What happens if I downgrade libpython in 18.04?

Question

I just upgraded to ubuntu 18.04, and I'm not sure what happened, but vim doesn't work anymore. I tried sudo aptitude install vim and i get this:

$ sudo aptitude install vim
The following NEW packages will be installed:
  libpython3.6{ab} vim vim-runtime{a} 
0 packages upgraded, 3 newly installed, 0 to remove and 4 not upgraded.
Need to get 8009 kB of archives. After unpacking 36,8 MB will be used.
The following packages have unmet dependencies:
 libpython3.6 : Depends: libpython3.6-stdlib (= 3.6.6-1~18.04) but 3.6.7-1+xenial1 is installed
open: 17; closed: 390; defer: 5; conflict: 7                                                                                                 .The following actions will resolve these dependencies:
 Keep the following packages at their current version:

1) libpython3.6 [Not Installed]
2) vim [Not Installed]

Does anyone know what to do without ruining my stuff?

Answer 1

This happens when you forget to use ppa-purge before upgrading. It's annoying, but not a serious problem. You have to identify all the Python 3.6 packages that did not come from the primary Ubuntu archive and reinstall the right versions.

One way of doing that is to use apt-show-versions:

sudo apt install apt-show-versions
sudo apt-show-versions -i    # rebuild index; only needed once

apt-show-versions|grep -v uptodate|grep -v not.installed

Then look for packages having python3.6 in the name.

Now the simpler option is to uninstall them all:

sudo apt remove --auto-remove libpython3.6-stdlib ...

Now you can proceed with sudo apt install vim (and reinstall any other packages you need that got removed when you removed the old PPA Python 3.6).