Antoine almost certainly thought about this with pathlib and may know about the change, or at least have some decent context on it.

I'm more inclined to think that os.path.isabs(r"\\server") should also return False, since it's not a path that can be opened directly unless you add more path before or after it.

Indeed, pathlib seems to support my understanding:

>>> Path('//server').is_absolute()
False
>>> Path('//server/share').is_absolute()
True
>>> Path('//server/share/').is_absolute()
True
>>> Path('//server/share/file').is_absolute()
True

If the regression appeared in 3.4, it should be easy enough to look at what changed.