How to fix Python 'ValueError: Namespace GnomeDesktop not available' on Ubuntu

Problem:

On Ubuntu, you are trying to run a Python script using the gi package and GnomeDesktop but you are seeing this stacktrace:

Traceback (most recent call last):
  File "myscript.py", line 48, in <module>
    gi.require_version('GnomeDesktop', '3.0')
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 130, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace GnomeDesktop not available

Solution

Install gir1.2-gnomedesktop-3.0:

sudo apt -y install gir1.2-gnomedesktop-3.0

and retry running your script.