Logging realtime stdout to a file in Python?

Question

I want to log the real time stdout from a different Class to a log file. Here is my demo code. Code works well. But it is waiting for the sample function to finish. I want to run it in parallel. So the log file will written in same time of stdout prints.

import sys
import time

class Logger(object):
    def __init__(self):
        self.terminal = sys.stdout
        self.log = open("log.txt", "a")

    def write(self, message):
        self.terminal.write(message)
        self.log.write(message)  

class sample:
  def __init__(self):
    print "TEST"
    time.sleep(5)

if __name__ == "__main__":
  a = sample()
  sys.stdout = Logger()

Did you try: self.log.flush()? after the write? Note that sys.stdout buffering can change depending on a number of factors/options, so you'd better flush() that as well. — Bakuriu
– Bakuriu, Commented Jun 3, 2014 at 9:15
@Bakuriu ..Thanks its worked. Also I have to call sys.stdout = Logger() before a = sample() — Nitheesh Chandrika
– Nitheesh Chandrika, Commented Jun 3, 2014 at 9:25

Community · Accepted Answer · 2017-05-23 11:57:14Z

5

Calls to file.write do not necessarily write the contents to the HD immediately. It depends on the buffer policy of the file object. If you want to force writing to disk at a certain moment in time, you can use the flush() method (see also this).

Note that sys.stdout flushing policy depends on the configuration of your installation and also on environmental variables, so if you want to guarantee "parallel" writes between standard output and the log file you must flush() both streams:

def write(self, message):
    self.terminal.write(message)
    self.log.write(message)  
    self.terminal.flush()
    self.log.flush()

edited May 23, 2017 at 11:57

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answered Jun 3, 2014 at 9:48

Bakuriu

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1 Comment

Lê Quang Duy Over a year ago

the os.fsync() in your reference link helps to solve my issue!

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