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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().