[Python] Python: contextlib.redirect_stdout sometimes doesn't work in real time

I am using Python3.5 on Ubuntu. The following script creates a file out and fills it with more and more lines of "Hello":

import contextlib
with contextlib.redirect_stdout(open('out','w')):
while True:
print('Hello')


However, if I make a tiny modification by adding a call to time.sleep(), the file out stays empty:

import contextlib
import time
with contextlib.redirect_stdout(open('out','w')):
while True:
print('Hello')
time.sleep(1)


If I further change the code by turning the loop into a finite loop, it fills out but only in a lump at the end of the loop.

Can anyone reproduce this and explain it?

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