subprocess.Popen using relative paths

Question

The docs for Popen mention that you can't specify your executable path relative to the 'change working directory' kwarg.

If cwd is not None, the child’s current directory will be changed to cwd before it is executed. Note that this directory is not considered when searching the executable, so you can’t specify the program’s path relative to cwd.

But python's behaviour on my system seems to directly contradict this claim:

/tmp$ mkdir a
/tmp$ cp /bin/ls /tmp/a/my_ls
/tmp$ mkdir b
/tmp$ touch /tmp/b/potato
/tmp$ cd /home/wim
~$ python
Python 2.7.5+ (default, Sep 19 2013, 13:48:49) 
[GCC 4.8.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from subprocess import check_output
>>> check_output(['../a/my_ls'], cwd='/tmp/b')
'potato\n'
>>> check_output(['../a/my_ls'])
OSError: [Errno 2] No such file or directory

Is using relative paths to cwd something that is platform dependent and shouldn't be relied upon? Or is this a documentation bug?

(this question spawns from a comment by glglgl here)

Answer 1

Yes, this is platform dependant.

On POSIX systems, the process is forked and in the child process a os.chdir(cwd) is executed before executing the executable.

On Windows however, the CreateProcess() API call is used and cwd is passed in as the lpCurrentDirectory parameter. No directory change takes place, and the CreateProcess() call does not consult that parameter when looking for the lpApplicationName to execute.

To keep your application cross-platform, you should not rely on the current working directory to be changed when looking up the executable.