python print function in real time

Question

I recently switched OS and am using a newer Python (2.7). On my old system, I used to be able to print instantaneously. For instance, suppose I had a computationally intense for loop:

for i in range(10):
  huge calculation
  print i

then as the code completed each iteration, it would print i

However, on my current system, python seems to cache the stdout so that the terminal is blank for several minutes, after which it prints:

1
2
3

in short succession. Then, after a few more minutes, it prints:

4
5
6

and so on. How can I make python print as soon as it reaches the print statement?

Answer 1

Try to call flush of stdout after the print

import sys

...
sys.stdout.flush()

Or use a command line option -u which:

Force stdin, stdout and stderr to be totally unbuffered.

Answer 2

Since Python 3.3, you can simply pass flush=True to the print function.

Answer 3

Import the new print-as-function as in Python 3.x:

from __future__ import print_function

(put the statement at the top of your script/module)

This allows you to replace the new print function with your own:

def print(s, end='\n', file=sys.stdout):
    file.write(s + end)
    file.flush()

The advantage is that this way your script will work just the same when you upgrade one day to Python 3.x.

Ps1: I did not try it out, but the print-as-function might just flush by default.

PS2: you might also be interested in my progressbar example.