What is the flush parameter of the print function?

Question

I realized that there is the parameter flush in function print python:

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

I know that:

• The parameter *objects sets the list of values to be displayed;
• The parameter sep sets the character between the values;
• The parameter end the amount that will be printed at the end;
• And the parameter file sets where values will be sent;

But the parameter is flush? What is his function?

Answer 1

The print uses stdout, as C. This is nothing more than the "file" in the operating system to which the text output of a program is sent, and so can be shown to the user.

By default, the stdout is bufferized; that is, it stores the data it receives without showing them until it receives the special code of new line \n.

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The function print Python by default automatically puts a new line character in any string you send to it. But sometimes this behavior is not desired and you want to use two prints to show something on the same line. In this case, you will use the argument end of function print to finish the string with something other than the new line character (or with an empty string, not to print anything other than what you sent).

For example, you can expect the following code to print multiple dots on the same line, one every 0.5s:

import time

for _ in range(5):
    print('.', end='')
    time.sleep(0.5)
print(' Pronto!')

But what really happens in most cases is that nothing happens for 2.5s and then all the dots appear at once.

This happens because the stdout by default shows nothing until it gets a new line, which we are not sending until the end.

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So how do we fix this problem?

Simple, just use the flush to force the result to appear immediately, even without a new line:

import time

for _ in range(5):
    print('.', end='', flush=True)
    time.sleep(0.5)
print(' Pronto!')

It is worth mentioning that the flush as a pro argument print is only available from Python 3.3. Before that, it had to be manually called:

import sys
import time

for _ in range(5):
    print('.', end='')
    sys.stdout.flush()
    time.sleep(0.5)
print(' Pronto!')

Answer 2

Beyond what Peter commented in his reply.

The flush may be useful in some cases such as when the program generates a exception, or where there is a lot of processing abruptly in the program, after the print.

In both cases, it’s as if the program didn’t give stdout time to process by not letting the print, then to ensure that the print takes place at the right time, simply add the flush=True