python - 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

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().