[New-bugs-announce] [issue11459] Python select.select does not correctly report read readyness

Novimir Pablant report at bugs.python.org
Thu Mar 10 06:04:28 CET 2011

New submission from Novimir Pablant <amicitas at gmail.com>:

I am trying to get the output from an external program into python using `subprocess.Popen` and `select.select`.   For some reason though select.select is at times telling me that stdout is not ready to read, even when it is (reading from it works).   

This problem exists in python 3.1 & 3.2 but not in python 2.7.

For my particular application I am connecting to external program via ssh.  I would not expect that the ssh connection would be an issue.

The program that I am calling produces some output and eventually asks for user input.  In the example below I only get a portion of the output, then at some point select.select tells me that read is not available.  If I try to read anyway I can keep getting more output.  As soon as I press a key (passing something to stdin),  select.select tells me that I can read again and I get the rest of the output.   

Any ideas?

    def wrapExternal(host=None):

       command = ["ssh", "-t", host, "some_program"]

       child = subprocess.Popen(command
       out = ''
       while True:
          r, w, x = select.select([child.stdout], [], [], 1.0)
          if child.poll() is not None:
          if r:
             out = child.stdout.read(1)
             print(out.decode(), end='')
       return child.returncode

components: Library (Lib)
messages: 130488
nosy: amicitas
priority: normal
severity: normal
status: open
title: Python select.select does not correctly report read readyness
type: behavior
versions: Python 3.1, Python 3.2

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