What does print()'s `flush` do?

Question

There is a boolean optional argument to the print() function flush which defaults to False.

The documentation says it is to forcibly flush the stream.

I don't understand the concept of flushing. What is flushing here? What is flushing of stream?

Answer 1

Normally output to a file or the console is buffered, with text output at least until you print a newline. The flush makes sure that any output that is buffered goes to the destination.

I do use it e.g. when I make a user prompt like Do you want to continue (Y/n):, before getting the input.

This can be simulated (on Ubuntu 12.4 using Python 2.7):

from __future__ import print_function

import sys
from time import sleep

fp = sys.stdout
print('Do you want to continue (Y/n): ', end='')
# fp.flush()
sleep(5)

If you run this, you will see that the prompt string does not show up until the sleep ends and the program exits. If you uncomment the line with flush, you will see the prompt and then have to wait 5 seconds for the program to finish

Answer 2

There are a couple of things to understand here. One is the difference between buffered I/O and unbuffered I/O. The concept is fairly simple - for buffered I/O, there is an internal buffer which is kept. Only when that buffer is full (or some other event happens, such as it reaches a newline) is the output "flushed". With unbuffered I/O, whenever a call is made to output something, it will do this, 1 character at a time.

Most I/O functions fall into the buffered category, mainly for performance reasons: it's a lot faster to write chunks at a time (all I/O functions eventually get down to syscalls of some description, which are expensive.)

flush lets you manually choose when you want this internal buffer to be written - a call to flush will write any characters in the buffer. Generally, this isn't needed, because the stream will handle this itself. However, there may be situations when you want to make sure something is output before you continue - this is where you'd use a call to flush().

Answer 3

Two perfect answers we have here,

Anthon made it very clear to understand, Basically, the print line technically does not run (print) until the next line has finished.

Technically the line does run it just stays unbuffered until the next line has finished running.

This might cause a bug for some people who uses the sleep function after running a print function expecting to see it prints before the sleep function started.

So why am I adding another answer?

The Future Has Arrived And I Would Like To Take The Time And Update You With It:

from __future__ import print_function

First of all, I believe this was an inside joke meant to show an error: Future is not defined ^_^

---

I'm looking at PyCharm's documentation right now and it looks like they added a flush method built inside the print function itself, Take a look at this:

def print(self, *args, sep=' ', end='\n', file=None): # known special case of print
"""
print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

Prints the values to a stream, or to sys.stdout by default.
Optional keyword arguments:
file:  a file-like object (stream); defaults to the current sys.stdout.
sep:   string inserted between values, default a space.
end:   string appended after the last value, default a newline.
flush: whether to forcibly flush the stream.
"""
pass

---

So we might be able to use: (Not sure if the usage order of the parameters should be the same)

from __present__ import print_function

from time import sleep

print('Hello World', flush=True)

sleep(5)

Or This:

print('Hello World', file=sys.stdout , flush=True)

As Anthon said:

If you run this, you will see that the prompt string does not show up until the sleep ends and the program exits. If you uncomment the line with flush, you will see the prompt and then have to wait 5 seconds for the program to finish

So let's just convert that to our current situation:

If you run this, you will see the prompt and then have to wait 5 seconds for the program to finish, If you change the line with flush to flush=False, you will see that the prompt string does not show up until the sleep ends and the program exits.

Answer 4

A practical example to understand print() with and without the flush parameter:

import time

for i in range(5):
    print(i, end=" ", flush=True)  # Print numbers as soon as they are generated
    # print(i, end=" ", flush=False)  # Print everything together at the end
    time.sleep(0.5)

print("end")

You can comment/uncomment the print lines to check how this affects the way the output is produced.

This is similar to the accepted answer, just a bit simpler and for Python 3.