The OS module interacts directly with the OS rather than abstracts it, so a lot of the functions in it have the "may not be available on all platforms" apology.

The OS module interacts directly with the OS rather than abstracts it, so a lot of the functions in it have the "may not be available on all platforms" apology. Subprocess uses OS under the hood but offers an abstraction that mostly works on all platforms, e.g. the way that "communicate" is implemented on Windows differs considerably from how its implemented on Unix.