What does sys.stdout.flush() do?

• python
• sys

Python's standard out is buffered (meaning that it collects some of the data "written" to standard out before it writes it to the terminal). Calling sys.stdout.flush() forces it to "flush" the buffer, meaning that it will write everything in the buffer to the terminal, even if normally it would wait before doing so.

Here's some good information about (un)buffered I/O and why it's useful:
http://en.wikipedia.org/wiki/Data_buffer
Buffered vs unbuffered IO

Consider the following simple Python script:

import time
import sys

for i in range(5):
    print i,
    #sys.stdout.flush()
    time.sleep(1)

This is designed to print one number every second for five seconds, but if you run it as it is now (depending on your default system buffering) you may not see any output until the script completes, and then all at once you will see 0 1 2 3 4 printed to the screen.

This is because the output is being buffered, and unless you flush sys.stdout after each print you won't see the output immediately. Remove the comment from the sys.stdout.flush() line to see the difference.

As per my understanding, When ever we execute print statements output will be written to buffer. And we will see the output on screen when buffer get flushed(cleared). By default buffer will be flushed when program exits. BUT WE CAN ALSO FLUSH THE BUFFER MANUALLY by using "sys.stdout.flush()" statement in the program. In the below code buffer will be flushed when value of i reaches 5.

You can understand by executing the below code.

chiru@online:~$ cat flush.py

import time
import sys

for i in range(10):
    print i
    if i == 5:
        print "Flushing buffer"
        sys.stdout.flush()
    time.sleep(1)

for i in range(10):
    print i,
    if i == 5:
        print "Flushing buffer"
        sys.stdout.flush()

chiru@online:~$

*****OUTPUT*****

chiru@online:~$ python flush.py

0 1 2 3 4 5 Flushing buffer
6 7 8 9 0 1 2 3 4 5 Flushing buffer
6 7 8 9

chiru@online:~$

As per my understanding sys.stdout.flush() pushes out all the data that has been buffered to that point to a file object. While using stdout, data is stored in buffer memory (for some time or until the memory gets filled) before it gets written to terminal. Using flush() forces to empty the buffer and write to terminal even before buffer has empty space.

import sys
for x in range(10000):
    print "HAPPY >> %s <<\r" % str(x),
    sys.stdout.flush()