Lets say, we have a btrfs partion with subvolumes @, mounted at /
and home, which was created directly inside @, such that after mounting @, it can be found at /home.

Then 
os.path.ismount("/home") returns True
I would not have expected that, because the program findmnt from the util-linux software package doesn't list it if I run
findmnt -A

I think that the two programs should agree on whether it's a mount point or not but am unsure whether findmnt or os.path.ismount should be altered because I don't know a definite definition of a mountpoint.


Note that I agree with the current behaviour for explicitely mounted subvolumes. An example would be if I have the subvolume @ mounted at / and a subvolume @home created at the toplevel of the btrfs filesystem which is then mounted at /home.
Here findmnt and os.path.ismount agree that this is a mount point.