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The return value of os.access(FILE, os.X_OK) is not consistent across operating system when executed as "root"
I have tested with Python 2.5 on Linux and Solaris, but there is a bug in python-nose reporting the same behavior with Python 2.6 on Solaris and AIX.
http://code.google.com/p/python-nose/issues/detail?id=423
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On Solaris:
$ ls -al 
-rw-rw-r-- 1 buildslave other 1079 May 1 16:25 nose_runner.py
$ ./bin/python 
Python 2.5.6 (r256:88840, Nov 1 2011, 12:19:05) 
[GCC 3.4.3 (csl-sol210-3_4-branch+sol_rpath)] on sunos5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.access('nose_runner.py', os.X_OK)
False
$ sudo ./bin/python 
Python 2.5.6 (r256:88840, Nov 1 2011, 12:19:05) 
[GCC 3.4.3 (csl-sol210-3_4-branch+sol_rpath)] on sunos5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.access('nose_runner.py', os.X_OK)
True
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On Linux:
$ ls -al 
-rw-rw-r-- 1 adi adi 1079 2012年05月02日 11:25 nose_runner.py
$ ./bin/python 
Python 2.5.5 (r255:77872, Sep 14 2010, 16:22:46) 
[GCC 4.4.5] on linux2
>>> import os
>>> os.access('nose_runner.py', os.X_OK)
False
$ sudo ./bin/python 
Python 2.5.5 (r255:77872, Sep 14 2010, 16:22:46) 
[GCC 4.4.5] on linux2
>>> import os
>>> os.access('nose_runner.py', os.X_OK)
False

This is not a Python bug. os.access() is just a wrapper around the POSIX access() function:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/faccessat.html
"If any access permissions are checked, each shall be checked individually, as described in XBD File Access Permissions, except that where that description refers to execute permission for a process with appropriate privileges, an implementation may indicate success for X_OK even if execute permission is not granted to any user."
So this seems to be a well-known portability problem accross Unix implementations. If you want to test the executable bits, just use os.stat().

Many thanks for your comment!
Cheers,
Adi