[Tutor] script hangs when attempting to read Popen3 output

[David Clymer]

Are python's popen* commands/classes really supposed to work like pipes?
When I try to do somthing that works like this:

continuing input >> popen3('cat') >> read output

the script hangs when I try to read the command output. It seems I can
provide multi-line input, but I cant read any lines from the output of a
command that is still recieving input. I have to close the input before
reading any output. That doesnt seem very "pipe-ish" to me. Am I doing
something wrong, or just expecting the wrong behavior?

The script that I am working on is below.

TIA for any enlightenment you can provide me.

-davidc


david at gorilla:~/dev/debian/packages/userutils/experimental$ cat
shellcommands.py

"""
  Utilities & wrappers for running shell commands
"""

import sys, os
from debug import debug
from popen2 import Popen3

def cmd(command,input=None, dbg=False):
  pcmd = Popen3(command)

  # dont bother writing if no input is available
  if input != None:
    debug('input >> ' + input, dbg)
    try:
      pcmd.tochild.write(input + "\n")
      pcmd.tochild.flush()
      if pcmd.poll():
        print "still running"
    except IOError, e:
      print e

  # this magical line allows me to read
  pcmd.tochild.close()

  output=''
  while True:
    try:
      output += pcmd.fromchild.next()
      print 'output: %s' % output
    except StopIteration:
      break
    except IOError, e:
      print e
      break

  debug('output << ' + output, dbg)

  return output


# -- try it out -- #

input=''

# if input is provided from the cmd line, lets use it
if len(sys.argv) > 1:
  input = sys.argv[1]

print cmd("cat", input, dbg=True)