How to fix 'ModuleNotFoundError: No module named 'apt_pkg'?

Question

I'm running code sudo apt update and fetch error

ModuleNotFoundError: No module named 'apt_pkg'.

Please, help me solve it.

sudo apt update


Hit:20 http://ppa.launchpad.net/xorg-edgers/ppa/ubuntu bionic InRelease       
Fetched 93,3 kB in 2s (42,4 kB/s)                   
Traceback (most recent call last):
  File "/usr/lib/cnf-update-db", line 8, in <module>
    from CommandNotFound.db.creator import DbCreator
  File "/usr/lib/python3/dist-packages/CommandNotFound/db/creator.py", line 11, in <module>
    import apt_pkg
ModuleNotFoundError: No module named 'apt_pkg'
Reading package lists... Done
E: Problem executing scripts APT::Update::Post-Invoke-Success 'if /usr/bin/test -w /var/lib/command-not-found/ -a -e /usr/lib/cnf-update-db; then /usr/lib/cnf-update-db > /dev/null; fi'
E: Sub-process returned an error code

Answer 1

In my case the problem was due to upgrading python version from 3.6 to 3.8.

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 2

sudo update-alternatives --config python3

Solved by:

Settings back the python version to 3.6

Answer 2

Just reinstalled python3-apt and the error disappeared

sudo apt remove python3-apt

restart and then install,

sudo apt install python3-apt

NOTE: Use only if, python3 is a new installation dependency. Else it can screw existing python3 based setups on your system. Use of --reinstall is a safer bet though.

Answer 3

In my case, the problem was that I removed original /usr/bin/python3 symlink on Ubuntu 18.04 and replaced it with one pointing to python3.8. Problem disappeared when I restored the original pointing to python3.6

My takeaway is: if you need custom version of some library or module, install it in an isolated environment, do not mess up with system settings. Otherwise you are at risk of breaking something which can be noticed only later when it's difficult to figure out what exactly is wrong.