I'm trying to determine if the operating system is Unix-based from a Python script. I can think of two ways to do this but both of them have disadvantages:

Check if platform.system() is in a tuple such as ("Linux", "Darwin"). The problem with this is that I don't want to provide a list of every Unix-like system every made, in particular there are many *BSD varieties.
Check if the function os.fchmod exists, as this function is only available on Unix. This doesn't seem like a clean or "Pythonic" way to do it.
The Pythonic way to do it is not to care what platform you are on.
If there are multiple different facilities to accomplish something depending on the platform, then abstract them behind a function or class, which should try a facility and move on to another if that facility is not available on the current platform.
How can I determine if the operating system a Python script is running on is Unix-like?