Python: why is this print automatically flushing to screen without flush()?

Question

I was reading about std.flush() in python. And I found this example a lot.

import sys,time
for i in range(10):
    print i,
    #sys.stdout.flush() 
    time.sleep(1)

It is often said that it makes a difference with/without the "sys.stdout.flush()". However, when I called this script from command prompt, it didn't make a difference in my case. Both printed numbers to the screen in real time. I used python 2.7.5 in windows.

Why is that happening?

p.s. In another example which printed the output through subprocess.PIPE instead of to the screen directly, I did observe a difference of the buffering.

What am I missing?

Answer 1

Using flush will generally guarantee that flushing is done but assuming the reverse relationship is a logical fallacy, akin to:

1. Dogs are animals.
2. This is an animal.
3. Therefore this is a dog.

In other words, not using flush does not guarantee flushing will not happen.

Interestingly enough, using Python 2.7.8 under Cygwin in Win81, I see the opposite behaviour - everything is batched up until the end. It may be different with Windows-native Python, it may also be different from within IDLE.

Answer 2

See stdio buffering. In brief:

Default Buffering modes:

• stdin is always buffered
• stderr is always unbuffered
• if stdout is a terminal then buffering is automatically set to line buffered, else it is set to buffered

For me, the example you gave prints:

In cmd:

• all the numbers upon exit in Cygwin's python
• one by one in Win32 python

In mintty:

• both all upon exit
• both one by one with -u option
• sys.stdout.isatty() returns False!

So, it looks like msvcrt's stdout is unbuffered when it points to a terminal. A test with a simple C program shows the same behaviour.