CODEWITHSUNDEEP
pythonstdoutflushlogfile
Marmstrong
Marmstrong

Reputation: 1786

Python flush a print statement

I recenlty desired a bit of python code that would allow me to output to the console and a logfile with the same print statement. After googleing I found this website which offered an excellent solution. However, I would like to be able to flush the output buffer after each write in order to see it in the log file. How would I go about adding it into this class?

I have tried to following setups...

class output_file(object):
  def __init__(self, stdout, filename):
    silentremove(filename)
    self.stdout = stdout
    self.logfile = open(filename, "a")

def write(self, text):
    self.stdout.write(text)
    self.logfile.write(text)
    sys.stdout.flush()

  def flush(self):
    sys.stdout.flush()

  def close(self):
    self.stdout.close()
    self.logfile.close()

This was had a cyclic error which resulted in the flush function calling itself.

class output_file(object):
  def __init__(self, stdout, filename):
    silentremove(filename)
    self.stdout = stdout
    self.logfile = open(filename, "a")

  def write(self, text):
    self.stdout.write(text)
    self.logfile.write(text)
    self.stdout.flush()

  def flush(self):
    sys.stdout.flush()

  def close(self):
    self.stdout.close()
    self.logfile.close()

This didn't flush it at all.

Upvotes: 0

Views: 5083

Answers (1)

Gio
Gio

Reputation: 3340

The following reopens sys.stdout in unbuffered mode. Each stdout.write and print will be automatically flushed (i.e. printed to stdout) afterwards.

import sys
import os

# Flush STDOUT continuously
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

The third argument of os.fdopen is the buffering argument, 0 is unbuffered, 1 is line buffered and >1 will result in a buffer of (approximately) that size (in bytes), <0 will use the systems default.

UPDATE

With Python 3 unbuffered text is not allowed, i.e. the following 

sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

Will result in the error

ValueError: Can't have unbuffered text I/O

To use unbuffered I/O in python3, bytes can be used, i.e.

sys.stdout = os.fdopen(sys.stdout.fileno(), 'wb', 0)

Will work fine in python3.

Upvotes: 4

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