Affects Win32 only (tested under Ubuntu 9.10-64 and XP-32 with v2.6.4). If script output is piped, child processes created via multiprocessing.Process cannot write to stdout. Also, trying to call stdout.flush() in child processes causes IOError.

Normal behaviour
----------------
C:\>stdout.py
[Parent] stdout: <open file '<stdout>', mode 'w' at 0x00A54070>
[Parent] message via stdout

[Child] stdout: <open file '<stdout>', mode 'w' at 0x00A54070>
[Child] message via stdout

Piped behaviour (same result if redirecting to file)
---------------
C:\>stdout.py | cat
[Parent] stdout: <open file '<stdout>', mode 'w' at 0x00A54070>
[Parent] message via stdout

[Child] stdout: <open file '<stdout>', mode 'w' at 0x00A54070>
Process Process-1:
Traceback (most recent call last):
  File "C:\Python26\lib\multiprocessing\process.py", line 232, in _bootstrap
    self.run()
  File "C:\Python26\lib\multiprocessing\process.py", line 88, in run
    self._target(*self._args, **self._kwargs)
  File "C:\stdout.py", line 18, in child
    stdout.flush()
IOError: [Errno 9] Bad file descriptor

I tried to reproduce / narrow-down the cause of this with a debug build in a VM but couldn't reproduce the behaviour (neither with debug nor with standard 2.6.4 binary). I have to conclude that there is something perculiar with my native Windows installation (since in both VM and natively Python is using exactly the same DLLs and ).

Apologies for wasting your time.

I have the same problem. Not only for piping but also when using redirection, e.g. 'stdout.py > log.txt'.
Interesting thing is that when I run it like 'python.exe stdout.py > log.txt' it works. I don't see why this should matter but apparently it does. ".py" files are registered to be open with '"C:\Python26\python.exe" "%1" %*'. 
My system: WinXp SP3, Python 2.6.5

That's (still...) a known issue with Windows file associations and 
redirects:

   http://support.microsoft.com/kb/321788

In theory it was fixed way back when. In practise...

On 30/06/2010 15:04, Vojtech Fried wrote:
>
> Vojtech Fried<vojtech.fried@gmail.com>  added the comment:
>
> I have the same problem. Not only for piping but also when using redirection, e.g. 'stdout.py>  log.txt'.
> Interesting thing is that when I run it like 'python.exe stdout.py>  log.txt' it works. I don't see why this should matter but apparently it does. ".py" files are registered to be open with '"C:\Python26\python.exe" "%1" %*'.
> My system: WinXp SP3, Python 2.6.5
>
> ----------
> nosy: +Vojtech.Fried
>
> _______________________________________
> Python tracker<report@bugs.python.org>
> <http://bugs.python.org/issue8056>
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Thanks, the registry change proposed in the article fixed it.