[Tutor] different behaviour in Idle shell vs Mac terminal

Steven D'Aprano steve at pearwood.info
Mon Jan 9 00:56:29 CET 2012

Adam Gold wrote:
> I have short piece of code I'm using to print a string to the terminal one letter at a time.  It works fine when I invoke the script from within Idle; each letter appears after the preceding one according to the designated time interval.  However if I run it in the Mac terminal ('python3 ./script.py'), there's a pause and then the whole string prints in one go.  Here's the relevant code:
> import sys
> import time
> text = "this text is printing one letter at a time..."
> for char in text:
>     sys.stdout.write(char)
>     time.sleep(0.03)
> I'm thinking this may be a tty issue (is stdout going to the right terminal?)

It's a buffering issue.

> P.S. if it's relevant, this is part of a simple financial maths program and it's used to display the results after certain inputs have been gathered.

To annoy your users? I'm not sure why you think it's a good idea to pretend 
that the computer has to type the letters one at a time. This isn't some 
stupid imaginary program in a Hollywood movie, I assume it is meant to 
actually be useful and usable, and trust me on this, waiting while the program 
pretends to type gets old *really* fast.

(What are you trying to emulate? A stock ticker or something? Do those things 
still even exist? I haven't seen one since the last time I watched the Addams 
Family TV series. The old one, in black & white.)

But if you must, after writing each character, call sys.stdout.flush() to 
flush the buffer.


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