Why is my python output delayed to the end of the program?

Question

I've got an extremely simple application:

import sys
from time import sleep

for i in range(3):
    sys.stdout.write('.')
    sleep(1)

print('Welcome!')

I expect it to print out a dot every second (3 times), after which it should display "Welcome!". Unfortunately, it simply waits three seconds, and then prints out everything at once. I'm on a mac running regular Python 2.7 and I have no clue why this code behaves like this. Any suggestions?

Answer 1

It's because sys.stdout is buffered. Use flush:

import sys
from time import sleep

for i in range(3):
    sys.stdout.write('.')
    sys.stdout.flush()
    sleep(1)

print('Welcome!')

Answer 2

You can call python with -u to make stdin, stdout, and stderr totally unbuffered. This would save you from having to manually flush them.

On Unix, call your script like python -u myscript.py

Or you can put it in the shebang: #!/usr/bin/python -u

Answer 3

stdout is a buffered stream. The buffer is flushed implicitly when it reaches a newline character.

If you want to flush the buffer without writing a newline character, you must do so explicitly by calling sys.stdout.flush()

Another alternative is to write to stderr, which is not buffered.