org.freedesktop.PowerManagement always available?

[c.buhtz at posteo.jp]

Hello,
I am an upstream maintainer not knowing much about DBus. But I took over 
that old project [1] which is using DBus to inhibit hibernation on 
desktop systems.
Now I need to make some decisions regarding that piece of code I don't 
fully understand. So please apologize my dumb questions.

The code is iterating over 3 different session managers to execute this 
inhibit method; Gnome, Mate and Freedesktop.

1. My initial question is if this make even sense to you, trying three 
different managers?

2. To my research the freedesktop session manager is somehow connected 
to tjhe "upower" package which is available on most GNU/Linux systems. 
If "upower" is installed (maybe because it is a dependency of my 
application) can I rely on the fact, that the freedesktop manager will 
always be available? So I could get rid of the two other managers and 
the iteration?

Thank you very much.

Regards,
Christian Buhtz

[1] -- <https://github.com/bit-team/backintime/>
[2] -- 
<https://github.com/bit-team/backintime/blob/e59f93c55e79cc3c177e4a7f1fb581280c7d9006/common/tools.py#L1983-L2004>