Comment # 1 on bug 988328 from Dominique Leuenberger

(In reply to Mario Manno from comment #0)
> User-Agent:       Mozilla/5.0 (X11; Linux x86_64; rv:47.0) Gecko/20100101
> Firefox/47.0
> Build Identifier: 
> 
>   File "/usr/bin/caffeine", line 3
> SyntaxError: Non-ASCII character '\xc2' in file /usr/bin/caffeine on line 3,
> but no encoding declared; see http://python.org/dev/peps/pep-0263/ for
> details

This is likely a bug in caffeine - and needs to be fixed (but, as you said: is
not causing the actual failure)

> After adding '# encoding: utf-8' to the source file, it's still not starting:
> 
> /usr/bin/caffeine:24: PyGIWarning: Gtk was imported without specifying a
> version first. Use gi.require_version('Gtk', '3.0') before import to ensure
> that the right version gets loaded.

This is only a warning - and does not stop the application from starting.

But I get more output when trying:
Traceback (most recent call last):
  File "/usr/bin/caffeine", line 36, in <module>
    ewmh = EWMH()
  File "/usr/lib/python3.5/site-packages/ewmh.py", line 50, in __init__
    self.display = _display or display.Display()
  File "/usr/lib/python3.5/site-packages/Xlib/display.py", line 80, in __init__
    self.display = _BaseDisplay(display)
  File "/usr/lib/python3.5/site-packages/Xlib/display.py", line 62, in __init__
    display.Display.__init__(*(self, ) + args, **keys)
  File "/usr/lib/python3.5/site-packages/Xlib/protocol/display.py", line 129,
in __init__
    raise error.DisplayConnectionError(self.display_name, r.reason)
Xlib.error.DisplayConnectionError: Can't connect to display ":0": b'No protocol
specified\n'


=> this in turn is an issue of python3-xlib itself (not caffeine), a minimal
test case to show the issue:

> from Xlib.display import Display
> display = Display()

run this small program through python and the same error appears.

Passing the bug to the python3-xlib maintainer(s)