subprocess.Popen not creating a pipe

Gabriel Genellina gagsl-py2 at
Thu Feb 5 18:16:21 CET 2009

En Sun, 01 Feb 2009 18:00:36 -0200, Andrew Parker <gbofspam at>

> On Sun, Feb 1, 2009 at 1:46 PM, Andrew Parker <gbofspam at> wrote:
>> I'm having some fun with Popen.  I have the following line:
>>    process = subprocess.Popen(command, stdout=subprocess.PIPE,
>> stderr=subprocess.STDOUT)
>>    print process.stdout
>> Under normal circumstances, this displays:
>>    <open file '<stdout>', mode 'w' at 0xb7f8e068>
>> However, I have a binary that I use to kick off this script, and when
>> that runs, it displays:
>>    None
>> So, two questions:
>>    1.  What the heck is this binary doing that upsets Popen so much?
>>    2.  What can *my script* do to get around this problem.
>> Unfortunately I'm stuck using this binary, so its the python where I
>> have to solve this.
> so, tracing through subprocess.Popen, I see that os.pipe() is being
> invoked for stdout.  This is returning (0, 3), and I assume the 0 is
> conflicting with what python is assuming is stdin.  Calling pipe()
> before Popen gets around my problem, as the pipe the Popen gets then
> returns (4,5) which Popen seems happy with.

Seems that "your binary" is closing stdin before running your script, so
the first available file descriptor is 0.

> Sounds like a bug.  Should I report this, or is it expected/known  
> behaviour?

It's hardly a Python error, and I don't think the subprocess module should
consider this very special case, but you may report it at anyway.

Gabriel Genellina

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