Why do python print statements that contain 'end=' arguments behave differently in while-loops?

Question

I\'m running python version 2.7.3 on MacOSX.

Consider this block of code:

from __future__ import print_function
import time
x = 0
while x < 5:
            


        

Answer 1

This is just python buffering stdout. This answer has some more info.

You can flush it like this:

import sys
from __future__ import print_function
import time
x = 0
while x < 5:
    print(x, end='')
    x += 1
    sys.stdout.flush()
    time.sleep(1)

Alternatively start python python -u and it won't be buffered.

Answer 2

Because the output stream is line-buffered - since it's not explicitly being flushed in between print statements, it waits until it sees a newline to flush the output.

You can force flushing via sys.stdout.flush().

Alternatively if you run Python with the -u flag, it will disable line buffering.