Question

I would like to run an executable that performs some processing on a dataset located on a remote filer. As part of the design, I’d like the location of the filer to be flexible and something that’s passed to my python program at runtime.

I’ve put together the following bit of code that illustrates my problem, but using the python command, so anyone can run this:

#!/usr/bin/env python
import os
import subprocess

def runMySubProcess(cmdstr, iwd):
    p = subprocess.Popen(cmdstr,
        shell=True,
        cwd=iwd,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE)
    (stdout, stderr) = p.communicate()
    if stderr:
        raise IOError, stderr
    return stdout

if __name__ == '__main__':
    print runMySubProcess('python -h', 'C:\\')
    print runMySubProcess('python -h', '\\\\htpc\\nas')

This works great as long as iwd is on a share that’s be mapped to a drive letter on the machine. But if iwd is a UNC path the subprocess.Popen() call ends up with stderr output, which in turn throws the IOError exception:

Traceback (most recent call last):
  File "test.py", line 19, in <module>
    print runMySubProcess('dir', '\\\\htpc\\nas')
  File "test.py", line 14, in runMySubProcess
    raise IOError, stderr
IOError: '\\htpc\nas'
CMD.EXE was started with the above path as the current directory.
UNC paths are not supported.  Defaulting to Windows directory.

Is there a way to make this subprocess call work without resorting to parsing iwd and making a temporary drive mount on the machine that exists while the subprocess command executes? I want to avoid having to manage the creation and cleanup of drive mounts. And of course, I’d rather not have to deal with (albeit unlikely) case where all drive letters are currently in use on the machine.

Answer 1

The problem is not with Popen,, but with cmd.exe, which does not allow the working directory to be a UNC path. It just does not; try it. You may have better luck specifying shell=False on your Popen() call, assuming that whatever executable you’re running can handle a UNC path, but of course if what you’re trying to run is a command that’s built in to cmd.exe you don’t have a choice.