How to flush output of Python print?

Question

I would like to force Python's print function to output to the screen.

Answer 1

import sys
sys.stdout.flush()

print by default prints to sys.stdout

References:

• http://docs.python.org/reference/simple_stmts.html#the-print-statement
• http://docs.python.org/library/sys.html
• http://docs.python.org/library/stdtypes.html#file-objects

Answer 2

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 doc.

Answer 3

Using the -u command-line switch works, but it is a little bit clumsy in my opinion. I usually use a custom stdout, like this:

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)

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

Answer 4

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 docs 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.

Answer 5

Why not try using an unbuffered file?

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

OR

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