Do sudo update-alternatives --remove-all gccc.

replace

[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2

[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3

with

[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2

[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3

e.g. installing into /usr/local/bin instead of /usr/bin.

and ensure the /usr/local/bin is before /usr/bin in PATH.

i.e.

[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin

Ensure this always is the case by adding

export PATH=/usr/local/bin:$PATH

to the end of your ~/.bashrc file. Prefixing the PATH environment variable with custom bin folder such as /usr/local/bin or /opt/<some install>/bin is generally recommended to ensure that customizations are found before the default system ones.

I would try cleaning it up manually. I've never done this so make sure you backup beforehand.

• Remove the link from /etc/alternatives
• Remove the relevant file from the admin directory
  • /var/lib/dpkg/alternatives/ on ubuntu (debian may be the same but check the man pages under the FILES section)
  • /var/lib/alternatives/ on CentOS 6&7

Python 2.X packages are named differently than Python 3.X packages in the default Ubuntu repositories, python-... for Python 2.X packages vs. python3-... for Python 3.X packages. The command to start the Python interpreter follows a similar pattern, python for the default Python 2.X version vs. python3 for the default Python 3.X version.

Don't remove the default Python 3.x version because removing it can break a lot of things like the terminal, the Software app and many other apps. For more information see this question: Removed Python 3 and now Ubuntu Software Center, terminal and other applications don't work. The good news is that even if you break those things it is possible to restore the original Python 3.x version by booting Ubuntu into recovery mode, and then Ubuntu will work normally again.

Instead of replacing the default Python 3.x version entirely the recommended way of installing another Python 3.x version is to keep the existing Python 3 and install the new Python 3 version alongside it. Then you can use a program called update-alternatives to select which one of the Python versions you want to use.

Add Python 3.8 to update-alternatives so that you can switch between Python 3.5 and Python 3.8 by running update-alternatives --config python3.

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8
update-alternatives --config python3

After you are done using Python 3.8 you can switch the it back to the default Python 3.5 version.

• List installed versions of Python: update-alternatives --list python

• Switch between Python versions: update-alternatives --config python

  From the terminal command-line Press <enter> to keep the current choice[*], or type selection number:

Listen, when playing with python on your OS you gotta be careful since it can break many applications and even the OS itself... anyway :

• 1)Install python3.7 (you've done this already so lets proceed)

• 2)Add Python 3.6 & Python 3.7 to update-alternatives

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

  sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 2

• 3)Update Python 3 to point to Python 3.7 with this command

  sudo update-alternatives --config python3

reference to the original guide -

https://www.itsupportwale.com/blog/how-to-upgrade-to-python-3-7-on-ubuntu-18-10/

I have answered my own question. First, I did have alternatives, but as Ubuntu defaults to Python2 (please Ubuntu change that -- the world has moved on), the command is sudo update-alternatives --config python3:

$ sudo update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.8   2         auto mode
  1            /usr/bin/python3.6   1         manual mode
  2            /usr/bin/python3.8   2         manual mode

Note the line "providing /usr/bin/python3." That means the Python symlinks for Python 3 are in /usr/bin/python3. So I add the new one:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/local/bin/python3.10 3

That makes Python 3.10 priority 3, the highest priority. Then:

$ python3 --version
Python 3.10.0   

I hope that helps someone in the future.

To solve the error: no alternatives for python3 error message open the terminal and run the following command:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 2
sudo update-alternatives --config python3

You are definitely able to do an "n to n" link as you state, just add the python3 group symlink you created as a potential target to python:

alternatives --install /usr/bin/python python /usr/bin/python3 300
alternatives --set python /usr/bin/python3

Each --install command will add or update an alternative, which you can see with alternatives --display python

Compare the python and python3 symlinks in /etc/alternatives/ (e.g. with ls -lF /etc/alternatives/python*.

My guess is that python points to python3.8 while python3 points to python3.10. Or vice-versa.

Is there any particular reason why you want python to run 3.8 instead of 3.10? if not, run update-alternatives again and set it to either auto (preferred) or to 3.10 (you'll run into the same problem in future when 3.11 or 3.12 or whatever is the latest auto version).

Or just make sure that the alternatives symlinks for python and python3 point to the same version of python.