How to flush output of print function?

On Python 3, print can take an optional flush argument

print("Hello world!", flush=True)

On Python 2 you'll have to do

import sys
sys.stdout.flush()

after calling print. By default, print prints to sys.stdout (see the documentation for more about file objects).

Running python -h, I see a command line option:

-u : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x see man page for details on internal buffering relating to '-u'

Here is the relevant doc.

Since Python 3.3, you can force the normal print() function to flush without the need to use sys.stdout.flush(); just set the "flush" keyword argument to true. From the documentation:

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

Print objects to the stream file, separated by sep and followed by end. sep, end and file, if present, must be given as keyword arguments.

All non-keyword arguments are converted to strings like str() does and written to the stream, separated by sep and followed by end. Both sep and end must be strings; they can also be None, which means to use the default values. If no objects are given, print() will just write end.

The file argument must be an object with a write(string) method; if it is not present or None, sys.stdout will be used. Whether output is buffered is usually determined by file, but if the flush keyword argument is true, the stream is forcibly flushed.