How can I flush the output of the print function?

In Python 3, print can take an optional flush argument:

print("Hello, World!", flush=True)

In Python 2, after calling print, do:

import sys
sys.stdout.flush()

By default, print prints to sys.stdout (see the documentation for more about file objects).

Running python -h, I see a command line option:

-u : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x see man page for details on internal buffering relating to '-u'

Here is the relevant documentation.

Using the -u command-line switch works, but it is a little bit clumsy. It would mean that the program would potentially behave incorrectly if the user invoked the script without the -u option. I usually use a custom stdout, like this:

class flushfile:
  def __init__(self, f):
    self.f = f

  def write(self, x):
    self.f.write(x)
    self.f.flush()

import sys
sys.stdout = flushfile(sys.stdout)

... Now all your print calls (which use sys.stdout implicitly), will be automatically flushed.

Use an unbuffered file:

f = open('xyz.log', 'a', 0)

Or

sys.stdout = open('out.log', 'a', 0)

Dan's idea doesn't quite work:

#!/usr/bin/env python
class flushfile(file):
    def __init__(self, f):
        self.f = f
    def write(self, x):
        self.f.write(x)
        self.f.flush()

import sys
sys.stdout = flushfile(sys.stdout)

print "foo"

The result:

Traceback (most recent call last):
  File "./passpersist.py", line 12, in <module>
    print "foo"
ValueError: I/O operation on closed file

I believe the problem is that it inherits from the file class, which actually isn't necessary. According to the documentation for sys.stdout:

stdout and stderr needn’t be built-in file objects: any object is acceptable as long as it has a write() method that takes a string argument.

so changing

class flushfile(file):

to

class flushfile(object):

makes it work just fine.

I first struggled to understand how the flush option was working. I wanted to do a 'loading display' and here is the solution I found:

for i in range(100000):
    print('{:s}\r'.format(''), end='', flush=True)
    print('Loading index: {:d}/100000'.format(i+1), end='')

The first line flushes the previous print and the second line prints a new updated message. I don't know if an one-line syntax exists here.