On Ubunut, time to time, when running do-release-upgade on Python you may got hit with:

Your python3 install is corrupted. Please fix the '/usr/bin/python3' symlink.

The root cause is the python3 no longer match the default python version of the distro. The update progream is part of the distroy, and it requires that same python version.

Example, your python3 may point to python3.8, where as the current distro release require python3.6(which is its default python in the apt).

On top of that, python3 also doesn’t like /etc/alternatives/python3 when using with update-alternative.

The only way I found to make this move forward is manually set python3 symlink to the right python version directly, without going throuh other symlink

Upon that, doing upgrade will work and the side effect of the update is a newer Python versionfor that distro release is installed and ultimately correct any python version issue.

sudo ln -s /usr/bin/python3.6 /usr/bin/python3

If you don’t have python3.6 it can be re-installed:

apt install --reinstall python3
# This remove all /etc/alternative/python3 link
update-alternatives --remove-all python3
ln -sf /usr/bin/python3.6 /usr/bin/python3

Note, on a good system, this will never happens. This happen when people start to install python with some third party package or deadsnakes ppa.

Strongly recomend to stay away from that deadsnakes PPA and use pyenv tomanage python version instead.