Is the Python fcntl module different on different Linux kernels?

Question

I have a Python script that does locking thru the file system calls using the Python fcntl module.

The script works fine on an Ubuntu 10.10 release.

But if I run the same script on a Centos 6.2 host, the script bombs with the error:

*AttributeError: 'module' object has no attribute 'F_WRLCK'*

The actual code is:

    if Cmd['mode'] == 'r':
        lockType = fcntl.F_RDLCK
    else:
        lockType = fcntl.F_WRLCK

On the Centos host, the script bombs on the last line.

Why doesn't the fcntl module have a F_WRLCK attribute on the Centos 6.2 kernel but does on the Ubuntu?

FYI, both the Ubuntu 10.10 and Centos 6.2 are running Python 2.6.6.

Is there a workaround or solution to this? I need to use fcntl and not flock.