Test buffering behaviour of print(), sys.stdout.write() and logging without TTY (PIPE mode)

test-stdout-buffering.py

'''Test behaviour various printing methonds without TTY.

If you run it with a TTY, all is as expected: print() and sys.stdout flush
thir buffers after each NewLine '\n':
    
    python test-stdout-buffering.py


However, if you take the TTY away and run it in PIPE mode, then Python's
buffers much more agressively across '\n'!

    true  |  python    test-stdout-buffering.py 2>&1  |  cat


Of course you can tell Python to never buffer:

    true  |  python -u test-stdout-buffering.py 2>&1  |  cat


See also:
  - https://stackoverflow.com/questions/107705
'''

import logging, sys
from time import sleep
# Interestingly, gevent doesn't help! Looks like print() is not gevent'ed
#from gevent import sleep

logging.basicConfig(level=logging.DEBUG)
log = logging.getLogger(__name__)

print('unbuffered print() before sleep', flush=True)
print('buffered print() before sleep')
sys.stdout.write('sys.stdout.write() before sleep\n')
log.info('log(): before sleep')

sleep(2)

print('unbuffered print() after sleep', flush=True)
print('buffered print() after sleep')
sys.stdout.write('sys.stdout.write() after sleep\n')
log.info('log(): after sleep')

This is what happens in normal TTY-mode (buffered, but buffer flashed after each NewLine \n):

$ python3 test-stdout-buffering.py

unbuffered print() before sleep
buffered print() before sleep
sys.stdout.write() before sleep
INFO:__main__:log(): before sleep

# SLEEP HERE

unbuffered print() after sleep
buffered print() after sleep
sys.stdout.write() after sleep
INFO:__main__:log(): after sleep

And this without TTY:

$ true  |  python3 test-stdout-buffering.py 2>&1  |  cat

unbuffered print() before sleep
INFO:__main__:log(): before sleep

# SLEEP HERE

buffered print() before sleep
sys.stdout.write() before sleep
unbuffered print() after sleep
INFO:__main__:log(): after sleep
buffered print() after sleep
sys.stdout.write() after sleep

If only I had read Disable output buffering before, in particular tzp's comment:

output buffering works differently depending on if the output goes to a tty or another process/pipe. If it goes to a tty, then it is flushed after each \n, but in a pipe it is buffered. In the latter case you can make use of these flushing solutions.