How can I flush the output of the print function?

Applying unbuffered output to scripts

You can run your script with the -u flag for unbuffered output on-all-the-time:

python -u my_script.py
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To globally unbuffer session, use PYTHONUNBUFFERED environment variable:

export PYTHONUNBUFFERED=x
python my_script.py
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Judicious use of sys.stdout

Understanding sys.stdout is pivotal as it's connected to the standard output. By default, sys.stdout is line-buffered. This means it's flushed when a newline char is printed. Full buffering flushes only when the buffer is full. If you need immediate output, flushing is essential.

import sys
print("Part one. ")  # Part one is in buffer limbo.
sys.stdout.flush()  # Manual call to limbo, Part one is out!
print("Part two, Blazing fast.")  # Immediately after Part one, Part two is out like Flash!
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Nifty tricks to boost print output

Redefining print in Python 2

You can automatically include flushing within the print function:

def print_now(*args):
    print(*args)
    # Flushing off memory cobwebs, ensuring zippy output.
    sys.stdout.flush()

print_now("Hey look, I got flushed!")
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Module-wide auto-flush with functools.partial

Modify print across entire module using functools.partial, can be handy in debugging.

from functools import partial
print = partial(print, flush=True)

print("I get auto-flushed, every single time!")
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Redirect the course of print output

Print output can flow to different streams using the file argument:

with open('log.txt', 'a') as f:
    print("This message is now in a bottle (log file)!", file=f)  
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Customize your print outs

Use sep and end to customize output formatting:

print("One","Two", sep=" then ", end=". Easy as ABC\n")  # Output: One then Two. Easy as ABC
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Caveats to watch out for

• Script exécution speed: Consistent flushing can put brakes on your script due to reduced buffering efficiency.
• I/O blocking: In case the script is writing to an output that can block, over flushing could lead to script hanging until the I/O operation is complete.
• Resource management: Output without buffering translates to more write operations to the file or terminal which is resource-wise less efficient.