Usage of sys.stdout.flush() method
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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.
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
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.
[email protected]:~$ 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()
[email protected]:~$ 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
import sys for x in range(10000): print "HAPPY >> %s <<\r" % str(x), sys.stdout.flush()
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.
You can see the differences b/w these two
import sys for i in range(1,10 ): sys.stdout.write(str(i)) sys.stdout.flush() for i in range(1,10 ): print i
In the first case, the characters are output one by one after each is written, because of the flush. In the second case, the characters are buffered by Python until it thinks it’s got something worth the effort to write, and then written all in a batch.
If you add, say, a
time.sleep(0.2) in the loops, this becomes more obvious.