Flushing stdout

[Steve Holden]

Graham Ashton wrote:
> Hi. I'm having trouble flushing sys.stdout. I've written a small example
> to illustrate my problem (see below). In short, I expect it to ping
> "received hello", sleep for 5 seconds and then print "received world".
> Instead I get nothing for 5 seconds and then both statements pop out at
> once.
> 
You aren't actually having trouble flushing sys.stdout at all ...

> As you'll no doubt gather from the example, the problem I'm really trying
> to solve is sending message back from one process to another via a pipe.
> Changing the buffering option in the os.pipe() call doesn't have any
> effect either.
> 
Nope. The holdup is elsewhere.

> Any thoughts? Thanks.
> 
> P.S. I've not found -u and PYTHONUNBUFFERED to help me.
> 
Nope, they won't ...

> P.P.S. I'm running 2.3.3 on Linux, libc 2.2.5-11.5.
> 
> ---cut---
> #!/usr/bin/env python
> 
> import os, sys, time
> 
> if len(sys.argv) > 1:
>     sys.stdout.write("hello\n")
>     sys.stdout.flush()
>     time.sleep(5)
>     sys.stdout.write("world\n")
> else:
>     command = "python %s foo" % os.path.basename(sys.argv[0])
>     for line in os.popen(command, "r", 1):
>         print "received", line,
> 
The iteration over the pipe's contents is waiting until it sees the end 
of the output before it yields anything. Try this:

import os, sys, time

if len(sys.argv) > 1:
     sys.stdout.write("hello\n")
     sys.stdout.flush()
     time.sleep(5)
     sys.stdout.write("world\n")
else:
     command = "python %s foo" % os.path.basename(sys.argv[0])
     f = os.popen(command, "r", 1)
     while 1:
         line = f.readline()
         if not line:
             break
         print "received", line,

You can probably find a tidier way to bundle this now you know what's up.

regards
  Steve
-- 
http://www.holdenweb.com
http://pydish.holdenweb.com
Holden Web LLC +1 800 494 3119