile readline           if not line or not line endswith   n                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  end

User · Answer

Non Blocking  If you are on linux  as windows does not support calling select on files  you can use the subprocess module along with the select module   import time import subprocess import select  f   subprocess Popen   tail   -F  filename            stdout subprocess PIPE stderr subprocess PIPE  p   select poll   p register f stdout   while True      if p poll 1           print f stdout readline       time sleep 1    This polls the output pipe for new data and prints it when it is available  Normally the time sleep 1  and print f stdout readline   would be replaced with useful code   Blocking  You can use the subprocess module without the extra select module calls   import subprocess f   subprocess Popen   tail   -F  filename            stdout subprocess PIPE stderr subprocess PIPE  while True      line   f stdout readline       print line   This will also print new lines as they are added  but it will block until the tail program is closed  probably with f kill

[python] How can I tail a log file in Python?

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