Dialog with a process via subprocess.Popen blocks forever

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Thu Mar 1 13:17:25 CET 2007

En Wed, 28 Feb 2007 18:27:43 -0300, <bayer.justin at googlemail.com> escribió:

> Hi,
> I am trying to communicate with a subprocess via the subprocess
> module. Consider the following example:
>>>> from subprocess import Popen, PIPE
>>>> Popen("""python -c 'input("hey")'""", shell=True)
> <subprocess.Popen object at 0x729f0>
>>>> hey
> Here hey is immediately print to stdout of my interpreter, I did not
> type in the "hey". But I want to read from the output into a string,
> so I do
>>>> x = Popen("""python -c 'input("hey\n")'""", shell=True, stdout=PIPE,  
>>>> bufsize=2**10)
>>>> x.stdout.read(1)
> # blocks forever
Blocks, or is the child process waiting for you to input something in  

> Is it possible to read to and write to the std streams of a
> subprocess? What am I doing wrong?

This works for me on Windows XP. Note that I'm using a tuple with  
arguments, and raw_input instead of input (just to avoid a traceback on  

py> x=Popen(("python", "-c", "raw_input('hey')"), shell=True, stdout=PIPE)
py> x.stdout.read(1)
py> x.stdout.read()

I typed that 1234 (response to raw_input).

You may need to use python -u, or redirect stderr too, but what your real  
problem is?

Gabriel Genellina

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