I don't understand popen2 :(

[marco at atmosp.physics.utoronto.ca]

Hello All,

I'm a python newbie, happy so far, but somewhat bewildered :(

My problem is with popen2.popen3. It sometimes gets stuck when
reading the process std_out/err. Here is an example:

>>> (po, pi, pe) = popen2.popen3('ssh computer "ls"')
>>> po_out = po.read()
>>> pe_err = pe.read()
>>> pe_err
''
>>> po_out
[Directory listing OK]

Alternatively (notice the read order):

>>> (po, pi, pe) = popen2.popen3('ssh computer "ls"')
>>> pe_err = pe.read()
>>> po_out = po.read()
>>> pe_err
''
>>> po_out
[Directory listing OK]

Fine. Now I try something slightly different:

>>> (po, pi, pe) = popen2.popen3('ssh computer "find /home/marco"')
>>> po_out = po.read()
>>> pe_err = pe.read()
>>> pe_err
''
>>> po_out
[snip find results]

which is fine, BUT if I do:

>>> (po, pi, pe) = popen2.popen3('ssh computer "find /home/marco"')
>>> pe_err = pe.read()

[freezes!! I then Ctrl-C]

Traceback (innermost last):
  File "<stdin>", line 1, in ?
KeyboardInterrupt

>>> po_out = po.read()
>>> po_out
[snip find results]

So, in this last case if I pe.read() first
my script freezes, and I have to Ctrl-C. Sure, I
could always po.read() first, but my biggest problem
is that sometimes it's the other way around! That is,
performing po.read() first causes a freeze! Unfortunately,
I cannot reliably reproduce this last behaviour, but here
is the result from a script I had to Ctrl-C:

prompt> ./myprog.py

[frozen -- Ctrl-C]

Traceback (innermost last):
  File "./myprog.py", line 1245, in ?
    get_files()
  File "./myprog.py", line 1102, in get_files
    po_out = po.read()
KeyboardInterrupt

line #
 1101     (po, pi, pe) = popen2.popen3(scp_from_remote_IP)
 1102     po_out = po.read()
 1103     pe_err = pe.read()

Sadly, the scp operation *does* seem to work most of the time,
why does it sometimes get stuck? Thoughts? Ideas?

I'm using:
Python 1.5.2 (#1, Feb  1 2000, 16:32:16)  [GCC egcs-2.91.66 19990314/Linux
(egcs- on linux-i386)]

Thanks for any help!
(Oh, and please CC me a reply if possible -- Deja's undergoing
transition pains to Google right now)

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