Capturing stderr and stdout of a subprocess as a single stream

Gabriel Genellina gagsl-py at
Sun Jan 7 21:17:19 CET 2007

On 7 ene, 16:33, "Fuzzyman" <fuzzy... at> wrote:
> Hello all,
> Before I ask the question a couple of notes :
> * This question is for implementing a script inside the Wing IDE. For
> some reason using the subprocess module doesn't work so I need a
> solution that doesn't use this module.
> * The platform is Windows and I'm happy with a Windoze only solution.
> :-)
> I would like to execute subprocesses asynchronously and capture stdout
> / stderr as a single stream.
> If I use "os.popen3(executable)" it gives me separate pipes for stdout
> and stderr, but reads from them are blocking.
> The output on stdout and stderr may be interleaved and I would like to
> display them *as* they arrive. That means I can't just read from them
> and output the results.
> The only solution I can think of is to read from both a character at a
> time on two separate threads, putting the data into a queue. A separate
> thread could pull characters off the queue and display them. (I don't
> need to differentiate between stdout and stderr when I display.)
> Can anyone think of a better solution ?
> My current code works, but *doesn't* capture stderr :
> from threading import Thread
> pipe = os.popen(executable)
> def DisplayOutput():
>     while True:
>         output =
>         if not output:
>             break
>         display(output)
> Thread(target=DisplayOutput).start()
> All the best,
> Fuzzyman

Try using popen4 instead. But since you already have a thread, it may
be better to use popen2 and two threads to read from stdout and stderr
to avoid a potential deadlock; they can put read lines into a Queue,
and DisplayOutput just get these lines in order. (See the warnings in
the popen2 module documentation).

Gabriel Genellina

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