I suddenly had a problem on the laptop (ubuntu 18.04, Bionic), and I’ve seen similar problems on Cinnamon. One manifestation is that when I try to start terminal using the gnome-terminal.desktop app, i.e. the icon in the dock, the launch fails, with no stated reason. A look in the log reveals a problem in Python loading _gi.

I resolved to look at it later. I was able to start the terminal from the desktop by right clicking, and open terminal.

Later I had a similar problem trying to launch gnome-tweak. It failed in the same way, trying to start up the python app it attempted to load _gi.

I have only the vaguest clue what is going on here. I looked at the python code. Indeed they are attempting:

From . import _gi. 

And the directory /usr/lib/python3/dist-packages/gi does not have a library named _gi. It does however have two library files:

_gi.cpython-36m-x86_64-linux-gnu.so
   and 
_gi_cairo.cpython-36m-x86_64-linux-gnu.so

Perusing the net, I find some suggestion that these libraries, with the 36m designation are somehow specific to or found by Python 3.6, and that if I want this to work under Python 3.7, I have to copy these files, and change 36m to 37m.

Ok, what have I got to lose? I did that simple thing – with just a simple copy in the above named directory. Lo and behold, it fixed the symptom that I was experiencing.

I don’t really like fixing things with magic when I don’t know what is happening, but now is not the time to go on a voyage of discovery with Python.